General Description
The MAX16807 evaluation kit (EV kit) is an 8-channel,
constant-current LED driver, capable of driving 50mA
through each channel and adapting the channel-supply
voltage. Each channel can be used to drive a string of
LEDs with a total forward voltage of up to 32V. The EV
kit features a MAX16807 IC that integrates eight con-
stant-current sinking outputs and a high-performance,
current-mode pulse-width modulator (PWM) controller
for implementing a DC-DC power converter that gener-
ates the supply voltage to drive LED strings connected
to each channel by the user. The sink current for the
eight channels is configurable using a single resistor.
The MAX16807 EV kit operates with supply voltages up
to 16V. The EV kit circuit also features PWM dimming
and shutdown control input PC pads. The MAX16807
EV kit is a fully assembled and tested board.
Features
♦Up to 16V Supply Voltage Range
♦50mA Output Current per Channel
♦Single Resistor Current Adjust for Eight Channels
♦Up to 32V LED Forward String Voltage
♦Boost Converter to Generate LED Voltage
♦Adaptive LED Voltage Control Increases
Efficiency
♦PWM Dimming and Shutdown Control Inputs
♦Proven PCB Layout
♦Fully Assembled and Tested
Evaluates: MAX16807
MAX16807 Evaluation Kit
________________________________________________________________
Maxim Integrated Products
1
19-0818; Rev 0; 5/07
Component List
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
Ordering Information
PART TEMP RANGE IC PACKAGE
MAX16807EVKIT+
0°C to +70°C* 28 TSSOP-EP**
DESIGNATION
QTY
DESCRIPTION
C1, C2 2
22µF ±20%, 50V electrolytic
capacitors (D-case)
Panasonic EEEFK1H220P
C3, C4, C12,
C13, C15 5
0.1µF ±10%, 50V X7R ceramic
capacitors (0603)
Murata GRM188R71H104K
TDK C1608X7R1H104K
C5 1
560pF ±5%, 50V C0G ceramic
capacitor (0603)
Murata GRM1885C1H561J
TDK C1608C0G1H561J
C6 1
150pF ±5%, 50V C0G ceramic
capacitor (0603)
Murata GRM1885C1H151J
TDK C1608C0G1H151J
C7 1
10pF ±5%, 50V C0G ceramic
capacitor (0603)
Murata GRM1885C1H100J
TDK C1608C0G1H100J
C8 1
100pF ±10%, 50V C0G ceramic
capacitor (0603)
Murata GRM1885C1H101K
TDK C1608C0G1H101K
DESIGNATION
QTY
DESCRIPTION
C9 1
1µF ±10%, 50V X7R ceramic
capacitor (1206)
Murata GRM31MR71H105K
TDK C3216X7R1H105K
C10, C11 2
22µF ±20%, 35V electrolytic
capacitors (C-case)
Panasonic EEEFK1V220R
C14 1
1µF ±10%, 16V X5R ceramic
capacitor (0603)
Murata GRM188R61C105K
TDK C1608X5R1C105K
C16 1
0.01µF ±10%, 50V X7R ceramic
capacitor (0603)
Murata GRM188R71H103K
TDK C1608X7R1H103K
C17–C24 8
1000pF ±10%, 50V X7R ceramic
capacitors (0603)
Murata GRM188R71H102K
TDK C1608X7R1H102K
C25 0 Not installed, ceramic capacitor
(0603)
+
Denotes a lead-free and RoHS-compliant EV kit.
*
This limited temperature range applies to the EV kit PCB only.
The MAX16807 IC temperature range is -40°C to +125°C.
**
EP = Exposed paddle.
