SC4538
High Efficiency Boost Converter
for up to 10 LEDs
POWER MANAGEMENT
Features
Input voltage range — 2.8V to 5.5V
Programmable LED current up to 30mA
Current-mode PWM control — 800kHz
Soft-start to reduce in-rush current
PWM dimming — 100Hz to 50kHz
PWM dimming at ISET — Analog and filtered
Over-voltage protection — 38V (minimum)
Under-voltage lockout (UVLO)
Thermal shutdown
Shutdown current — <0.1μA (typical)
Ultra-thin package — 2mm x 2mm x 0.6mm
Lead-free package, WEEE and RoHS compliant
Description
The SC4538 is designed to regulate current for a series
string of white LEDs in LCD backlighting applications
where small size and high efficiency are priorities. This
device integrates an 800kHz current-mode PWM boost
converter and a 30mA programmable low dropout current
sink regulator.
An external resistor sets the LED current up to 30mA.
PWM dimming (100Hz to 50kHz) may be applied directly
to the enable (EN) pin, or dimming can be controlled by
applying an analog signal to the ISET circuit. The boost
circuit can output up to 38V (guaranteed) to drive up to 10
LEDs in series. The current regulator protects against
shorts between the BL and OUT pins and also eliminates
backlight glow during shutdown when using LEDs with
high leakage. Under-voltage lockout and thermal shut-
down provide additional protection. A small external
capacitor and series resistor control soft-start and loop
compensation. Over-voltage detection protects the
SC4538 if the BL pin is floating or shorted to ground.
The SC4538 is available in an MLPD-8 pin 2mm x 2mm x
0.6mm package, with a rated temperature range of -40°C
to +85°C.
Applications
DSLR, DSC, and Video Cameras
Cellular handsets
Portable media players
Personal navigation systems
Satellite radio
Handheld video games
Typical Application Circuit
D
L1
2.8 to
5.5V
LX
IN
C
IN
C
OUT
OUT
SC4538
Enable and
PWM dimming
R
ISET
C
COMP
EN
ISET
R
COMP
COMP
BL
GND
June 20, 2008
© 2008 Semtech Corporation
1
SC4538
Pin Configuration
Ordering Information
Device
SC4538ULTRT
(1) (2)
SC4538EVB
Package
MLPD-UT-8 2x2
Evaluation Board
LX
EN
ISET
COMP
1
2
3
4
TOP
VIEW
8
7
6
IN
GND
BL
OUT
Note:
(1) Available in tape and reel only. A reel contains 3,000 devices.
(2) Lead-free package only. Device is WEEE and RoHS compliant.
T
5
MLPD: 2mm x 2mm 8 Lead
θ
JA
= 74°C/W
Marking Information
FE0
yw
FE0 = Marking code for SC4538
yw = Date Code
2
SC4538
Absolute Maximum Ratings
Supply Voltage (V) . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 to +20.0
LX Voltage, Output Voltage (V) . . . . . . . . . . . . . -0.3 to +45
Current Sink Voltage (V) . . . . . . . . . . . . . . . . . . . . -0.3 to +45
Enable Voltage (V) . . . . . . . . . . . . . . . . . . . . . . -0.3 to (V
IN
+ 0.3)
Compensation Voltage (V) . . . . . . . . . . . . . . . . . . -0.3 to +2.0
Current Set Voltage (V) . . . . . . . . . . . . . . . . . . . . . . -0.3 to +2.0
ESD Protection Level (kV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
(1)
Recommended Operating Conditions
Supply Voltage (V) . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.8 to 5.5
Output Voltage (V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 to 38
Ambient Temperature Range (°C) . . . . . . . . . . . . . -40 to +85
Thermal Information
Thermal Resistance, Junction to Ambient
(2)
(°C/W) . . . . . 74
Maximum Junction Temperature (°C) . . . . . . . . . . . . . . +150
Storage Temperature Range (°C) . . . . . . . . . . . . -65 to +150
Peak IR Reflow Temperature (10s to 30s) (°C) . . . . . . . +260
Exceeding the above specifications may result in permanent damage to the device or device malfunction. Operation outside of the parameters
specified in the Electrical Characteristics section is not recommended.
NOTES:
(1) Tested according to JEDEC standard JESD22-A114-B.
(2) Calculated from package in still air, mounted to 3 x 4.5 (in), 4 layer FR4 PCB with thermal vias under the exposed pad per JESD51 standards.
Electrical Characteristics
Unless otherwise noted: V
IN
= 3.6V, C
IN
= 2.2μF, C
OUT
= 1μF, C
COMP
= 47nF, R
COMP
= 1.27kΩ, R
ISET
= 5.76kΩ, L = 22μH, T
A
= -40 to 85ºC, typical values are at T
A
= 25ºC.
