LM3503
LM3503 Dual-Display Constant Current LED Driver with Analog Brightness
Control
Literature Number: SNVS329D
LM3503
Dual-Display Constant Current LED Driver with Analog
Brightness Control
General Description
The LM3503 is a white LED driver for lighting applications.
For dual display backlighting applications, the LM3503 pro-
vides a complete solution. The LM3503 contains two internal
white LED current bypass FET (Field Effect Transistor)
switches. The white LED current can be adjusted with a DC
voltage from a digital to analog converter or RC filtered PWM
(pulse-width-modulated) signal at the Cntrl pin.
With no external compensation, cycle-by-cycle current limit,
output over-voltage protection, input under-voltage protec-
tion, and dynamic white LED current control capability, the
LM3503 offers superior performance over other step-up
white LED drivers.
Features
nDrives up to 4, 6, 8 or 10 White LEDs for Dual Display
Backlighting
n>80% Peak Efficiency
nOutput Voltage Protection Options: 16V, 25V, 35V & 44V
nInput Under-Voltage Protection
nInternal Soft Start Eliminates Inrush Current
n1 MHz Constant Switching Frequency
nAnalog Brightness Control
nWide Input Voltage Range: 2.5V to 5.5V
nLow Profile Packages: <1 mm Height
— 10 Bump MicroSMD
— 16 Pin LLP
Applications
nDual Display Backlighting in Portable devices
nCellular Phones and PDAs
Typical Application
20128662
August 2006
LM3503 Dual-Display Constant Current LED Driver with Analog Brightness Control
© 2006 National Semiconductor Corporation DS201286 www.national.com
Connection Diagrams
10-Bump Thin MicroSMD Package (TLP10) 16-Lead Thin Leadless Leadframe Package (SQA16A)
20128602
Top View
20128603
Top View
LM3503
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Pin Descriptions/Functions
Bump # Pin # Name Description
A1 9 Cntrl White LED Current Control Connection
B1 7 Fb Feedback Voltage Connection
C1 6 V
OUT2
Drain Connections of the NMOS and PMOS Field Effect Transistor (FET) Switches
(Figure 1: N2 and P1). Connect 100nF at V
OUT2
node if V
OUT2
is not used
D1 4 V
OUT1
Over-Voltage Protection (OVP) and Source Connection of the PMOS FET Switch
(Figure 1: P1)
D2 2 and 3 Sw Drain Connection of the Power NMOS Switch (Figure 1: N1)
D3 15 and 16 Pgnd Power Ground Connection
C3 14 Agnd Analog Ground Connection
B3 13 V
IN
Input Voltage Connection
A3 12 En2 NMOS FET Switch Control Connection
A2 10 En1 PMOS FET Switch Control Connection
1 NC No Connection
5 NC No Connection
8 NC No Connection
11 NC No Connection
DAP DAP Die Attach Pad (DAP), to be soldered to the printed circuit board’s ground plane for
enhanced thermal dissipation.
Cntrl (Bump A1): White LED current control pin. Use this
pin to control the feedback voltage with an external DC
voltage.
Fb (Bump B1):Output voltage feedback connection.
V
OUT2
(Bump C1):Drain connections of the internal PMOS
and NMOS FET switches (Figure 1: P1 and N2). It is recom-
mended to connect 100nF at V
OUT2
if V
OUT2
is not used for
LM3503-35V & LM3503-44V versions.
V
OUT1
(Bump D1):
Source connection of the internal PMOS FET switch (Figure
1: P1) and OVP sensing node. The output capacitor must be
connected as close to the device as possible, between the
V
OUT1
pin and ground plane. Also connect the Schottky
diode as close as possible to the V
OUT1
pin to minimize trace
resistance and EMI radiation.
Sw (Bump D2):
Drain connection of the internal power NMOS FET switch
(Figure 1: N1). Minimize the metal trace length and maxi-
mize the metal trace width connected to this pin to reduce
EMI radiation and trace resistance.
Pgnd (Bump D3): Power ground pin. Connect directly to the
ground plane.
Agnd (Bump C3):Analog ground pin. Connect the analog
ground pin directly to the Pgnd pin.
V
IN
(Bump B3): Input voltage connection pin. The C
IN
ca-
pacitor should be as close to the device as possible, be-
tween the V
IN
pin and ground plane.
En2 (Bump A3): Enable pin for the internal NMOS FET
switch (Figure 1: N2) during device operation. When V
En2
is
≥1.4V, the internal NMOS FET switch turns off and the SUB
display is turned on. The En2 pin has an internal pull down
circuit, thus the internal NMOS FET switch is normally in the
on state of operation with the SUB display turned off. When
V
En2
is ≤0.3V, the internal NMOS FET switch turns on and
the SUB display is turned off. If both V
En1
and V
En2
are ≤
0.3V the LM3503 will shutdown. If V
OUT2
is not used, En2
must be floating or grounded and En1 used to enable the
device.