Evaluates: MAX16807
MAX16807 Evaluation Kit
2 _______________________________________________________________________________________
Component Suppliers
SUPPLIER PHONE WEBSITE
Central Semiconductor 631-435-1110 www.centralsemi.com
Coilcraft, Inc. 847-639-6400 www.coilcraft.com
Diodes Inc. 805-446-4800 www.diodes.com
IRC, Inc. 361-992-7900 www.irctt.com
Murata Mfg. Co., Ltd 770-436-1300 www.murata.com
TDK Corp. 847-803-6100 www.component.tdk.com
Panasonic Corp. 800-344-2112 www.panasonic.com
Vishay 203-268-6261 www.vishay.com
Component List (continued)
DESIGNATION
QTY
DESCRIPTION
D1 1
40V, 1A Schottky diode (SMA)
Central Semiconductor
CMSH1-40ML LEAD FREE
D2–D5 4 6.2V dual zener diodes (SOT23)
Diodes Inc. AZ23C6V2-7-F
D6 1
30V, 30mA Schottky diode (SOD523)
Diodes Inc. SDM03U40
D7 1
75V, 300mA fast switching diode
(SOD323)
Diodes Inc. 1N4148WS
D8 1 33V zener diode (SOD323)
Diodes Inc. MMSZ5257BS
J1 1 10-pin header
L1 1 33µH, 2.3A inductor
Coilcraft MSS1038-333ML
N1 1
40V, 3.5A n-channel MOSFET
(SOT23)
Vishay Si2318DS-E3
Q1 1
40V, 600mA npn small signal
transistor (SOT523)
Diodes Inc. MMBT2222AT
R1 1 200kΩ ±1% resistor (0603)
R2 1 8.45kΩ ±1% resistor (0603)
DESIGNATION
QTY
DESCRIPTION
R3 1 15Ω ±5% resistor (0603)
R4, R13 2 22kΩ ±1% resistors (0603)
R5 1 1.2kΩ ±1% resistor (0603)
R6 1 17.4kΩ ±1% resistor (0603)
R7 1 365Ω ±1% resistor (0603)
R8 0 Not installed, resistor (1206)
R9 1 0.11Ω ±1%, 0.5W resistor (1206)
IRC, Inc. LRC-LR1206LF-01-R110-F
R10 1 330kΩ ±1% resistor (0603)
R11 1 75kΩ ±1% resistor (0603)
R12, R15 2 10kΩ ±1% resistors (0603)
R14 1 2.21kΩ ±1% resistor (0603)
U1 1
MAX16807AUI+ (28-pin TSSOP-EP)
U2 1
Dual Schmitt trigger inverter
(SC70-6)
TI SN74LVC2G14DCKT
U3 1
-50V, -100mA pnp digital transistor
(SC59)
Diodes Inc. DDTA114WKA
U4 1
50V, 100mA npn digital transistor
(SC59)
Diodes Inc. DDTC114WKA
—- 1 PCB: MAX16807 Evaluation Kit+
Note: Indicate that you are using the MAX16807 when contacting these component suppliers.
Evaluates: MAX16807
MAX16807 Evaluation Kit
_______________________________________________________________________________________ 3
Quick Start
Recommended Equipment
• One 16V, 2A adjustable power supply
• One 5V power supply
• One voltmeter
• Eight LED strings with a total forward voltage rating
≤32V (optional)
• One PWM signal generator (optional)
Procedure
The MAX16807 EV kit is a fully assembled and tested sur-
face-mount printed circuit board (PCB). Follow the steps
below to verify board operation. Caution: Do not enable
the power supply until all connections are made.
1) Adjust the 16V power supply output to 12V.
Connect this power supply between the EV kit’s VIN
and GND pads.
2) Connect the 5V power supply between the EV kit’s
VBIAS and GND pads.
3) Connect the SHDN pad to the VIN pad.
4) Enable both power supplies.
5) Use the voltmeter to verify that the voltage at head-
er J1, pins J1-1 and J1-2, referencing GND, mea-
sures approximately 36V.
6) Disable the VIN power-supply output.
7) Connect the anode of each LED string to VLED (pins
J1-1and J1-2). Connect the cathode of each LED
string to channels OUT0–OUT7 (pins J1-3–J1-10).
8) Enable the VIN power-supply output.
9) Verify that all LEDs are illuminating.
10) Connect a PWM signal with amplitude of 5V and a fre-
quency between 100Hz and 2kHz to the PWM input
PC pad. The LED brightness should increase as the
PWM signal’s duty cycle increases and vice versa.
Detailed Description
This EV kit evaluates the MAX16807 IC, which has two
major sections. The first section consists of eight con-
stant-current LED drivers for LED strings. Each driver
can sink up to 55mA through an LED string when ON
and block up to 36V when OFF. The second section is
a high-performance, current-mode PWM controller that
controls a power converter to generate a voltage for dri-
ving the LED strings. The EV kit uses the PWM con-
troller to drive a boost-converter circuit, which takes a
9V to 16V input and generates up to 36V LED voltage at
header pins J1-1 and J1-2 (VLED). To drive the LED
string with constant current, connect the LED string
between the VLED output and any of the eight con-
stant-current sink outputs. The sink current of each out-
put is configured to 50mA with resistor R7.
The LED voltage generated by the boost converter on
the MAX16807 EV kit is adaptive. The LED string with
the highest total forward voltage dominates the control
loop. The boost-converter voltage is adjusted so that
the driver associated with that string receives just
enough voltage as needed for current drive. All other
strings with lower total forward voltages will have
excess supply voltage, which is then dropped in the
associated driver. This feedback mechanism ensures
that the linear current-control circuit dissipates the mini-
mum possible power.
The MAX16807 EV kit does not require an external
microcontroller to enable the eight LED drivers. The EV
kit circuit is configured to enable all LED drivers by
tying the MAX16807 DIN (data-in) and LE (latch-
enable) pins to a logic-high signal and automatically
supplies approximately a 50kHz clock signal. Inverter
U2 is configured to generate the clock signal and to
implement the PWM dimming function. A 5V (VBIAS)
supply is also required to power the MAX16807 con-
stant-current output drivers and the inverter.
