Innovative PowerTM - 1 - www.active-semi.com
Copyright © 2012 Active-Semi, Inc.
SIMPLIFIED APPLICATION CIRCUIT
ACT6357/ACT6358
High-Efficiency, 40V Step-Up WLED Bias Supplies
FEATURES
• High-Efficiency DC/DC WLED Bias Supply
• Internal 40V, 0.55Ω Power MOSFET
• Up to 10 WLEDs per String
• Two Peak Current Options:
− ACT6357: 0.5A
− ACT6358: 1A
• Supports Analog and PWM LED Dimming
• Integrated Over-Voltage Protection (OVP)
• Programmable Soft-Start Function
• Thermal Shutdown
• Cycle-by-Cycle Over Current Protection
• Tiny TDFN33-8 Package
APPLICATIONS
• TFT LCD Displays
• Smart Phones
• Portable Media Players
• GPS/Personal Navigation Devices
GENERAL DESCRIPTION
The ACT6357 and ACT6358 step-up DC/DC con-
verters drive white LEDs with an externally pro-
grammable constant current. These devices feature
integrated, 40V power MOSFETs that are capable
of driving up to ten white LEDs in series, providing
inherent current matching for uniform brightness.
WLED brightness adjustment is easily achieved via
a dual-function pin, which accepts either a PWM or
an analog dimming control signal.
The ACT6357 and ACT6358 feature a variety of
protection circuits, including integrated over voltage
protection (OVP), programmable soft-start, cycle-
by-cycle current limiting, and thermal shutdown pro-
tection circuitry.
The ACT6357 has 500mA current limit, while the
ACT6358 has 1A current limit. Both parts are avail-
able in a small 3mm x 3mm 8-pin TDFN33-8.
Rev 1, 15-Nov-12
ACT6357/ACT6358
Rev 1, 15-Nov-12
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Copyright © 2012 Active-Semi, Inc.
PART NUMBER CURRENT
LIMIT
TEMPERATURE
RANGE PACKAGE PACKAGING
ACT6357NH-T 0.5A -40°C to 85°C TDFN33-8 TAPE & REEL
ACT6358NH-T 1A -40°C to 85°C TDFN33-8 TAPE & REEL
PINS
8
8
PIN NAME DESCRIPTION
1
2 IN Supply Input
3 EN
Enable Control. Drive to a logic high to enable the device. Connect to a logic low to disable the
device. EN should not be left floating; connect EN to IN when unused.
4 BC
Brightness Control. Multifunction pin accepts either a PWM or analog control signal. When using a
PWM control signal, the best results are achieved when the PWM frequency is in the 100Hz to
10kHz range and when the PWM high voltage is 1.8V or higher. When using an analog control sig-
nal, the best results are achieved when the control voltage is in the 0V to 1.8V range.
5 FB
Feedback Input. Connect this pin to the cathode of the bottom LED, and a current feedback resistor
between this pin and G to set the LED bias current.
6 SS
Soft Start Control Input. Connect a capacitor from this pin to G to program the soft start duration. SS
is internally discharged when IC the is disabled.
7 OV
Over Voltage Protection Input. The IC is automatically disabled when the voltage at this pin exceeds
1.21V. Connect OV to the center point of a resistive voltage divider connected across the LED string.
Ground G
8 SW Switch Output. Connect this pin to the inductor and the Schottky diode.
EP EP Exposed Pad. Connect to ground.
PIN CONFIGURATION
PIN DESCRIPTIONS
ORDERING INFORMATION
ACT6357
ACT6358
1
2
3
4
8
7
6
5
G
IN
BC
EN
SW
SS
FB
OV
TDFN33-8
ACT6357/ACT6358
Rev 1, 15-Nov-12
Innovative PowerTM - 3 - www.active-semi.com
Copyright © 2012 Active-Semi, Inc.
ABSOLUTE MAXIMUM RATINGSc
PARAMETER VALUE UNIT
SW to G -0.3 to 42 V
IN, EN to G -0.3 to 6 V
FB, OV, BC, SS to G -0.3 to VIN + 0.3 V
Continuous SW Current Internally Limited
Junction to Ambient Thermal Resistance (θJA) 42.5 °C/W
1.9 W
Operating Junction Temperature -40 to 150 °C
Storage Temperature -55 to 150 °C
Lead Temperature (Soldering, 10 sec) 300 °C
Maximum Power Dissipation
c: Do not exceed these limits to prevent damage to the device. Exposure to absolute maximum rating conditions for long periods may
affect device reliability.
ACT6357/ACT6358
Rev 1, 15-Nov-12
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Copyright © 2012 Active-Semi, Inc.
(VIN = VEN = 3.3V, TA = 25°C, unless otherwise specified.)
