Display Backlight Troubleshooting - Nokia 2255 Baseband Description And Troubleshooting

2255 (RM-97)
Baseband Description and Troubleshooting
The 2255 display, keypad, flashlight, and CallLED light are power by a single LED driver
and controlled by both UEMC and UPP.
GENIO(19) coming out of UPP turns on the LED driver. Then the Dlight comes up,
activating the current mirror and turning on the display and keypad backlight.
To turn on the flashlight, UPP turns on the LED driver by enabling GENIO (19) high and
UEMC turns on the current mirror by enabling Klight.
To turn on the CallLED, UPP turns on LED driver by enabling GENIO(19) and turning on
the current mirror by enabling GEN IO (18).

Display Backlight Troubleshooting

When troubleshooting the display backlight, make these common checks (see
• Perform visual inspection of LCD connector and LED Driver circuitry
• If the display backlight does not turn on, check VLED +(~8V)
• If there, then you can assume the driver is working properly and the LED inside
the display might be faulty. Change display.
• If the VLED + is not ok, check Vin (~Vbat) are present on driver inputs. If not
check power supply connection
• If Vin is Ok then check that the output of the LED driver Vout is (~9.5V)
• If Vout is ok, check the current mirror transistors for wrong orientation, bad
solder or misalignments. This sets the driver output.
• If Vout is not ok check the FB voltage (~.5v).
• Check that the output setting resistors are ok and the right value
• If the LED brightness is low or to bright check that the Rset resistor is 33 Ohms
providing (~17mA) to the display LEDs.
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©2005 Nokia Corporation
Company Confidential
Figure 19):
ISSUE 1 10/2005