SD42509
1A WHITE LED DRIVER
DESCRIPTION
The SD42509 is a step-down PWM control LED driver with a built-in
internal power MOSFET. It achieves 1A continuous output current over
a wide input supply range. It provides thermal shutdown circuit, current
limiting circuit.
SD42509 adopts current mode contro
l.
Current mode operation
provides fast transient response and simple loop stabilization design.
SD42509 has high efficiency up to more than 90%, and has excellent
constant current characteristic when the input/output voltage changed,
with the excellent current accuracy within
±1%.
FEATURES
* Maximum 1A output current
* 0.25Ω built-in power MOSFET
* 280kHz fixed frequency
* Excellent constant current accuracy
±1%
* High efficiency up to 90%
* Thermal shutdown
* Cycle-by-cycle over current protection
* 6-20V input voltage range
* Drive LED string
ORDERING INFORMATION
Device
SD42509
Package
SOP-8-225-1.27
APPLICATIONS
* Power LED driver
BLOCK DIAGRAM
HANGZHOU SILAN MICROELECTRONICS CO.,LTD
Http://www.silan.com.cn
REV:1.4
2009.02.26
Page 1 of 8
SD42509
ABSOLUTE MAXIMUM RATING
Characteristics
Supply Voltage
Switch Voltage
Boost Voltage
Feedback Voltage
Comp Voltage
LED Voltage
Junction Temperature
Lead Temperature
Storage Temperature
Input Voltage
Operating Temperature
Symbol
V
IN
V
SW
V
BS
V
FB
V
COMP
V
LED
T
j
T
L
T
STG
V
IN
T
OPR
Ratings
22
-1~V
IN
+1
V
SW
+6
-0.3~V
IN
-0.3~6
-0.3~V
IN
150
260
-65~150
6~20
-40~125
Unit
V
V
V
V
V
V
°C
°C
°C
V
°C
ELECTRICAL CHARACTERISTICS
(Unless otherwise specified, V
IN
=12V, T
amb
=25°C)
Characteristics
Operating Current
Upper Switch Leakage
Current Limit
Maximum Duty Cycle
Oscillator Frequency
COMP Clamp Voltage
Switch Transistor on Resistance
Sampling Voltage Threshold
Symbol
I
IN
I
L
I
LIM
D
max
f
osc
V
COMP
R
ONH
V
FB
－V
LED
Open without load
V
FB
=0V
Test Condition
V
FB
≥3.5V
V
SW
=0V
1.3
--
250
1.9
--
--
Min.
Typ.
2.0
0
1.6
93
280
2.2
0.25
88
Max.
2.3
10
2.0
--
310
2.5
--
--
Unit
mA
μA
A
%
kHz
V
Ω
mV
PIN CONFIGURATION
HANGZHOU SILAN MICROELECTRONICS CO.,LTD
Http://www.silan.com.cn
REV:1.4
2009.02.26
Page 2 of 8
SD42509
PIN DESCRIPTIONS
Pin No.
1
2
3
4
5
6
7
8
Pin Name
FB
LED
COMP
NC
BS
V
IN
SW
GND
I/O
I
O
I/O
-
I
I
O
-
Description
Current sense pin ,connect inductance and sense resistor.
LED positive connecting end, the other end connects to the sense resistor.
Compensation pin, connects to external capacitor
No Connected.
Bootstrap pin, used to increase the power MOSFET voltage
Input supply voltage.
Power output.
Ground.
FUNCTION DESCRIPTION
The SD42509 is a current mode LED driver. The voltage on compensation pin COMP is proportional to the
current delivered to the load.
At the beginning of a cycle: the upper transistor M1 is off; the COMP pin voltage is higher than the current
sense amplifier output; and the current comparator’s output is low. The rising edge of the 280KHz CLK signal
sets the RS Flip-Flop. Its output turns on M1 thus connecting the SW pin and inductor to the input supply. The
inductor current is sensed and amplified by the current sense amplifier. Ramp compensation is summed to
current sense amplifier output and compared to the Gm amplifier output by the current comparator. When the
current sense amplifier plus ramp compensation signal exceeds the comp pin voltage, the RS Flip-Flop is reset
and the chip reverts to its initial M1 off state. If the current sense amplifier plus slope compensation signal does
not exceed the comp voltage, then the falling edge of the CLK resets the Flip-Flop.
