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PDF SC4538 Data sheet ( Hoja de datos )
| Número de pieza | SC4538 | |
| Descripción | High Effi ciency Boost Converter | |
| Fabricantes | Semtech Corporation | |
| Logotipo |  |
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SC4538
High Efficiency Boost Converter
POWER MANAGEMENT
for up to 10 LEDs
Features
Description
Input voltage range — 2.8V to 5.5V
Current-mode PWM control — 800kHz
Programmable LED current up to 30mA
Soft-start to reduce in-rush current
PWM dimming — 100Hz to 50kHz
Analog and filtered-PWM dimming at ISET
www.DataSheeOt4vUe.cr-ovmoltage protection — 38V (min.)
Under-voltage lockout (UVLO)
Thermal shutdown
Shutdown current — <1μA
Ultra-thin package — 2mm x 2mm x 0.6mm
Lead-free package, WEEE and RoHS compliant
Applications
White LED Display Backlight Driver
Smartphones and PDAs
Cellular Handsets
MP3 and Portable Media Players
DSC, GPS, Satellite Radio, Game Players
The SC4538 is designed to regulate current for a series
chain 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 SC4538
drives a string of up to 10 white LEDs with a forward
voltage of 3.8V or less. 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 shutdown provide additional protection. A small
external capacitor and series resistor control soft-start and
loop compensation. Over-voltage detection protects the
SC4538 if the LED pin is floating or shorted to ground.
The SC4538 is available in an MLPD-8 pin 2mmx 2xmm
0.6mm package, with a rated temperature range of -40°C
to +85°C.
Typical Application Circuit
2.8 to
5.5V
CIN
Enable and
PWM dimming
D
L1
LX
IN OUT
SC4538
EN
COUT
RISET
CCOMP
RCOMP
ISET
COMP
GND
BL
July 11, 2007
1
1 page  
SC4538
Typical Characteristics
All data taken with V = 20V, R = 5.76kΩ, V = 3.6V, and efficiency (η) = P /P unless otherwise noted.
OUT ISET
IN
LED IN
Efficiency vs. I
BL
100
Efficiency vs. Output Voltage
90
5.5VIN
80 86
60
5.5VIN
www.DataSheet4U.com
40
20
3.6VIN
2.8VIN
82
78
74
3.6VIN
2.8VIN
0
0.0
90
86
6.0 12.0 18.0 24.0
I (mA)
BL
Efficiency vs. Input Voltage
70
30.0 0.0
90
86
8.0 16.0 24.0 32.0
VOUT (V)
Efficiency vs. Input Voltage
6 LED
82
VOUT Efficiency
78
74
LED Efficiency
82
4 LED
78
10 LED
74
70
2.8
90
86
82
78
74
70
2.8
3.3 3.9 4.4 5.0
V (V)
IN
Efficiency vs. Input Voltage
20mA
10mA
30mA
3.3 3.9 4.4 5.0
V (V)
IN
70
5.5 2.8
6.0
3.3 3.9 4.4 5.0
VIN (V)
Normalized I vs. Output Voltage
BL
4.0
2.0
0.0
-2.0
-4.0
-6.0
5.5 0.0
8.0 16.0 24.0 32.0
VOUT (V)
40.0
5.5
40.0
5
5 Page 
SC4538
Applications Information (continued)
slowly charges the COMP node voltage, the duty cycle of
the boost switch ramps from 0% to its final value once in
regulation. The gradual increase of the duty cycle slowly
charges the output capacitor and limits in-rush current
during start up. Soft-start is implemented only when the
power is cycled on the part.
Thermal Shutdown
A thermal shutdown system is included for protection in
www.DataSheet4U.com
the event the junction temperature exceeds 155°C. In
thermal shutdown, the on-chip power switch is disabled.
Switching and sinking resumes when the temperature
drops by 20°C.
Current Limit
The power switch of the boost is protected by an internal
current limit function that opens the switch when the
current exceeds the maximum current switch value up to
850mA.
Inductor Selection
The inductor value should be within the range of 4.7μH to
22μH. The DCR needs to be considered when picking the
inductor to ensure decent efficiency. The largest inductor
package that can be tolerated in the circuit area should be
used since the DCR generally decreases with package
size.
The saturation current of the inductor should be much
higher than the peak current of the internal boost switch
to ensure that the inductor never enters saturation during
normal operation of the part. The equation to calculate
the peak inductor current is:
IL(Peak)
IIN ǻIL
2
where
ǻIL
VIN u D
L u fosc
D 1 VIN
VOUT
IIN
VOUT u IOUT
Ș u VIN
D is the duty cycle for continuous operation. Efficiency (η)
can be approximated by using the curves provided in the
Typical Characteristics section. Table 1 has inductors that
have been proven to work with SC4538.
Table 1 -- Recommended Inductors
Part Number
Value DCR
Rated
Toler-
(μH) (Ω) Current (A) ance
Dimensions
(L x W x H)
(mm)
Coilcraft
LPS4018-223ML
22 0.360
0.70
±20% 3.9 x 3.9 x 1.7
Murata
LQH43CN150K03
Murata
LQH32CN150K03
15 0.320 0.570
15 0.580 0.300
±10% 4.5 x 3.2 x 2.6
±20% 3.9 x 2.5 x 1.55
Capacitor Selection
The input capacitor should be at least 2.2μF. A larger
capacitor will reduce the voltage ripple on the input. The
output capacitor values can range from 0.22μF to 1μF. The
compensation capacitor value should be 47nF. Capacitors
of X5R type material or better can be used for many of the
capacitors mentioned above. See Table 2 for capacitors
that can be used.
Table 2 -- Recommended Capacitors
Part Number
Input Capacitor
Value
(μF)
Rated
Voltage (V)
Type Case Size
Murata
GRM188C70J22KE20
2.2 6.3 X7S 0603
Output Capacitor
Murata
GRM21BR71H105KA12L
1.0 50 X7R 0805
Compensation Capacitor
Taiyo Yuden
EMK105BJ473KV-F
0.047 16 X7R 0402
Diode Selection
A Schottky diode with a forward voltage of 40V and a
forward current rating of 1A should be used with this
device for optimum performance. The Central
Semiconductor Corporation CMOSH-4E is a suitable diode
for this application.
11
11 Page | ||
| Páginas | Total 14 Páginas | |
| PDF Descargar | [ Datasheet SC4538.PDF ] | |
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