Power Supplies
The MAX16807 EV kit requires an 8.8V to 16V power
supply connected across VIN and GND PC pads, and
a 5V power supply connected across VBIAS and GND
PC pads for normal operation. The 8.8V to 16V power
supply is used to provide power to the MAX16807 IC
(U1) and to the DC-DC step-up power converter. The
5V power supply is used to provide power to the con-
stant-current LED driver of the MAX16807 and to the
dual Schmitt trigger inverter (U2). The VBIAS power
supply also provides a logic-high voltage signal to the
DIN and LE pins.
Evaluates: MAX16807
MAX16807 Evaluation Kit
4 _______________________________________________________________________________________
LED Driver
The MAX16807 feature an 8-channel constant-current
LED driver, with each channel capable of sinking up to
55mA of LED current. LED strings can be connected
between VLED (J1-1 and J1-2) and the constant-current
sink outputs to drive regulated current through each LED
string. The current through all eight channels is con-
trolled through resistor R7, which is connected from the
SET pin to ground. The current through each string is
configured to 50mA and the maximum VLED voltage to
33V. The EV kit can drive LED strings with a total for-
ward voltage of up to 32V.
The MAX16807 4-wire (DIN, CLK, LE, and OE) serial
interface controls the eight constant-current outputs.
The MAX16807 EV kit circuit connects DIN and LE to
5V and uses a clock signal, generated by inverter U2,
to clock eight logic 1s into the IC’s internal shift regis-
ter, thus enabling all eight channels. The output enable
(OE) pin is configured to provide PWM dimming. An
inverted PWM signal, generated by the inverter U2, drives
the OE pin. When the PWM signal is low (LED drivers off)
it also influences the feedback with the network formed
by R13 and D6. See the
Adaptive LED Voltage Control
section for more details.
Output-Current Setting
The amplitude of the output currents for all eight chan-
nels is set by resistor R7. The minimum value for resis-
tor R7 is 324Ω, which sets the output currents to 55mA.
The maximum value of R7 is 4.99kΩ, which sets the
output current to 3.6mA. The MAX16807 EV kit sets the
output current to 50mA with a 365ΩR7 resistor. To set
a different output current, use the following equation:
where IOUT is the desired output current.
PWM Dimming
The MAX16807 EV kit features a PWM input PC pad
that can be used to control the LED brightness by
adjusting the duty cycle of the PWM input signal.
Applying a logic-high signal at the PWM input enables
the output current and a logic-low signal turns off the
output current. The PWM signal is conditioned through
inverter U2 before reaching the MAX16807 OE pin.
Connect a PWM signal with peak amplitude of 3V to 5V
and a frequency in the range of 100Hz to 2kHz to the
EV kit PWM input PCB pad. Vary the duty cycle to
adjust the LED brightness. The LED brightness increas-
es when the duty cycle increases and vice versa.
SHDN
Input
The MAX16807 EV kit features a SHDN input PC pad to
enable or disable the MAX16807 IC. Connect 5V or VIN to
the SHDN pad to enable the IC. Connect the SHDN pad
to ground or leave disconnected to disable the IC. The IC
can also be enabled by connecting VIN to test point TP3.
Adaptive LED Voltage Control
To reduce power dissipation in the IC, the MAX16807
EV kit features adaptive voltage control of VLED based
on the operating voltage of the LED strings. The con-
stant-current outputs can sink stable currents with
channel voltages as low as 0.8V. The voltage at each of
the outputs will be the difference between VLED and
the total forward voltage of the LED string connected to
that output. The MAX16807 EV kit implements a feed-
back mechanism to sense the voltage at each of the
outputs. Using dual zener diodes (D2–D5), the circuit
selects the lowest voltage among all the output chan-
nels. The PWM boost converter will then adjust VLED
until this output channel is 0.8V. All other strings will
have sufficient voltage, as their total forward voltages
are equal or less. This feedback mechanism ensures
that the IC dissipates the minimum possible power. For
adaptive control to function efficiently, connect LED
strings to all eight channels and use equal number of
LEDs of the same VFrating in each string. Use the fol-
lowing equation to calculate the value of resistor R10 to
set the minimum voltage at the outputs:
where 2.5V is the feedback reference, VDZ is the forward
voltage drop (0.65V) of the zener diodes (D2–D5), VS
(0.8V) is the required sink output voltage, and VFLED is the
total nominal operating voltage of the LED strings. Select
the value of R10 such that R12 is approximately 10kΩ.
Zener diodes D2–D5 also provide output overvoltage
protection. If an LED string becomes partially or fully
short-circuited, making the sink output voltage rise
above 17.5V, the 15V zener diode connected to that
output conducts in reverse direction and limits the
VLED voltage. Under this condition, the other LED
strings might not turn on.
R7 =18V
IOUT
Evaluates: MAX16807
MAX16807 Evaluation Kit
_______________________________________________________________________________________ 5
When the outputs are off, the LED drivers are high
impedance and the feedback network now combines
R13 and D6 to provide a path for the feedback current
and to control VLED. Use the following equation to cal-
culate the value of R13 to get the required LED supply
voltage during PWM off time:
where 2.5V is the feedback reference voltage, 0.4V is
the total voltage dropped by diode D6, and VLEDOFF is
the desired LED supply voltage during PWM off time.