The Gm amplifier compares the output current to the threshold current (threshold current is set by internal).
When the output current is higher than threshold current, the COMP pin’s voltage is lower down. Since the
COMP pin’s voltage is proportional to the peak inductor current a decrease in its voltage decreases current
delivered to the output. When the output current is lower than threshold current, the COMP pin’s voltage is up
while the output current acreases. The output current is stable at the set value by the adjusting of the loop.
1． Output Current Setting
The output current is determined by the sampling resistor and setting voltage. The sampling voltage of VFB-
VLED is 88mV, and adjust the output current by adjusting the sampling resistor Rs (refer to Typical Application
Circuit).
Iout
=
V
FB
−
V
LED
R
S
2． Bootstrap Circuit
SD42509 has Bootstrap circuit, which provides grid drive voltage to power MOSFET; the chip has diode to
charge the bootstrap capacitor by 5V standard voltage.
3． Current Limiting Function
SD42509 has internal current limiting function, and the voltage on COMP is clamped at about 2.2V, the output
current of the power MOSFET is limited at about 1.6A by current comparator.
HANGZHOU SILAN MICROELECTRONICS CO.,LTD
Http://www.silan.com.cn
REV:1.4
2009.02.26
Page 3 of 8
SD42509
COMPONENTS SELECTION
1．
Input Capacitor Selection
The input capacitor provides the pulse current when the power MOSFET is on, and charge the capacitor when
the power MOSFET is off, thus to keep the stability of the input voltage. The input capacitor is recommended to
be more than 10μF, which can reduce the peak current drawn from input source and the switch noise. The input
capacitor should be near to the input pin when in real routing.
2． Output Capacitor Selection
Parallel connecting a capacitor between the two ends of LED can reduce the output voltage ripple accordingly
reduce the ripple current of LED, while this capacitor will not effect the operating frequency and efficiency, but the
start time will be longer by reducing the rising speed of the voltage on LED. The larger the output capacitor is, the
smaller the current ripple on LED is. It is recommend use the capacitor of 4.7μF or larger.
3． Inductance Selection
The inductance is used to keep the output current constant, the bigger the inductance is, the smaller the output
current ripple is; while the bigger the physical size is, the bigger the series-wound resistor is. The selected
effective current (RMS current rating)of inductance current should be bigger than the maximum output current,
and the saturation current should be 30% bigger than maximum output current. In order to improve the efficiency,
the series-wound resistor (DCR) of inductance should be smaller than 0.2Ω.
The relation between inductance and ripple:
L
=
V
OUT
*
V
IN
−
V
OUT
V
IN
* f *
ΔI
Where: L: Inductance value
F: Oscillator frequency
ΔI
is ripple current
When select inductance, you should consider the combination of various factors to select suitable inductance.
When output current is 1A, 47μH inductance is recommended.
4． Diode Selection
SD42509 is a non-synchronous step-down adjuster, so the diode should provide continuous current when the
power MOSFET is off. Because the forward voltage of Schottky diode is small, and the reverse continuous
current time is short, so it is usually used for continuous current. During the power MOSFET is conducting, the
diode will withstand high voltage, so the reverse voltage of selected diode should be bigger than the input voltage.
The average current through the diode is I
D:
I
D
=
(1
−
D) * I
LED
I
LED
is the current of LED
When the input voltage is high with a small duty, ID increases, so the selected maximum continuous current
diode should be bigger than the output current.
HANGZHOU SILAN MICROELECTRONICS CO.,LTD
Http://www.silan.com.cn
REV:1.4
2009.02.26
Page 4 of 8
SD42509
TYPICAL APPLICATION CIRCUIT
6½20V input voltage, 1A output LED driver
12V input voltage, 350mA output three strings of LED driver
Note: The circuit and parameters are reference only, please set the parameters of the real application circuit
based on the real test .
HANGZHOU SILAN MICROELECTRONICS CO.,LTD
Http://www.silan.com.cn
REV:1.4
2009.02.26
Page 5 of 8
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