User Guide

RM-11

System Module CC Technical Documentation

Charger Interface Protection

In order to ensure safe operation with all chargers and in misuse situations, the charger

interface is protected using a transient voltage suppressor (TVS) and a 1.5A fuse. The TVS

device used in RM-11 is rated for 16 V@175 W.

Figure 6: Charger interface diagram

Table 4 includes the values for the TVS.

LED Driver Circuit

In RM-11, white LEDs are used for LCD and keypad lighting. Two LEDs are used for LCD

lighting and six are used for the keyboard. A step-up DC-DC converter (TK11851) is used

as the white LED driver.

The display LEDs are driven in serial mode to achieve stable backlight quality. This means

that constant current flows through LCD LEDs. Serial resistance RLCD is used to define

the proper current. The feedback signal (FB) is used to control the current. The driver

increases or decreases 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 is not used to control the current. The

driver is controlled by the UEM via the 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.

Table 4: Charger interface TVS characteristics

Characteristic Value

Breakdown voltage (VBR) 17.8Vmin (at IT 1.0mA)

Reverse standoff voltage (VR) 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