VLEDOFF should be set to the worst-case LED string VF
voltage, plus additional headroom for the LED drivers
that must be greater than 0.8V, as well as a reserve
voltage (about +1V). This reserve voltage allows the
MAX16807 to provide current for very short PWM dim-
ming pulses. With pulses as low as 2µs, the VLED con-
trol loop is not able to react, and the output capacitors
provide all the current. For longer PWM dimming puls-
es, the control loop will react and the supply will oper-
ate at the adaptive voltage level.
During an open LED condition, the 33V zener diode
(D8) limits the maximum VLED supply voltage to 35.5V.
If VLED attempts to increase beyond this level, D8 con-
ducts in reverse direction and pulls the FB pin high,
which causes the boost regulator to cut back on the
PWM signal and reduce the output voltage.
Boost Converter
The EV kit boost-converter circuit is configured to gener-
ate up to 33V of LED voltage (VLED) and operate at a
switching frequency of 350kHz in continuous conduction
mode (CCM). The MAX16807’s current-mode PWM con-
troller drives external MOSFET N1. The MOSFET is
turned on at the beginning of every switching cycle and
turned off when the current through the inductor (L1)
reaches the peak value set by the error-amplifier output
voltage. Inductor current is sensed by the MAX16807 CS
pin using the voltage across current-sense resistor R9.
The RC filter, consisting of R5 and C8, removes voltage
spikes in the current-sense signal produced by the turn-
on gate current of MOSFET N1, and the reverse-recov-
ery current of D1. Without filtering, these current spikes
can cause the MAX16807 to turn off N1 prematurely.
The filter time constant is configured to 120ns.
During normal operating conditions, the feedback loop
and compensation network (R1, R10, R11, C6, and C7)
control the peak current. The error amplifier compares
a scaled-down version of the VLED voltage with the
MAX16807 highly accurate 2.5V reference. The error
amplifier and compensation network then amplify the
error signal, and the current comparator compares this
signal to the sensed current voltage to create a PWM
drive output.
Power-Circuit Design
Initially, decide the input supply voltage range, the max-
imum voltage (VLED) that is required to drive the LED
strings, plus 1V (minimum voltage across the constant-
current sink = 0.8 + VLED ripple peak), and the output
current IOUT (the sum of all the LED string currents).
Calculate maximum duty cycle DMAX using the following
equation:
where VDis the forward drop of the rectifier diode D1
(~0.6V), VINMIN is the minimum input supply voltage (in
this case, 9V), and VFET is the drain-to-ground voltage
of the MOSFET N1 when it is on.
Select the switching frequency (FSW) depending on the
space, noise, dynamic response, and efficiency con-
straints. Select the maximum peak-to-peak ripple on
the inductor current (ILPP). For the MAX16807 EV kit,
FSW is 350kHz and ILPP is ±30% of the average induc-
tor current. Use the following equations to calculate the
maximum average inductor current ILAVG and peak
inductor current ILPEAK:
Since ILPP is ±30% of the average inductor current
ILAVG:
Calculate the minimum inductance value LMIN with the
inductor current ripple set to the maximum value:
Choose an inductor that has a minimum inductance
that is greater than this calculated value.
Calculate the current-sense resistor (R8 in parallel with
R9) using the equation below:
where 0.3V is the maximum current-sense signal voltage.
The factor 0.75 is for compensating the reduction of
RIL
CS PEAK
=×03 075..
LVIN V D
FIL
MIN MIN FET MAX
SW PP
=−×
×
()
IL IL
IL IL IL
PP AVG
PEAK AVG PP
=××
=+
03 2
2
.
IL I
D
AVG OUT
MAX
=−1
DVLED V VIN
VLED V V
MAX DMIN
D FET
=+−
+−
R1 3 =×
()
R10 2.5V - 0.4V
VLED -2.5V
OFF
Evaluates: MAX16807
MAX16807 Evaluation Kit
6 _______________________________________________________________________________________
maximum current-sense voltage due to the addition of
slope compensation. Check this factor and adjust after
the slope compensation is calculated. Slope compensation
is explained in detail in a later section.
The saturation current limit of the selected inductor
(ILSAT) should be greater than the value given by the
following equation. Selecting an inductor with 10%
higher ILSAT rating is a good choice.
Calculate the output capacitor COUT (parallel combina-
tion of C1, C2, and C15) using the following equation:
where VLEDPP is the peak-to-peak ripple in the LED
supply voltage. The value of the calculated output
capacitance will be much lower than what is actually
necessary for feedback loop compensation. See the
Feedback Compensation
section to calculate the output
capacitance based on the compensation requirements.
Calculate the input capacitor CIN (parallel combination
of C9, C10, and C11) using the following equation:
where VINPP is the peak-to-peak input ripple voltage.
This equation assumes that input capacitors supply
most of the input ripple current.
