User Guide

RH-25
System Module CCS Technical Documentation
Page 12 ©Nokia Corporation Confidential Issue 1 10/2003
interface is protected using transient voltage suppressor (TVS) and 1.5A fuse. The TVS
device used in RH-25 is rated for 16V@175W.
Figure 5: Charger interface TVS characteristics
Breakdown voltage (VBR) 17.8Vmin (at IT 1.0mA)
Reverse standoff voltage (VR) 1 6V
Max reverse leakage current at VR (IR)5uA
Max peak impulse current (Ipp) 7A (at Ta=25*C, current waveform: 10/1000us)
Max clamping voltage at Ipp (Vc) 26V
LED Driver Circuit
In RH-25, white LEDs are used for LCD and keypad lighting. Two LED are used for LCD
lighting and six for keyboard. A step-up DC-DC converter (TK11851) is used as white LED
driver.
The display LEDs are driven in serial mode to achieve stable backlight quality. This means
that constant current flow through LCD LEDs. Serial resistance Rlcd is used to define the
proper current. The feedback signal, FB, is used to control the current. Driver will increase
or decrease the output voltage for LEDs to keep the current stable.
Keyboard LEDs are driven in 2-serial/3 parallel modes. Serial resistance R is used to limit
the current through LEDs. The feedback signal, FB, is not used to control the current.
Driver is controlled by the UEM via DLIGHT output. This signal is connected to driver EN-
pin (on/off). It is possible to control the LED brightness by PWM to achieve smooth on/off
operation.