TPS60231 Datasheet, PDF (13/16 Pages) Texas Instruments – WHITE LED CHARGE PUMP CURRENT SOURCE WITH PWM BRIGHTNESS CONTROL

TPS60231
www.ti.com
SLVS544 – OCTOBER 2004
APPLICATION INFORMATION (continued)
TYPICAL APPLICATION USING 2 WHITE LEDs AND 6 GREEN LEDs FOR LCD BACKLIGHT AND
KEYBOARD LIGHTING
The TPS60231 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:
Rg
+
VFLEDD1
Ig
*
VFg
V_Fg— Forward voltage of a green LED
I_g— Current per green LED
VIN = 2.7 V
to 6.5 V
1 mF
VIN
C1+
0.47 mF
C1−
C2+
0.47 mF
C2−
VOUT
D1
D2
D3
EN1
EN2
GND
ISET
1 mF
2 White
LEDs With
25 mA Each
6 Green
LEDs With
4 mA Each
R_g = 220 W
6.2 kW
Sets Current to 25 mA
Per Current Sink
(With EN2 = EN1 = 1)
Figure 14. LED Connections for LCD Backlight and Keyboard Lighting
PROPOSED LAND PATTERN FOR PCB PRODUCTION
Refer to the application note SLUA271 for the proposed land pattern of the QFN package.
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