ELECTRICAL CHARACTERISTICS
PARAMETER TEST CONDITIONS MIN TYP MAX UNIT
Power Switch Voltage Rating 40 V
Input Voltage 2.6 5.5 V
Under Voltage Lockout Threshold VIN Rising 2.1 2.25 2.45 V
Under Voltage Lockout Hysteresis 80 mV
Supply Current
Not Switching 0.1 0.25
mA
Switching 0.25 0.5
Supply Current in Shutdown EN = G 0.1 10 µA
Maximum On Time VIN = 3.3V 2.6 4.0 5.8 µs
Maximum On Time Constant (K) K = tMAXON × VIN 13.2 µs × V
Minimum Off Time 220 320 450 ns
FB Feedback Voltage
VBC = 3.3V 275 290 305
mV VBC = 1.25V 197 207 217
VBC = 0.625V 98 106 114
∆VFB/∆VBC Ratio 0.16 V/V
FB Input Current VFB = 1V 0 200 nA
BC Input Impedance VBC = 0 to 1.25V 400 kΩ
Switch Current Limit
ACT6357 320 500 750
mA
ACT6358 620 1000 1500
Switch On Resistance VIN = 3.3V 0.55 0.9 Ω
Switch Leakage Current VSW = 38V, EN = G 10 µA
Over Voltage Protection Threshold VOV Rising 1.11 1.21 1.31 V
OV Input Current VOV = 1.5V 0 200 nA
EN Logic High Threshold 1.4 V
EN Logic Low Threshold 0.4 V
EN Input Current VEN = 0V or 5V 0 1 µA
Thermal Shutdown Temperature 160 °C
Thermal Shutdown Hysteresis 20 °C
ACT6357/ACT6358
Rev 1, 15-Nov-12
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Copyright © 2012 Active-Semi, Inc.
FUNCTIONAL BLOCK DIAGRAM
Control Scheme
The ACT6357 and ACT6358 use a minimum off-
time, current-mode control scheme to achieve ex-
cellent performance under high duty-cycle operating
conditions. This control scheme initiates a switching
cycle only when needed to maintain output voltage
regulation, resulting in very high efficiency opera-
tion.
During each switching cycle, the N-channel power
MOSFET turns on, increasing the inductor current.
The switching cycle terminates when either the in-
ductor current reaches the current limit (500mA for
the ACT6357, 1A for the ACT6358) or when the
cycle lasts longer than the maximum on-time of
4µs. Once the MOSFET turns off, it remains off for
at least the minimum off-time of 320ns, then an-
other switching begins when the error comparator
detects that the output is falling out of regulation
again.
Soft-Start
The ACT6357 and ACT6358 include a programma-
ble soft-start function, which can be used to opti-
mize an application between start-up time and
start-up inrush current. Soft start is achieved by
connecting a capacitor CSS between the SS pin and
G. The soft start duration can be calculated from
the following equation:
where tSS is the required soft start duration. In a
typical application, use 0.1µF to generate 20ms soft
start time.
Over Voltage Protection
Both the ACT6357 and ACT6358 include internal
over-voltage protection circuitry that monitors the
OV pin voltage. Over-voltage protection is critical
when one of the LEDs in the LED string fails as an
open circuit. When this happens the feedback volt-
age drops to zero, and the control switches at maxi-
mum on time causing the output voltage to keep
rising until it exceeds the maximum voltage rating of
the power MOSFET. The ACT6357 and ACT6358's
over-voltage protection detects this condition and
switching ceases if the voltage at the OV pin
reaches 1.21V.
To set the maximum output voltage, connect a re-
sistor divider from the output node to G, with center
tap at OV, and select the two resistors with the fol-
lowing equation:
where VOV is the over voltage detection threshold,
ROV1 is the resistor between OV and G, and ROV2 is
the resistor from the output to the OV pin. As a first
estimate, the OV threshold can often be set to 4V
times the number of LEDs in the string.
⎥
⎦
⎤
⎢
⎣
⎡−
⎟
⎠
⎞
⎜
⎝
⎛
×= 1
V21.1
V
RR OV
1OV2OV
s
Fμ5
tC SSSS ×=
ACT6357/ACT6358
Rev 1, 15-Nov-12
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Copyright © 2012 Active-Semi, Inc.
Setting the LED Current
The LED current is programmed by appropriate
selection of the feedback resistor RFB connected
between FB and G. To set the LED current, choose
the resistor according to the equation:
where VFB is the FB feedback voltage (typically
207mV at VBC = 1.25V) and ILED is the desired maxi-
mum LED current. Once the LED current is selected
via RFB, it may be adjusted via the BC pin to provide
a simple means of LED dimming. The BC pin sup-
ports both analog as well as PWM dimming control.