En1 (Bump A2): Enable pin for the internal PMOS FET
switch (Figure 1: P1) during device operation. When V
En1
is
≤0.3V, the internal PMOS FET switch turns on and the MAIN
display is turned off. When V
En1
is ≥1.4V, the internal PMOS
FET switch turns off and the MAIN display is turned on. If
both V
En1
and V
En2
are ≤0.3V the LM3503 will shutdown.
The En1 pin has an internal pull down circuit, thus the
internal PMOS FET switch is normally in the on state of
operation with the MAIN display turned off. If V
OUT2
is not
used, En2 must be grounded and En1 use to enable the
device.
LM3503
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Ordering Information
Voltage
Option
Order Number Package
Marking
Supplied As
16 LM3503ITL-16 SBHB 250 Units, Tape-and-Reel
16 LM3503ITLX-16 SBHB 3000 Units, Tape-and-Reel
16 LM3503SQ-16 L00045B 1000 Units, Tape-and-Reel
16 LM3503SQX-16 L00045B 4500 Units, Tape-and-Reel
25 LM3503ITL-25 SBJB 250 Units, Tape-and-Reel
25 LM3503ITLX-25 SBJB 3000 Units, Tape-and-Reel
25 LM3503SQ-25 L00046B 1000 Units, Tape-and-Reel
25 LM3503SQX-25 L00046B 4500 Units, Tape-and-Reel
35 LM3503ITL-35 SBKB 250 Units, Tape-and-Reel
35 LM3503ITLX-35 SBKB 3000 Units, Tape-and-Reel
35 LM3503SQ-35 L00047B 1000 Units, Tape-and-Reel
35 LM3503SQX-35 L00047B 4500 Units, Tape-and-Reel
44 LM3503ITL-44 SDNB 250 Units, Tape-and-Reel
44 LM3503ITLX-44 SDNB 3000 Units, Tape-and-Reel
44 LM3503SQ-44 L00053B 1000 Units, Tape-and-Reel
44 LM3503SQX-44 L00053B 4500 Units, Tape-and-Reel
LM3503
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Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
V
IN
Pin −0.3V to +5.5V
Sw Pin −0.3V to +48V
Fb Pin −0.3V to +5.5V
Cntrl Pin −0.3V to +5.5V
V
OUT1
Pin −0.3V to +48V
V
OUT2
Pin −0.3V to V
OUT1
En1 −0.3V to +5.5V
En2 −0.3V to +5.5V
Continuous Power Dissipation Internally Limited
Maximum Junction Temperature
(T
J-MAX
) +150˚C
Storage Temperature Range −65˚C to +150˚C
ESD Rating (Note 2)
Human Body Model:
Machine Model:
2kV
200V
Operating Conditions (Notes 1, 6)
Junction Temperature (T
J
) Range −40˚C to +125˚C
Ambient Temperature (T
A
) Range −40˚C to +85˚C
Supply Voltage, V
IN
Pin 2.5V to 5.5V
En1 and En2 Pins 0V to 5.5V
Cntrl Pin 0V to 3.5V
Thermal Properties (Note 4)
Junction-to-Ambient Thermal Resistance (θ
JA
)
Micro SMD Package 65˚C/W
Leadless Leadframe Package 49˚C/W
Electrical Characteristics (Notes 6, 7) Limits in standard typeface are for T
J
= +25˚C. Limits in bold type-
face apply over the full operating junction temperature range (−40˚C ≤T
J
≤+125˚C). Unless otherwise specified,V
IN
= 2.5V.