Selection of Power Semiconductors
The switching MOSFET (N1) should have a voltage rat-
ing sufficient to withstand the maximum output voltage
together with the diode drop of D1, and any possible
overshoot due to ringing caused by parasitic inductances
and capacitances. Use a MOSFET with voltage rating
higher than that calculated by the following equation:
where the factor of 1.3 provides a 30% safety margin.
The continuous drain-current rating of the selected
MOSFET, when the case temperature is at +70°C,
should be greater than that calculated by the equation
below. The MOSFET must be mounted on a board, as
per manufacturer specifications, to dissipate the heat.
The MOSFET will dissipate power due to both switching
losses, as well as conduction losses. Use the following
equation to calculate the conduction losses in the
MOSFET:
where RDSON is the on-state drain-source resistance of
the MOSFET with an assumed junction temperature of
+100°C.
Use the following equation to calculate the switching
losses in the MOSFET:
where IGON and IGOFF are the gate currents of the
MOSFET (with VGS equal to the threshold voltage)
when it is turned on and turned off, respectively, and
CGD is the gate-to-drain MOSFET capacitance. Choose
a MOSFET that has a higher power rating than that cal-
culated by the following equation when the MOSFET
case temperature is at +70°C:
The MAX16807 EV kit uses a Schottky diode as the
boost-converter rectifier (D1). A Schottky rectifier diode
produces less forward drop and puts the least burden
on the MOSFET during reverse recovery. If a diode with
considerable reverse-recovery time is used, it should be
considered in the MOSFET switching-loss calculation.
The Schottky diode selected should have a voltage rat-
ing 20% above the maximum boost-converter output
voltage. The current rating of the diode should be
greater than IDin the following equation:
Slope Compensation
When the boost converter operates in CCM with more
than 50% duty cycle, subharmonic oscillations will
occur if slope compensation is not implemented.
Subharmonic oscillations do not allow the PWM duty
cycle to settle to a peak current value set by the volt-
age feedback loop. The duty cycle oscillates back and
forth about the required value (usually at half the
switching frequency). Subharmonic oscillations will die
out if a sufficient negative slope is added to the inductor
peak current. This means that for any peak current set
IIL
D
DAVG
MAX
=−
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟×
2
112.
PP P
TOT COND SW
=+
PIL VLED C F
II
SW AVG GD SW
GON GOFF
=×××
×+
⎛
⎝
⎜⎞
⎠
⎟
2
2
11
PIL
DRDS
COND AVG
MAX ON
=×
2
ID IL
D
RMS AVG
MAX
=
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟×
2
13.
V VLED V
DS D
=+
()
×13.
CIL
F VIN
IN PP
SW PP
=××8
CDI
VLED F
OUT MAX OUT
PP SW
=×
×
IL IL
SAT PEAK
=×11.
by the feedback loop, the output pulse will terminate
sooner than normally expected. The minimum slope
compensation that should be added to stabilize the
current loop is half of the worst-case (maximum) falling
slope of inductor current.
Adding a ramp, with positive slope in sync with the
switching frequency, to the current-sense signal can
produce the desired function. The greater the duty
cycle, the greater the added voltage, and the greater
the difference between the set current and the actual
inductor current. In the MAX16807 EV kit, the oscillator
ramp signal is buffered using Q1 and added to the cur-
rent-sense signal with proper scaling to implement the
slope compensation. Follow the steps below to calcu-
late the component values for slope compensation.
Calculate the worst-case falling slope of the inductor
current using the following equation:
From the inductor current falling slope, find its equiva-
lent voltage slope across the current-sense resistor
RCS (R8 parallel with R9) using the following equation:
The minimum voltage slope that should be added to
the current-sense waveform is half of VSLOPE for ensur-
ing stability up to 100% duty cycle. As the maximum
continuous duty cycle used is less than 100%, the mini-
mum required compensation slope becomes:
where the factor 1.1 provides a 10% margin. Resistors
R5 and R6 determine the attenuation of the buffered
voltage slope from the emitter of Q1. The forward drop
of signal diode D7, together with the VBE of Q1, almost
cancel the 1.1V offset of the ramp waveform. Calculate
the approximate slope of the oscillator ramp using the
following equation:
where 1.7V is the ramp amplitude and FSW is the
switching frequency.
Select the value of R5 so that the input bias current of
the current-sense comparators does not add consider-
able error to the current-sense signal. The value of R6
for the slope compensation is given by the equation:
Feedback Compensation
Like any other circuit with feedback, the boost convert-
er that generates the voltage for the LED strings needs
to be compensated for stable control of its output volt-
age. When the boost converter is operated in CCM,
there exists a right-half-plane (RHP) zero in the power-
circuit transfer function. This zero adds a 20dB/decade
gain, together with a 90° phase lag, which is difficult to
compensate. The easiest way to avoid this zero is to roll
off the loop gain to 0dB, at a frequency less than half of
the RHP zero frequency, with a -20dB/decade slope.
For a boost converter, the worst-case RHP zero fre-
quency (FZRHP) is given by the following equation:
where DMAX is the maximum duty cycle, L is the induc-
tance of the inductor, and IOis the output current,
which is the sum of all the LED string currents.