Analog Dimming Control
To implement analog dimming, apply a voltage be-
tween 0.1V to 1.25V to BC. The resulting LED cur-
rent as a function of VBC is given by:
BC may be overdriven, but driving VBC higher than
1.8V produces a constant LED current given by:
Direct PWM Dimming Control
The ACT6357 and ACT6358 support direct PWM
dimming control, allowing LED current to be ad-
justed via a PWM signal without the need for an
external RC network. For PWM dimming, drive BC
with a logic-level PWM signal to scale the LED cur-
rent proportionally with the PWM duty cycle, with
resulting LED current given by:
For best results, use PWM frequencies in the
100Hz to 10kHz range.
Inductor Selection
The ACT6357 and ACT6358 were designed for op-
eration with inductors in the 4.7µH to 47µH range,
and achieve best results under most operating con-
ditions when using 22µH to 33µH. Keep in mind
that larger-valued inductors generally result in con-
tinuous conduction mode operation (CCM) and
yield higher efficiency due to lower peak currents,
while smaller inductors typically yield a smaller foot-
print but at the cost of lower efficiency, resulting
from higher peak currents (and their associated I2R
losses). For best results, choose an inductor with a
low DC-Resistance (DCR) and be sure to choose
an inductor with a saturation current that exceeds
the current limit (500mA for the ACT6357 and 1A
for the ACT6358).
Capacitor Selection
The ACT6357 and ACT6358 only require a tiny
0.47µF output capacitor for most applications. For
circuits driving 6 or fewer LEDs, a 4.7µF input ca-
pacitor is generally suitable. For circuits driving
more than 6 LEDs, a 10µF input capacitor may be
required.
When choosing a larger inductor which results in
CCM operation, stability and ripple can be improved
by adding a small feed-forward capacitor from OUT
to FB. About 3000pF is a good starting point for
most applications, although a larger value can be
used to achieve best result in applications with 6 or
fewer LEDs
Ceramic capacitors are recommended for most ap-
plications. For best performance, use X5R and X7R
type ceramic capacitors, which possess less degra-
dation in capacitance over voltage and temperature.
Diode Selection
The ACT6357 and ACT6358 require a Schottky
diode as the rectifier. Select a low forward voltage
drop Schottky diode with forward current (IF) rating
that exceeds the peak current limit (500mA for the
ACT6357 and 1A for the ACT6358) and a peak re-
petitive reverse voltage (VRRM) rating that exceeds
the maximum output voltage, typically set by the OV
threshold.
Shutdown
The ACT6357 and ACT6358 feature low-current
shutdown modes. In shutdown mode, the control
circuitry is disabled and the quiescent supply cur-
rent drops to less than 1µA. To disable the
ACT6357 and ACT6358, simply drive EN to a logic
low. To enable the ICs, drive EN to a logic high or
connect it to the input supply.
Low Input Voltage Applications
In applications that have low input voltage range,
such as those powered from 2-3 AA cells, the
ACT6357 and ACT6358 may still be used if there is
a suitable system supply (such as 3.3V) available to
power the controller. In such an application, the in-
ductor may be connected directly to the battery,
while the IC power is supplied by the system sup-
ply.
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
×=
FB
BC
LED R
V
16.0I
FB
LED R
mV290
I=
DUTY
R
V
I
FB
FB
LED ×
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
=
LED
FB
FB I
V
R=
ACT6357/ACT6358
Rev 1, 15-Nov-12
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Copyright © 2012 Active-Semi, Inc.
TYPICAL PERFORMANCE CHARACTERISTICS
(VVIN = 3.6V, TA = 25°C, unless otherwise specified.)
ACT6357 Efficiency vs. Load Current
Efficiency (%)
Load Current (mA)
50
ACT6357/ACT6358-001
5 0
60
ACT6357 Efficiency vs. Load Current
Efficiency (%)
ACT6357/ACT6358-002
70
80
90
100
10 15 20 25
50
60
70
80
90
100
4 LEDs 4 LEDs
ACT6357 Efficiency vs. Load Current
Efficiency (%)
ACT6357/ACT6358-003
50
60
70
80
90
100
6 LEDs
30
Load Current (mA)
5 0 10 15 20 25 30
Load Current (mA)
5 0 10 15 20 25 30
VIN = 3.6V L = 33µH
L = 22µH
L = 33µH
VIN = 3.2V
VIN = 5V
VIN = 3.6V
VIN = 3.6V
L = 33µH
L = 22µH
ACT6357 Efficiency vs. Load Current
Efficiency (%)
Load Current (mA)
50
ACT6357/ACT6358-004
5 0
60
ACT6357 Efficiency vs. Load Current
Efficiency (%)
ACT6357/ACT6358-005
70
80
90
100
10 15 20 25
50
60
70
80
90
100
6 LEDs
8 LEDs
ACT6357 Efficiency vs. Load Current
Efficiency (%)
ACT6357/ACT6358-06
50
60
70
80
90
100
8 LEDs
30
Load Current (mA)
5 0 10 15 20 25 30
Load Current (mA)
5 0 10 15 20 25 30
VIN = 5V
L = 33µH
VIN = 3.6V L = 33µH
VIN = 3.2V
VIN = 3.6V
L = 33µH
L = 22µH
VIN = 5V
VIN = 3.2V
VIN = 3.6V
ACT6357/ACT6358
Rev 1, 15-Nov-12
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Copyright © 2012 Active-Semi, Inc.