Symbol Parameter Conditions Min Typ Max Units
V
IN
Input Voltage 2.5 5.5 V
I
Q
Non-Switching
Switching
Shutdown
Cntrl = 1.6V
Fb = 0V, Sw Is Floating
En1=En2=0V
0.5
1.9
0.1
1
3
3
mA
mA
µA
V
Fb
Feedback Voltage Cntrl = 3.5V 0.5 0.55 0.6 V
I
CL
NMOS Power Switch
Current Limit
16, Fb = 0V
25, Fb = 0V
35, Fb = 0V
44,FB = 0V
250
400
450
450
400
600
750
750
650
800
1050
1050
mA
I
Fb
Feedback Pin Output
Bias Current
Fb = 0.25V, Cntrl = 1.6V 64 500 nA
F
S
Switching Frequency 0.8 11.2 MHz
R
DS(ON)
NMOS Power Switch
ON Resistance
(Figure 1: N1)
I
Sw
= 500 mA, (Note 8)
0.55 1.1 Ω
R
PDS(ON)
PMOS ON Resistance
Of V
OUT1
/V
OUT2
Switch (Figure 1: P1)
I
PMOS
= 20 mA, En1 = 0V, En2 = 1.5V
510 Ω
R
NDS(ON)
NMOS ON Resistance
Of V
OUT2
/Fb Switch
(Figure 1: N2)
I
NMOS
= 20 mA, En1 = 1.5V, En2 = 0V
2.5 5Ω
D
MAX
Maximum Duty Cycle Fb = 0V 90 95 %
I
Sw
Sw Pin Leakage
Current (Note 3)
Sw = 42V, En1 = En2 =0V 0.01 5µA
I
V
OUT1(OFF) V
OUT1
Pin Leakage
Current (Note 3)
V
OUT1
= 14V, En1 = En2 = 0V (16)
V
OUT1
= 23V, En1 = En2 = 0V (25)
V
OUT1
= 32V, En1 = En2 = 0V (35)
V
OUT1
= 42V, En1 = En2 = 0V (44)
0.1
0.1
0.1
0.1
3
3
3
3
µA
I
V
OUT1(ON) V
OUT1
Pin Bias
Current (Note 3)
V
OUT1
= 14V, En1 = En1 = 1.5V (16)
V
OUT1
= 23V, En1 = En2 = 1.5V (25)
V
OUT1
= 32V, En1 = En2 = 1.5V (35)
V
OUT1
= 42V, En1 = En2 = 1.5V (44)
40
50
50
85
80
100
100
140
µA
LM3503
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Electrical Characteristics (Notes 6, 7) Limits in standard typeface are for T
J
= +25˚C. Limits in bold
typeface apply over the full operating junction temperature range (−40˚C ≤T
J
≤+125˚C). Unless otherwise specified,V
IN
=
2.5V. (Continued)
Symbol Parameter Conditions Min Typ Max Units
I
V
OUT2 V
OUT2
Pin Leakage
Current (Note 3)
Fb = En1 = En2 = 0V, V
OUT2
=V
OUT1
= 42V 0.1 3µA
UVP Under-Voltage
Protection
On Threshold
Off Threshold 2.2
2.4
2.3
2.5 V
OVP Over-Voltage
Protection (Note 5)
On Threshold (16)
Off Threshold (16)
On Threshold (25)
Off Threshold (25)
On Threshold (35)
Off Threshold (35)
On Threshold (44)
Off Threshold (44)
14.5
14.0
22.5
21.5
32.0
31.0
40.5
39.0
15.5
15
24
23
34
33
42
41
16.5
16.0
25.5
24.5
35.0
34.0
43.5
42.0
V
V
En1
PMOS FET Switch
and Device Enabling
Threshold (Figure 1:
P1)
Off Threshold 0.8 0.3
V
On Threshold 1.4 0.8
V
En2
NMOS FET Switch
and Device Enabling
Threshold (Figure 1:
N2)
Off Threshold 0.8 0.3
V
On Threshold 1.4 0.8
V
Cntrl
V
Cntrl
Range V
IN
= 3.6V 0.2 3.5 V
I
En1
En1 Pin Bias Current
(Note 3)
En1 = 2.5V
En1=0V
7
0.1
14 µA
I
En2
En2 Pin Bias Current
(Note 3)
En2 = 2.5V
En2=0V
7
0.1
14 µA
I
CNTRL
Cntrl Pin Bias Current
(Note 3)
Cntrl = 2.5V 814 µA
Note 1: Absolute maximum ratings indicate limits beyond which damage to the device may occur. Electrical characteristic specifications do not apply when
operating the device outside of its rated operating conditions.
Note 2: The human body model is a 100 pF capacitor discharged through a 1.5 kΩresistor into each pin. The machine model is a 200 pF capacitor discharged
directly into each pin.
Note 3: Current flows into the pin.
Note 4: The maximum allowable power dissipation is a function of the maximum junction temperature, TJ(MAX), the junction-to-ambient thermal resistance, θJA, and
the ambient temperature, TA. See Thermal Properties for the thermal resistance. The maximum allowable power dissipation at any ambient temperature is calculated
using: PD(MAX) =(T
J(MAX)–TA)/ θJA. Exceeding the maximum allowable power dissipation will cause excessive die temperature. For more information on this topic,
please refer to Application Note 1187(An1187): Leadless Leadframe Package (LLP) and Application Note 1112(AN1112) for microSMD chip scale package.
Note 5: The on threshold indicates that the LM3503 is no longer switching or regulating LED current, while the off threshold indicates normal operation.
Note 6: All voltages are with respect to the potential at the GND pin.
Note 7: Min and Max limits are guaranteed by design, test, or statistical analysis. Typical numbers are not guaranteed, but do represent the most likely norm.
