SGM3124 Datasheet, PDF (14/17 Pages) SG Micro Corp – White LED Charge Pump Current Source with PWM Brightness Control

SGM3124
White LED Charge Pump Current
Source with PWM Brightness Control
APPLICATION INFORMATION
Typical Application Using 2 White LEDs and 6 Green LEDs for LCD Backlight and Keyboard
Lighting
The SGM3124 can be used to power any kind of LED. It is also possible to mix white LEDs with color LEDs which have
a lower forward voltage. The LED with the highest forward voltage (typically the white LED) has to be connected to D1,
because the output voltage of the charge pump is regulated in such a way to keep the voltage drop from D1 to GND at
400mV (with EN1 = EN2 = 1). Therefore the output voltage of the charge pump is regulated to:
V_OUT = V_D1 + V_FLEDD1
V_OUT — Output voltage at VOUT
V_D1 — Voltage from D1 to GND (Vsource at D1 pin, see electrical characteristics)
V_FLEDD1 — Forward voltage of the LED connected to D1
Resistor R_g is used to provide current sharing between the 6 green LEDs. The upper value is calculated using:
V_Fg — Forward voltage of a green LED
I_g — Current per green LED
R_g = ^{VFLEDD1  VFg}
l_g
C2
0.47µF
C1
0.47µF
Li-ion
CIN
Battery
1µF
C2- C1- C1+ C2+
VIN
VOUT
D1
EN1 SGM3124 _D2
EN2
D3
ISET GND PGND
1µF
2 White
LEDs With
30 mA Each
6 Green LEDs
With 5 mA Each
5.1kΩ
R_g = 220Ω
Set Current to 30 mA
Per Current Sink (With
EN2 = EN1 = 1)
SG Micro Corp
www.sg-micro.com
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