ACT6358 Efficiency vs. Load Current
Efficiency (%)
ACT6357/ACT6358-007
50
60
70
80
90
100
6 LEDs
ACT6358 Efficiency vs. Load Current
Efficiency (%)
ACT6357/ACT6358-009
50
60
70
80
90
100
8 LEDs
Load Current (mA)
5 0 10 15 20 25 30
Load Current (mA)
5 0 10 15 20 25 30
ACT6358 Efficiency vs. Load Current
Efficiency (%)
Load Current (mA)
50
ACT6357/ACT6358-008
5 0
60
70
80
90
100
10 15 20 25
6 LEDs
30
L = 33µH
VIN = 3.2V
VIN = 5V
VIN = 3.6V
VIN = 3.6V
L = 33µH
L = 22µH
TYPICAL PERFORMANCE CHARACTERISTICS
(VVIN = 3.6V, TA = 25°C, unless otherwise specified.)
ACT6358 Efficiency vs. Load Current
Efficiency (%)
Load Current (mA)
50
ACT6357/ACT6358-010
5 0
60
ACT6358 Efficiency vs. Load Current
Efficiency (%)
ACT6357/ACT6358-011
70
80
90
100
10 15 20 25
50
60
70
80
90
100
8 LEDs
10 LEDs
ACT6358 Efficiency vs. Load Current
Efficiency (%)
ACT6357/ACT6358-012
50
60
70
80
90
100
10 LEDs
30
Load Current (mA)
5 0 10 15 20 25 30
Load Current (mA)
5 0 10 15 20 25 30
VIN = 5V
L = 33µH
VIN = 3.6V
L = 33µH
L = 22µH
L = 33µH
VIN = 3.2V
VIN = 5V
VIN = 3.6V
VIN = 3.6V
L = 33µH
L = 22µH
VIN = 3.2V
VIN = 3.6V
ACT6357/ACT6358
Rev 1, 15-Nov-12
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Copyright © 2012 Active-Semi, Inc.
LED Current vs. BC Voltage
LED Current (mA)
0
BC Voltage (V)
ACT6357/ACT6358-014
LED Current vs. BC Duty Cycle
ILED (mA)
Duty Cycle (%)
ACT6357/ACT6358-013
4
0
8
12
16
20
24
28
30
5
10
15
20
25
30
60 40 20 0 80 100 5.5 5
0 4 3 2 1
TYPICAL PERFORMANCE CHARACTERISTICS
(VVIN = 3.6V, TA = 25°C, unless otherwise specified.)
Soft-Start Time vs. Capacitance
Soft-Start Time (ms)
Capacitance (µF)
0
ACT6357/ACT6358-015
0.5 0
100
200
300
400
600
1 1.5 2 2.5
500
10kHz
1kHz
100Hz
4 LEDs
10 LEDs
6 LEDs
ACT6357/ACT6358
Rev 1, 15-Nov-12
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Copyright © 2012 Active-Semi, Inc.
PACKAGE OUTLINE
TDFN33-8 PACKAGE OUTLINE AND DIMENSIONS
D
E
D2
eb
L
E2
K
PIN #1 INDEX AREA
D/2 x E/2
PIN #1 INDEX AREA
D/2 x E/2
SYMBOL
DIMENSION IN
MILLIMETERS
DIMENSION IN
INCHES
MIN MAX MIN MAX
A 0.700 0.800 0.028 0.031
A1 0.000 0.050 0.000 0.002
A3 0.200 REF
D 2.850 3.150 0.112 0.124
E 2.850 3.150 0.112 0.124
D2 2.100 2.500 0.083 0.098
E2 1.350 1.750 0.053 0.069
b 0.250 0.350 0.010 0.014
e 0.650 TYP 0.026 TYP
L 0.300 0.500 0.012 0.020
0.008 REF
K 0.200 --- 0.008 ---
Active-Semi, Inc. reserves the right to modify the circuitry or specifications without notice. Users should evaluate each
product to make sure that it is suitable for their applications. Active-Semi products are not intended or authorized for use
as critical components in life-support devices or systems. Active-Semi, Inc. does not assume any liability arising out of
the use of any product or circuit described in this datasheet, nor does it convey any patent license.
Active-Semi and its logo are trademarks of Active-Semi, Inc. For more information on this and other products, contact
sales@active-semi.com or visit http://www.active-semi.com.
is a registered trademark of Active-Semi.