Note 8: NMOS Power On Resistance measured at ISW= 250mA for sixteen voltage version.
LM3503
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Block Diagram
20128604
FIGURE 1. Block Diagram
LM3503
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Detailed Description of Operation
The LM3503 utilizes an asynchronous current mode pulse-
width-modulation (PWM) control scheme to regulate the
feedback voltage over specified load conditions. The DC/DC
converter behaves as a controlled current source for white
LED applications. The operation can best be understood by
referring to the block diagram in Figure 1 for the following
operational explanation. At the start of each cycle, the oscil-
lator sets the driver logic and turns on the internal NMOS
power device, N1, conducting current through the inductor
and reverse biasing the external diode. The white LED cur-
rent is supplied by the output capacitor when the internal
NMOS power device, N1, is turned on. The sum of the error
amplifier’s output voltage and an internal voltage ramp are
compared with the sensed power NMOS, N1, switch voltage.
Once these voltages are equal, the PWM comparator will
then reset the driver logic, thus turning off the internal NMOS
power device, N1, and forward biasing the external diode.
The inductor current then flows through the diode to the
white LED load and output capacitor. The inductor current
recharges the output capacitor and supplies the current for
the white LED load. The oscillator then sets the driver logic
again repeating the process. The output voltage of the error
amplifier controls the current through the inductor. This volt-
age will increase for larger loads and decrease for smaller
loads limiting the peak current in the inductor and minimizing
EMI radiation. The duty limit comparator is always opera-
tional, it prevents the internal NMOS power switch, N1, from
being on for more than one oscillator cycle and conducting
large amounts of current. The light load comparator allows
the LM3503 to properly regulate light/small white LED load
currents, where regulation becomes difficult for the
LM3503’s primary control loop. Under light load conditions,
the LM3503 will enter into a pulse skipping pulse-frequency-
mode (PFM) of operation where the operational frequency
will vary with the load. As a result of PFM mode operation,
the output voltage ripple magnitude will significantly in-
crease.
The LM3503 has two control pins, En1 and En2, used for
selecting which segment of a single white LED string net-
work is active for dual display applications. En1 controls the
main display (MAIN) segment of the single string white LED
network between pins V
OUT1
and V
OUT2
. En2 controls the
sub display (SUB) segment of the single string white LED
network between the V
OUT2
and Fb. If both V
En1
and V
En2
are ≤0.3V, the LM3503 will shutdown, for further description
of the En1 and En2 operation, see Figure 3. During shut-
down the output capacitor discharges through the string of
white LEDs and feedback resistor to ground. The LED cur-
rent can be dynamically controlled by a DC voltage on the
Cntrl pin. When V
Cntrl
= 0V the white LED current may not be
equal to zero because of offsets within the LM3503 internal
circuitry. To guarantee zero white LED current the LM3503
must be in shutdown mode operation.
The LM3503 has dedicated protection circuitry active during
normal operation to protect the integrated circuit (IC) and
external components. Soft start circuitry is present in the
LM3503 to allow for slowly increasing the current limit to its
steady-state value to prevent undesired high inrush current
during start up. Thermal shutdown circuitry turns off the
internal NMOS power device, N1, when the internal semi-
conductor junction temperature reaches excessive levels.
The LM3503 has a under-voltage protection (UVP) compara-
tor that disables the internal NMOS power device when
battery voltages are too low, thus preventing an on state
where the internal NMOS power device conducts large
amounts of current. The over-voltage protection (OVP) com-
parator prevents the output voltage from increasing beyond
the protection limit when the white LED string network is
removed or if there is a white LED failure. OVP allows for the
use of low profile ceramic capacitors at the output. The
current through the internal NMOS power device, N1, is
monitored to prevent peak inductor currents from damaging
the IC. If during a cycle (cycle=1/switching frequency) the
peak inductor current exceeds the current limit for the
LM3503, the internal NMOS power device will be turned off
for the remaining duration of that cycle.
20128605
FIGURE 2. Operational Characteristics Table
LM3503
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Typical Performance Characteristics (See Typical Application Circuit : L=DO1608C-223 and
D=B150-13. Efficiency: η=P
OUT
/P
IN
= [(V
OUT
–V
Fb
)*I
OUT
]/[V
IN
*I
IN
]. T
A
= +25˚C, unless otherwise stated.)