The boost converter used in the MAX16807 EV kit is
operated in peak current-mode control. There are two
feedback loops within a current-mode controlled con-
verter: an inner loop that controls the inductor current,
and an outer loop that controls the output voltage. The
amplified voltage error produced by the outer voltage
loop is the input to the inner current loop that controls
the peak inductor current.
The internal current loop converts the double-pole/sec-
ond-order system, formed by the inductor and the out-
put capacitor COUT, to a first-order system having a
single pole consisting of the output filter capacitor and
the output load. As the output load is a constant current
(very high Thevenin impedance), this pole is located
near the origin (0Hz). The phase lag created by the
output pole for any frequency will be 90°. However, as
the power circuit DC gain is limited by other factors, the
gain starts falling at -20dB/decade from a non-zero fre-
quency before which the power circuit gain will be stable.
Total gain of the feedback loop at DC is given by the
following equation:
where GPis the power-circuit DC gain and GEA is the
error-amplifier open-loop DC gain, typically 100dB. GFB
is the gain of the feedback network for adaptive control
of the VLED, which is seen from VLED to the error-
amplifier input (FB pin). The adaptive control senses
GGGG
TOT P EA FB
=× ×
FD
ZRHP MAX
=−
××
VLED
2LI
O
()12
π
RVR
VC R
SLOPE
SLOPE
615=−
⎛
⎝
⎜⎞
⎠
⎟×
VR F
SLOPE SW
=×17.
VC VD
D
SLOPE SLOPE MAX
MAX
=×−×().2111
VILR
SLOPE SLOPE CS
=×
IL VLED V VIN
L
SLOPE MAX D MIN
MIN
=+−()
Evaluates: MAX16807
MAX16807 Evaluation Kit
_______________________________________________________________________________________ 7
Evaluates: MAX16807
MAX16807 Evaluation Kit
8 _______________________________________________________________________________________
the voltages at the eight constant-current sink outputs
and adjusts the feedback to control these voltages to a
minimum value. As the LEDs carry constant current, the
voltage across the LEDs does not change with varia-
tions in VLED. Any change in VLED directly reflects to
the constant-current sink outputs and to the error-
amplifier input, making GFB equal to unity.
The DC gain of the power circuit is expressed as the
change in the output voltage (ΔVLED), with respect to
the change in error-amplifier output voltage (ΔEAOUT).
As the boost converter in the MAX16807 EV kit drives a
constant-current load, the power circuit DC gain is cal-
culated by the following equation:
Calculate the power circuit DC gain using the following
equation:
where RCS is the current-sense resistor, FSW is the
switching frequency, and the factor 3 is to account for
the attenuation of error-amp output before it is fed to
the current-sense comparator.
The power-circuit gain will be the lowest at the mini-
mum input supply voltage and highest at the maximum
input supply voltage. Any input supply voltage between
9V and 16V can be used for the power-circuit gain cal-
culation, since the final compensation values obtained
will be the same.
Calculate the frequency FP2, at which the power-circuit
gain starts falling, at -20dB/decade using the following
equation:
where COUT is the output filter capacitor, which is the
parallel combination of C1, C2, and C15. Adjust the
output capacitance such that the product of FP2 and
GPis below FZRHP / 6. The value of output capacitance
obtained this way will be much greater than the value
obtained using the maximum output voltage ripple
specification.
The compensation strategy is as follows. The gain-
frequency response of the feedback loop should cross
0dB at or below half of the RHP zero frequency, with a
slope of -20dB/decade for the feedback to be stable
and have sufficient phase margin. The compensation
network from the COMP pin to the FB pin of MAX16807
(formed by R1, C6, C7, and R11) offers one dominant
pole (P1), a zero (Z1), and a high-frequency pole (P3).
There are two very-low-frequency poles and a zero in
the loop before the crossover frequency. The function of
the zero (Z1) is to compensate for the output pole and
reduce the slope of the loop gain from -40dB/decade to
-20dB/decade, and also to reduce the phase lag by 90°.
Choose the crossover frequency to be half of the worst-
case RHP zero frequency:
Place the zero (Z1) at one-third of the crossover fre-
quency so that the phase margin starts improving from
a sufficiently lower frequency:
Use the following equation to calculate the dominant
pole location so that the loop gain crosses 0dB at FC:
As the open-loop gain of the error amplifier can have
variations, the dominant pole location can also vary
from device to device. In the MAX16807 EV kit, the
dominant pole location is decided by the error-amplifier
gain and so the combined effect is a constant gain-
bandwidth product.
