Keypad Backlight; Keypad Backlight Troubleshooting - Nokia 2255 Baseband Description And Troubleshooting

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2255 (RM-97)

Baseband Description and Troubleshooting

Keypad Backlight

When troubleshooting keypad backlight, make these common checks (see

Figure

21):

• Perform visual inspection of all the components including LEDs

• If Keypad LEDs do not turn on check Vout +(~9.5V). If ok then you can assume

the driver is working properly and the LED's or the current mirror circuitry is

faulty.

• If Vout is not ok check Vin (~Vbat) is present on driver inputs. If not, check the

power supply connection

• If Vin is ok then check the output of the LED driver Vout is (~9.5V), if not ok

replace driver and inductor

• If Vout is ok then check the current mirror transistors for wrong orientation, bad

solder or misalignments

• If LED brightness is low or to bright check that the Rset resistor is 56 Ohms

providing (~10mA) to display LEDs

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Figure 20

and

ISSUE 1 10/2005

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Rm-97