I
Q
(Non-Switching) vs V
IN
Switching Frequency vs Temperature
20128606
20128607
I
Q
(Switching) vs V
IN
I
Q
(Switching) vs Temperature
20128608
20128609
10 LED Efficiency vs LED Current 8 LED Efficiency vs LED Current
20128610 20128611
LM3503
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Typical Performance Characteristics (See Typical Application Circuit : L=DO1608C-223 and
D=B150-13. Efficiency: η=P
OUT
/P
IN
= [(V
OUT
–V
Fb
)*I
OUT
]/[V
IN
*I
IN
]. T
A
= +25˚C, unless otherwise stated.) (Continued)
6 LED Efficiency vs LED Current 4 LED Efficiency vs LED Current
20128612 20128613
Cntrl Pin Current vs Cntrl Pin Voltage Maximum Duty Cycle vs Temperature
20128614
20128615
En1 Pin Current vs En1 Pin Voltage En2 Pin Current vs En2 Pin Voltage
20128663 20128664
LM3503
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Typical Performance Characteristics (See Typical Application Circuit : L=DO1608C-223 and
D=B150-13. Efficiency: η=P
OUT
/P
IN
= [(V
OUT
–V
Fb
)*I
OUT
]/[V
IN
*I
IN
]. T
A
= +25˚C, unless otherwise stated.) (Continued)
V
OUT1
Pin Current vs V
OUT1
Pin Voltage Power NMOS R
DS(ON)
(Figure 1: N1) vs V
IN
20128618 20128619
NMOS R
DS(ON)
(Figure 1: N2) vs V
IN
PMOS R
DS(ON)
(Figure 1: P1) vs V
IN
20128620 20128621
Feedback Voltage vs Cntrl Pin Voltage Current Limit (LM3503-16) vs Temperature
20128622 20128655
LM3503
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Typical Performance Characteristics (See Typical Application Circuit : L=DO1608C-223 and
D=B150-13. Efficiency: η=P
OUT
/P
IN
= [(V
OUT
–V
Fb
)*I
OUT
]/[V
IN
*I
IN
]. T
A
= +25˚C, unless otherwise stated.) (Continued)
Current Limit (LM3503-16) vs V
IN
Current Limit (LM3503-25) vs Temperature
20128659 20128657
Current Limit (LM3503-25) vs V
IN
Current Limit (LM3503-35/44) vs Temperature
20128660 20128658
Current Limit (LM3503-35/44) vs V
IN
Feedback Voltage (V
Cntrl
= 0.8V) vs Temp
20128625
20128624
LM3503
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Typical Performance Characteristics (See Typical Application Circuit : L=DO1608C-223 and
D=B150-13. Efficiency: η=P
OUT
/P
IN
= [(V
OUT
–V
Fb
)*I
OUT
]/[V
IN
*I
IN
]. T
A
= +25˚C, unless otherwise stated.) (Continued)
Feedback Voltage (V
Cntrl
= 1.6V) vs Temp V
IN
= 3.6V at 15mA & 4 Leds
20128626
20128650
V
IN
= 3.6V at 15mA & 2 Leds
Dimming Duty Cycle vs. LED Current
V
IN
=3.6V, 2LEDs on Main & Sub Display
20128653
20128661
LM3503
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Application Information
WHITE LED CURRENT SETTING
The white LED current is controlled by a DC voltage at the
Cntrl pin.
The relationship between the Cntrl pin voltage and Fb pin
voltage can be computed with the following:
V
Cntrl
: Cntrl Pin Voltage. Voltage Range: 0.2V ≤V
Cntrl
≤
3.5V.
V
Fb
: Feedback Pin Voltage.
LED CURRENT
The LED current is set using the following equation:
To determine the maximum output current capability of the
device, it is best to estimate using equations on page 16 and
the minimum peak current limit of the device (see electrical
table). Note the current capability will be higher with less
LEDs in the application.
Aside from varying the DC voltage at the Cntrl pin, white LED
dimming can be accomplished through the RC filtering of a
PWM signal. The PWM signal frequency should be at least a
decade greater than the RC filter bandwidth. Figure 3 is how
the LM3503 should be wired for PWM filtered white LED
dimming functionality. When using PWM dimming, it is rec-
ommended to add 1-2ms delay between the Cntrl signal and
the main Enable sginal (En1) to allow time for the output to
discharge. This will prevent potential flickering especially if
the Sub display is compose of 2 LEDs or less.
The equations below are guidelines for choosing the correct
RC filter values in relation to the PWM signal frequency.
Equation #1:
Equation #2:
F
RC
: RC Filter Bandwidth Cutoff Frequency.
F
PWM
: PWM Signal Frequency.
R: Chosen Filter Resistor.
C: Chosen Filter Capacitor.
For example, using the above equations to determine the
proper RC values. Assume the following condition:V
IN
= 3.6V,
C=0.01µF and F
PWM
= 500Hz, then F
RC
= 50Hz by relation to
equation 2. By rearranging equation 1 to solve for R; R =
318.5K ohms (standard value, R = 316K).
