Dell 2208WFP Service Manual page 9

+5VDC provides the power for U1; the control signals Brightness and ON/OFF come from I/F
board. ON/OFF signal connect to pin
pin of U1 and regulates the brightness of panel .TIMER makes up a network of delaying time
circuit and R23 make up a divided voltage network, C
. The operation frequency is determined by the external Resistor R16 and capacitor C8
connected to pin3 of U1. C7 is used for soft start and compensation.
The output drives, include DRV1, DRV2 (pins6,8,9,10 respectively) output square pulses to
drive MOSFET Q6, Q7 and each of them is consist of a N+P channel MOSFET. Q6 and Q7 work as
All Bridge structure, and it is high efficient, PWM switching.
During start up, VSEN (pin1) senses the voltage at the transformer secondary. When VSEN
reaches 3.0V, the output voltage is regulated. If no current is sensed for approximately 2seconds U1
will shut off.
The current flowing through CCFL is sensed and regulated through sense resistor R6, R19. The
feedback voltage connected to Pin1 (ISEN), then compared with a reference voltage (1.5V) via a
current amplifier, resulting that PWM drive U1 to work and output square pulses for all bridge
switches
2.1.2 Protection Circuit
Over-voltage and over-current protection are monitored by the voltage on VSEN(pin1).During
normal operation,if a CCFL is damaged or removed,the voltage at VSEN exceeds the user-defined, preset
voltage set by OVPT(pin15),the driver output duty cycle is regulated and the shutdown delay timer is
activated.OVPT sets the overall protection threshold voltage that is lower than3V(VSEN threshold. Once
the voltage at TIMER pin reaches 3V,the IC will shut down and latch. OVPT voltage setting is determined
by a resistor divider (R18/R8)connected to the OVPT.
The over-voltage protection feature is implemented by using an external capacitor divider
(C1/C15/C16/C17) to sense the output voltage. The divide-down voltage signal is sent to the IC
Pin1(VSEN) ,thus regulate the output voltage.
If a CCFL is removed, fails or is damaged during normal operation, CCFL current is no longer sensed
and the voltage on ISEN pin drops. The voltage at the SSTCMP(pin2)rises rapidly because there is no
current feedback at ISEN(pin14).When the voltage at SSTCMP reaches a threshold of approximately 2V
and ISEN is less than 0.7V,a current source charges the capacitor(C3) connected to TIMER(pin12).Once the
voltage level at the TIMER pin reaches a threshold of approximately 3V,the drive outputs shut down and
latch.To resume operation,toggle the ENA signal or restart VDDA.
2.2 Power PWM circuit
2.2.1) Block diagram:(fig.4)
of U1 and makes U1 enable. Brightness signal connect to
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Service Manual
are used to