Parameter
UVLO Threshold
UVLO Hysteresis
Quiescent Supply Current
Shutdown Supply Current
EN Logic High Voltage
EN Logic Low Voltage
EN Logic Input Current
Thermal Shutdown Temperature
Thermal Shutdown Hysteresis
Boost Converter Characteristics
Switching Frequency
Maximum Duty Cycle
Minimum On-Time
Switch Over-Current Protection
Symbol
V
UVLO
V
UVLO-HYS
I
Q
I
SHDN
V
IN
V
IL
I
IL,
I
IH
T
SD
Conditions
V
IN
rising
Min
2.40
Typ
2.60
120
Max
2.79
Units
V
mV
mA
Not switching
EN tied to GND
1.80
1.8
0.1
1
μA
V
0.8
V
IN
= 5.5V, V
EN
= 0V or 5.5V
T
J
rising
±0.01
155
20
±1
V
μA
°C
°C
f
SW
D
MAX
t
ON(MIN)
I
OCP
V
IN
= 3.2V, T
A
= 25°C
680
92
800
920
kHz
%
100
425
725
ns
mA
3
SC4538
Electrical Characteristics (continued)
Parameter
Boost Converter Characteristics (continued)
Switch Leakage Current
Switch Saturation Voltage
COMP Sourcing Current
I
COMP
COMP Sinking Current
OUT Over-Voltage Protection
OUT Internal Pull-Down Current
OUT Bias Current
I
OUT
OUT Leakage Current
PWM Dimming Frequency Range
(1)(2)
PWM Dimming Duty Cycle Range
(1)(2)
Current Sink Characteristics
BL Current Setting Range
(3)
I
BL
BL Current Setting Accuracy
BL Leakage Current
BL Current Line Regulation
BL Voltage
ISET Bias Voltage
ISET-to-I
BL
Gain
Start-Up Time
I
L(BL)
ΔI
BL
/ΔV
IN
V
BL
V
ISET
A
ISET
t
start-up
T
A
= 25°C
V
EN
= 0V, V
BL
= 2V
V
IN
= 3.0 to 5.5V
-3.5
0.01
±0.05
0.35
0.5
230
1.3
3.5
0.1
%
μA
mA/V
V
V
A/A
μs
1
30
mA
f
EN
D
EN
V
EN
= 0V, V
OUT
= V
IN
= 5.5V
Applied to EN pin
200Hz on EN pin
100
0
0.01
1
50k
100
μA
Hz
%
V
OVP
I
OVP
During OVP condition
V
EN
= V
IN
, V
OUT
= 20V
V
COMP
= 0.9V, T
A
= 25ºC
38
1
50
70
6
43
V
mA
μA
I
L(LX)
V
SAT
V
LX
= 5.5V
I
LX
= 0.3A
V
COMP
= 0.9V, T
A
= 25ºC
0.01
250
5
μA
1
450
μA
mV
Symbol
Conditions
Min
Typ
Max
Units
Notes:
(1) Guaranteed by design.
(2) See PWM Dimming description in the Applications Information section for limitations at high PWM dimming frequencies and low PWM
dimming duty cycles.
(3) Not recommended to program below 1mA with R
ISET
due to tolerance stackup. To produce output current less than 1mA, set the current >
1mA and use PWM dimming.
4
SC4538
Typical Characteristics
All data taken with V
OUT
= 20V (6 white LEDs), R
ISET
= 5.76kΩ (I
BL
= 20mA), V
IN
= 3.6V, L = 22μH, and efficiency (η) = P
LED
/P
IN
unless otherwise noted.
Efficiency vs. I
BL
100
3.6V
IN
80
2.8V
IN
60
5.5V
IN
40
Efficiency vs. Output Voltage
90
86
5.5V
IN
Efficiency (%)
Efficiency (%)
82
2.8V
IN
78
3.6V
IN
20
74
0
0
6
12
18
70
I
BL
(mA)
24
30
0
8
16
24
32
40
V
OUT
(V)
Efficiency vs. Input Voltage
90
90
Efficiency vs. Input Voltage
6 LEDs
4 LEDs
86
86
Efficiency (%)
82
V
OUT
78
LED
Efficiency (%)
82
10 LEDs
78
74
74
70
2.8
3.3
3.9
4.4
70
5.0
5.5
2.8
3.3
3.9
4.4
5.0
5.5
V
IN
(V)
V
IN
(V)
Efficiency vs. Input Voltage
90
20mA
86
Normalized I
BL
vs. Output Voltage
6
4
Normalized I
BL
(%)
5.0
5.5
Efficiency (%)
2
82
0
78
30mA
10mA
74
-2
-4
70
2.8
3.3
3.9
4.4
-6
0
8
16
V
IN
(V)
V
OUT
(V)
24
32
40
5
京公网安备 11010802033920号
EEWORLD