WHITE LED DIMMING
20128634
FIGURE 3. If V
OUT2
is not used, En2 must be grounded
LM3503
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Application Information (Continued)
PWM Dimming Duty Cycle vs. LED Current
The results are based on the 2LEDs on Main display and 2LEDs on Sub display
Duty 200Hz 500Hz 1KHz 10KHz 50KHz 100kHz
(%) R = 787k ohms R =316k ohms R = 158kohms R=16.2k ohms R=3.16k ohms R=1.62k ohms
10 0.78mA 1.59mA 2.23mA 3.42mA 3.58mA 3.61mA
20 1.85mA 3.46mA 4.78mA 7.09mA 7.41mA 7.48mA
30 2.88mA 5.35mA 7.33mA 10.77mA 11.25mA 11.34mA
40 3.96mA 7.24mA 9.88mA 14.48mA 15.12mA 15.24mA
50 5.05mA 9.12mA 12.45mA 19.1mA 19.06mA 19.16mA
60 6.08mA 11.03mA 15.03mA 21.86mA 22.98mA 23.10mA
70 7.13mA 12.94mA 17.61mA 25.71mA 26.9mA 27.05mA
80 8.17mA 14.83mA 20.20mA 29.53mA 30.83mA 31.00mA
90 9.24mA 16.73mA 22.79mA 33.32mA 34.78mA 35.00mA
CONTINUOUS AND DISCONTINUOUS MODES OF
OPERATION
Since the LM3503 is a constant frequency pulse-width-
modulated step-up regulator, care must be taken to make
sure the maximum duty cycle specification is not violated.
The duty cycle equation depends on which mode of opera-
tion the LM3503 is in. The two operational modes of the
LM3503 are continuous conduction mode (CCM) and dis-
continuous conduction mode (DCM). Continuous conduction
mode refers to the mode of operation where during the
switching cycle, the inductor current never goes to and stays
at zero for any significant amount of time during the switch-
ing cycle. Discontinuous conduction mode refers to the
mode of operation where during the switching cycle, the
inductor current goes to and stays at zero for a significant
amount of time during the switching cycle. Figure 4 illus-
trates the threshold between CCM and DCM operation. In
Figure 4 the inductor current is right on the CCM/DCM
operational threshold. Using this as a reference, a factor can
be introduced to calculate when a particular application is in
CCM or DCM operation. R is a CCM/DCM factor we can use
to compute which mode of operation a particular application
is in. If R is ≥1, then the application is operating in CCM.
Conversely, if R is <1, the application is operating in DCM.
The R factor inequalities are a result of the components that
make up the R factor. From Figure 4, the R factor is equal to
the average inductor current, I
L
(avg), divided by half the
inductor ripple current, ∆i
L
. Using Figure 4, the following
equation can be used to compute R factor:
20128637
FIGURE 4. Inductor Current Waveform
LM3503
www.national.com15
Application Information (Continued)
V
IN
: Input Voltage.
V
OUT
: Output Voltage.
Eff: Efficiency of the LM3503.
Fs: Switching Frequency.
I
OUT
: White LED Current/Load Current.
L: Inductance Magnitude/Inductor Value.
D: Duty Cycle for CCM operation.
∆i
L
: Inductor Ripple Current.
I
L
(avg): Average Inductor Current.
For CCM operation, the duty cycle can be computed with:
D: Duty Cycle for CCM Operation.
V
OUT
: Output Voltage.
V
IN
: Input Voltage.
For DCM operation, the duty cycle can be computed with:
D: Duty Cycle for DCM Operation.
V
OUT
: Output Voltage.
V
IN
: Input Voltage.
I
OUT
: White LED Current/Load Current.
Fs: Switching Frequency.
L: Inductor Value/Inductance Magnitude.
INDUCTOR SELECTION
In order to maintain inductance, an inductor used with the
LM3503 should have a saturation current rating larger than
the peak inductor current of the particular application. Induc-
tors with low DCR values contribute decreased power losses
and increased efficiency. The peak inductor current can be
computed for both modes of operation: CCM and DCM.
The cycle-by-cycle peak inductor current for CCM operation
can be computed with:
V
IN
: Input Voltage.
Eff: Efficiency of the LM3503.
Fs: Switching Frequency.
I
OUT
: White LED Current/Load Current.
L: Inductance Magnitude/Inductor Value.
D: Duty Cycle for CCM Operation.
I
PEAK
: Peak Inductor Current.
∆i
L
: Inductor Ripple Current.
I
L
(avg): Average Inductor Current.
The cycle-by-cycle peak inductor current for DCM operation
can be computed with:
V
IN
: Input Voltage.
Fs: Switching Frequency.
L: Inductance Magnitude/Inductor Value.
D: Duty Cycle for DCM Operation.
I
PEAK
: Peak Inductor Current.