Select the value of R11 such that the input bias current
of the error amplifier does not cause considerable drop
across it. The effective AC impedance seen from the
FB pin is the sum of R11 and R12. It is preferable to
keep R12 much less, compared to R11, to have better
control on the AC impedance. Find C6 using the follow-
ing equation:
CRR F
P
61
11 12 1
=××+×
2G (
EA
π)
FFF
PZRHP Z
11
=×
××
2G F
TOT P2
FF
ZC
1= 3
FF
CZRHP
= 2
FD
CRG
PMAX
OUT CS P
21
3
=−
××××
2
()
π
G
L F VLED
I
VR
P
SW
O
IN CS
=
×× × +
⎛
⎝
⎜
⎞
⎠
⎟××
1
V
IN2
2
2
3
GVLED
EA
POUT
=Δ
Δ
Evaluates: MAX16807
MAX16807 Evaluation Kit
_______________________________________________________________________________________ 9
The location of the zero (Z1), decided by R1 and C6, is
given by the following equation:
Place the high-frequency pole (P3), formed by C6, C7,
and R1, at half the switching frequency to provide further
attenuation to any high-frequency signal propagating
through the system. The location of the high-frequency
pole (FP3) is given by the following equation and should
be used to calculate the value of C7:
The MAX16807 EV kit uses electrolytic capacitors at the
output for filtering, so the zero produced by the ESR of
the capacitors can be low enough to be within or near
the crossover frequency. This zero should be compen-
sated using an additional pole (P4) placed at the ESR
zero location. The ESR zero frequency is calculated
using the following equation:
Use the following equation to calculate the value of
C25, to place the pole P4 at the ESR zero frequency:
If ceramic capacitors are used at the output for filtering,
the frequency of zero produced by the ESR and the
capacitance will be much above the crossover frequency
(0dB gain frequency) of the feedback loop, and hence,
need not be considered in the compensation design.
Layout Considerations
LED driver circuits based on the MAX16807 device use
a high-frequency switching converter to generate the
voltage for LED strings. Proper care must be taken while
laying out the circuit to ensure proper operation. The
switching-converter part of the circuit has nodes with
very fast voltage changes—producing high-frequency
electric fields and PCB traces with fast current
changes—resulting in high-frequency magnetic fields.
As the circuit converts power, the amplitude of these
fields will be high and can easily couple to sensitive parts
of the circuit, creating undesirable effects. Follow the
guidelines below to reduce noise as much as possible:
1) Connect the bypass capacitors from REF and VCC
as close as possible to the device and connect the
capacitor grounds to the analog ground plane
using vias close to the capacitor terminals. Connect
the AGND pin of the device to the analog ground
plane using a via close to the pin. Lay the analog
ground plane on the inner layer, preferably next to
the top layer. Use the analog ground plane to cover
the entire area under critical signal components for
the power converter.
2) Keep the oscillator timing capacitor and resistor
very close to the RTCT pin and make the connec-
tion as short as possible. Connect the ground of the
timing capacitor to the analog ground plane using a
via close to the capacitor terminal. Make sure that
no switching node is present near the RTCT node
and keep the area of the copper connected to the
pin small. Keep the REF connection to the timing
resistor short and away from any switching node.
3) Have a power ground plane for the switching-converter
power circuit under the power components (input filter
capacitor, output filter capacitor, inductor, MOSFET,
rectifier diode, and current-sense resistor). Connect
all the ground connections to the power ground
plane using vias close to the terminals.
4) There are two loops in the power circuit that carry
high-frequency switching currents. One loop is
when the MOSFET is on—from the input filter
capacitor positive terminal, through the inductor,
the MOSFET, and the current-sense resistor, to the
input capacitor negative terminal. The other loop is
when the MOSFET is off—from the input capacitor
positive terminal, through the inductor, the rectifier
diode, output filter capacitor, to the input capacitor
negative terminal. Analyze these two loops and
make the loop areas as small as possible.
Wherever possible, have a return path on the power
ground plane for the switching currents on the top-
layer copper traces, or through power components.
This will reduce the loop area considerably and
provide a low-inductance path for the switching
currents. Reducing the loop area also reduces radi-
ation during switching.
CFR
ZESR
25 1
212
=××π
FESR C
ZESR OUT
=××
1
2π
F
RCC
P3 1
1
11
6
1
7
=
×× +
⎛
⎝
⎜⎞
⎠
⎟
−
2π
FRC
Z1 1
16
=××
2π
Evaluates: MAX16807
MAX16807 Evaluation Kit
10 ______________________________________________________________________________________
5) The gate drive current of the MOSFET is another
high-frequency switching current to consider. There
are two major loops: one during the MOSFET turn-
on edge and the second during the turn-off edge.
The MOSFET turn-on loop is from the VCC bypass
capacitor positive terminal, through the MOSFET dri-
ver in the device, the gate drive resistor, the MOSFET
gate to source (CGS and CGD), and the current-
sense resistor to the VCC bypass capacitor nega-
tive terminal. There is no direct path for the current
from the current-sense resistor to return to the VCC
bypass capacitor through the ground plane, as the
VCC bypass capacitor is connected to the analog
ground plane and the current-sense resistor is con-
nected to the power ground plane. The best solution
is to connect the analog ground plane to the power
ground plane directly under the MOSFET gate drive
trace. This will ensure that the turn-off current also
has a return path on the ground plane.
6) The drain node of the MOSFET is a switching node.