The minimum inductance magnitude/inductor value for the
LM3503 can be calculated using the following, which is only
valid when the duty cycle is >0.5:
D: Duty Cycle.
D’: 1-D.
R
DS(ON)
: NMOS Power Switch ON Resistance.
Fs: Switching Frequency.
V
IN
: Input Voltage.
L: Inductance Magnitude/Inductor Value.
This equation gives the value required to prevent subhar-
monic oscillations. The result of this equation and the induc-
tor ripple currents should be accounted for when choosing
an inductor value.
Some recommended Inductor manufactures included but
are not limited to:
Coilcraft DO1608C-223 www.coilcraft.com
DT1608C-223
CAPACITOR SELECTION
Multilayer ceramic capacitors are the best choice for use
with the LM3503. Multilayer ceramic capacitors have the
lowest equivalent series resistance (ESR). Applied voltage
or DC bias, temperature, dielectric material type (X7R, X5R,
Y5V, etc), and manufacturer component tolerance have an
affect on the true or effective capacitance of a ceramic
capacitor. Be aware of how your application will affect a
particular ceramic capacitor by analyzing the aforemen-
tioned factors of your application. Before selecting a capaci-
tor always consult the capacitor manufacturer’s data curves
to verify the effective or true capacitance of the capacitor in
your application.
LM3503
www.national.com 16
Application Information (Continued)
INPUT CAPACITOR SELECTION
The input capacitor serves as an energy reservoir for the
inductor. In addition to acting as an energy reservoir for the
inductor the input capacitor is necessary for the reduction in
input voltage ripple and noise experienced by the LM3503.
The reduction in input voltage ripple and noise helps ensure
the LM3503’s proper operation, and reduces the effect of the
LM3503 on other devices sharing the same supply voltage.
To ensure low input voltage ripple, the input capacitor must
have an extremely low ESR. As a result of the low input
voltage ripple requirement multilayer ceramic capacitors are
the best choice. A minimum capacitance of 2.0 µF is required
for normal operation, so consult the capacitor manufactur-
er’s data curves to verify whether the minimum capacitance
requirement is going to be achieved for a particular applica-
tion.
OUTPUT CAPACITOR SELECTION
The output capacitor serves as an energy reservoir for the
white LED load when the internal power FET switch (Figure
1: N1) is on or conducting current. The requirements for the
output capacitor must include worst case operation such as
when the load opens up and the LM3503 operates in over-
voltage protection (OVP) mode operation. A minimum ca-
pacitance of 0.5 µF is required to ensure normal operation.
Consult the capacitor manufacturer’s data curves to verify
whether the minimum capacitance requirement is going to
be achieved for a particular application.
Some recommended capacitor manufacturers included but
are not limited to:
Taiyo-
Yuden
GMK212BJ105MD
(0805/35V)
www.t-yuden.com
muRata GRM40-035X7R105K
(0805/50V)
www.murata.com
TDK C3216X7R1H105KT
(1206/50V)
www.tdktca.com
C3216X7R1C475K
(1206/16V)
AVX 08053D105MAT
(0805/25V)
www.avxcorp.com
08056D475KAT
(0805/6.3V)
1206ZD475MAT
(1206/10V)
DIODE SELECTION
To maintain high efficiency it is recommended that the aver-
age current rating (I
F
or I
O
) of the selected diode should be
larger than the peak inductor current (I
Lpeak
). At the minimum
the average current rating of the diode should be larger than
the maximum LED current. To maintain diode integrity the
peak repetitive forward current (I
FRM
) must be greater than
or equal to the peak inductor current (I
Lpeak
). Diodes with low
forward voltage ratings (V
F
) and low junction capacitance
magnitudes (C
J
or C
T
or C
D
) are conducive to high efficiency.
The chosen diode must have a reverse breakdown voltage
rating (V
R
and/or V
RRM
) that is larger than the output voltage
(V
OUT
). No matter what type of diode is chosen, Schottky or
not, certain selection criteria must be followed:
1. V
R
and V
RRM
>V
OUT
2. I
F
or I
O
≥I
LOAD
or I
OUT
3. I
FRM
≥I
Lpeak
Some recommended diode manufacturers included but are
not limited to:
Vishay SS12(1A/20V) www.vishay.com
SS14(1A/40V)
SS16(1A/60V)
On
Semiconductor
MBRM120E
(1A/20V)
www.onsemi.com
MBRS1540T3
(1.5A/40V)
MBR240LT
(2A/40V)
Central
Semiconductor
CMSH1-40M
(1A/40V)
www.centralsemi.com
SHUTDOWN AND START-UP
On startup, the LM3503 contains special circuitry that limits
the peak inductor current which prevents large current
spikes from loading the battery or power supply. The
LM3503 is shutdown when both En1 and En2 signals are
less than 0.3V. During shutdown the output voltage is a
diode drop below the supply voltage. When shutdown, the
softstart is reset to prevent inrush current at the next startup.