Keep this node area small to reduce radiation and
capacitive coupling to other sensitive parts of the
circuit. However, the trace should be wide enough
to carry the large switching currents.
7) Keep the node area and trace length on the FB pin
small to reduce any noise pick up.
8) Connect the power ground plane for the constant-
current LED driver part of the circuit to the boost
converter output filter capacitor negative terminal.
Power Dissipation
The MAX16807 dissipates power during normal operat-
ing conditions. The heat transferred to the exposed pad
from the die should be properly dissipated to the board
to prevent the device from entering into thermal shut-
down. The exposed pad land area on the top layer
should be of the same size as that of the exposed pad.
Thermal vias are used to carry the heat from the
exposed pad to other layers of the board and spread it
across the board area through copper planes. Thermal
vias should have a maximum 0.4mm hole size and
should be placed at a distance of 1mm from center to
center. For a four-layer board, these vias should be con-
nected to the bottom ground plane and to one internal
ground plane. Do not use thermal relief for the thermal
vias; instead, use solid copper to get the minimum ther-
mal resistance.
Use the following equation to calculate the total power dis-
sipated in the MAX16807 device during normal operation:
where VS is the operating voltage of each of the LED dri-
ver outputs with respect to GND pins, IBis the input bias
current of the MAX16807 including the average of MOS-
FET drive current, and VIN is the input supply voltage. To
dissipate 1W of power, the exposed pad of the device
should be connected to a minimum of 2 square inches
of copper ground plane with 1oz copper thickness.
P VS IOUT I V
DN
NBIN
=×
∑+×
=−07
Evaluates: MAX16807
MAX16807 Evaluation Kit
______________________________________________________________________________________ 11
MAX16807
U1
VBIAS
VOUTA R12
10kΩ
1%
VCC
14
15 PGND
16 PGND
V+ DIN LE N.C.
OUT3
C14
1μF
C4
0.1μF
C9
1μF
C3
0.1μF
C10
22μF
35V
C11
22μF
35V
C15
0.1μF
R3
15Ω
R9
0.11Ω
1%
R14
2.21kΩ
1%
C16
0.01μF
R8
OPEN
C2
22μF
50V
C1
22μF
50V
L1
33μH
C13
0.1μF
VLED
OUT3
C24
1000pF
3
D5
R
21
L
17 13
23
OUT2
VLED
C23
1000pF
22
19
4VCC
2AGND
11 DOUT
1
3
N.C.
OUT
OE
CLK
R5
1.2kΩ
1%
C12
0.1μFR15
10kΩ
1%
U2
16
1A 1Y
32A
2GND
C8
100pF
R7
365Ω
C5
560pF
R1
200kΩ
1%
R11
75kΩ
1%
R13
22kΩ
1%
C25
OPEN
C6
150pF
C7
10pF
R6
17.4kΩ
1%
R4
22kΩ
1%
R2
8.45kΩ
1%
D7
Q1
3
2
1
10 26
18 CLK
CLK
VIN
SHDN
GND
GND
PWM
VBIAS
VIN
VLED
VBIAS
VBIAS
D1
3
2
N1 1
5REF
REF
TP1
TP2
TP4
VCC
VIN
TP3 U3
3
2
3
1
OUT
IN
IN
OUT
GND
GND
1
2
U4
CS
OE
4
2Y
5
VCC
OE
OUT2
OUT1
VLED
OUT1
C22
1000pF
3
D3
R
21
L
21
OUT0
VLED
C21
1000pF
20 OUT0
OUT7
VLED
OUT7
C20
1000pF
3
D4
R
21
L
9
OUT6
VLED
VOUTA VLED
VLED
J1-1
J1
J1-2
J1-3
J1-4
J1-5
J1-6
J1-7
J1-8
J1-9
J1-10
C19
1000pF
8OUT6
OUT5
VLED
OUT5
C18
1000pF
3
D2
R
21
L
7
OUT4
VLED
C17
1000pF
6
COMP 24
FB 25
RTCT 27
N.C. 28
SET 12
OUT4
REF
OUT3
OUT2
OUT1
OUT0
OUT7
OUT6
OUT5
OUT4
R10
330kΩ
1%
D8
D6
Figure 1. MAX16807 EV Kit Schematic
Evaluates: MAX16807
MAX16807 Evaluation Kit
12 ______________________________________________________________________________________
Figure 2. MAX16807 EV Kit Component Placement Guide—
Component Side
Figure 3. MAX16807 EV Kit PCB Layout—Component Side
Evaluates: MAX16807
MAX16807 Evaluation Kit
______________________________________________________________________________________ 13
Figure 5. MAX16807 EV Kit PCB Layout—Ground Layer 3Figure 4. MAX16807 EV Kit PCB Layout—Ground Layer 2
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
14
____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2007 Maxim Integrated Products is a registered trademark of Maxim Integrated Products, Inc.
Evaluates: MAX16807
MAX16807 Evaluation Kit
Figure 6. MAX16807 EV Kit PCB Layout—Solder Side