THERMAL SHUTDOWN
The LM3503 stops regulating when the internal semiconduc-
tor junction temperature reaches approximately 140˚C. The
internal thermal shutdown has approximately 20˚C of hyster-
esis which results in the LM3503 turning back on when the
internal semiconductor junction temperature reaches 120˚C.
When the thermal shutdown temperature is reached, the
softstart is reset to prevent inrush current when the die
temperature cools.
UNDER VOLTAGE PROTECTION
The LM3503 contains protection circuitry to prevent opera-
tion for low input supply voltages. When Vin drops below
2.3V, typically, the LM3503 will no longer regulate. In this
mode, the output voltage will be one diode drop below Vin
and the softstart will be reset. When Vin increases above
2.4V, typically, the device will begin regulating again.
OVER VOLTAGE PROTECTION
The LM3503 contains dedicated ciruitry for monitoring the
output voltage. In the event that the LED network is discon-
nected from the LM3503, the output voltage will increase
and be limited to 15.5V(typ.) for the 16V version, 24V(typ.)
for the 25V version, 34V(typ.) for 35V version and 42V(typ.)
for the 44V version. (see electrical table for more details). In
the event that the network is reconnected regulation will
resume at the appropriate output voltage.
LAYOUT CONSIDERATIONS
All components, except for the white LEDs, must be placed
as close as possible to the LM3503. The die attach pad
(DAP) must be soldered to the ground plane.
The input bypass capacitor C
IN
, as shown in the Typical
Application Circuit,, must be placed close to the IC and
connect between the V
IN
and Pgnd pins. This will reduce
copper trace resistance which effects input voltage ripple of
the IC. For additional input voltage filtering, a 100 nF bypass
LM3503
www.national.com17
Application Information (Continued)
capacitor can be placed in parallel with C
IN
to shunt any high
frequency noise to ground. The output capacitor, C
OUT
, must
be placed close to the IC and be connected between the
V
OUT1
and Pgnd pins. Any copper trace connections for the
C
OUT
capacitor can increase the series resistance, which
directly effects output voltage ripple and efficiency. The cur-
rent setting resistor, R1, should be kept close to the Fb pin to
minimize copper trace connections that can inject noise into
the system. The ground connection for the current setting
resistor network should connect directly to the Pgnd pin. The
Agnd pin should be tied directly to the Pgnd pin. Trace
connections made to the inductor should be minimized to
reduce power dissipation and increase overall efficiency
while reducing EMI radiation. For more details regarding
layout guidelines for switching regulators, refer to Applica-
tions Note AN-1149.
LM3503
www.national.com 18
Physical Dimensions inches (millimeters) unless otherwise noted
TLP10: 10-Bump Thin Micro SMD Package
X1 = 1.958 mm
X2 = 2.135 mm
X3 = 0.6 mm
NS Package Number TLP10
16-Lead Thin Leadless Leadframe Package
NS Package Number SQA16A
LM3503
www.national.com19
Notes
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves
the right at any time without notice to change said circuitry and specifications.
For the most current product information visit us at www.national.com.
LIFE SUPPORT POLICY
NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS
WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR
CORPORATION. As used herein:
1. Life support devices or systems are devices or systems
which, (a) are intended for surgical implant into the body, or
(b) support or sustain life, and whose failure to perform when
properly used in accordance with instructions for use
provided in the labeling, can be reasonably expected to result
in a significant injury to the user.
2. A critical component is any component of a life support
device or system whose failure to perform can be reasonably
expected to cause the failure of the life support device or
system, or to affect its safety or effectiveness.
BANNED SUBSTANCE COMPLIANCE
National Semiconductor follows the provisions of the Product Stewardship Guide for Customers (CSP-9-111C2) and Banned Substances
and Materials of Interest Specification (CSP-9-111S2) for regulatory environmental compliance. Details may be found at:
www.national.com/quality/green.
Lead free products are RoHS compliant.
National Semiconductor
Americas Customer
Support Center
Email: new.feedback@nsc.com
Tel: 1-800-272-9959
National Semiconductor
Europe Customer Support Center
Fax: +49 (0) 180-530 85 86
Email: europe.support@nsc.com
Deutsch Tel: +49 (0) 69 9508 6208
English Tel: +44 (0) 870 24 0 2171
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Support Center
Email: ap.support@nsc.com
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Fax: 81-3-5639-7507
Email: jpn.feedback@nsc.com
Tel: 81-3-5639-7560
www.national.com
LM3503 Dual-Display Constant Current LED Driver with Analog Brightness Control
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