Dell E190Sf Service Manual page 7

push-pull configuration..
SSTCMP(pin14) applys the soft-start function and the loop compensation funtion.C5 at SSTCMP
compensates the feedback loop and provides good transient response to load changes.
When the IC is enabled,it starts from the strinking mode and the TIMER(pin8) is activated.During lamp
ignition,if the voltage at pin SSTCMP exceeds an approximate 2.0V threshold,the drive frequency will swith
from stinking frequency to the operation frequency, The striking frequency is determined by (R4& R5)
connected to Pin 4(RT1)and Pin5(RT). The operating frequency is determined just by R4.
Applying a voltage level greater than 2V from IF BD to pin6(PWM) can enable the IC,Once the
voltage less than ~1.0V for longer than 20ms will disable the IC.
Once the lamps are ignited and the voltage at pins ISEN1(pin9),ISEN2(pin10),OLP1(pin12),and
OLP2(pin13) all exseed approximately 0.8V,the IC enters the normal operation mode and the PWM
dimming control is actived. During start up, ISEN1 (pin9and pin10) senses the voltage on the transformer
secondary. If no current is sensed approximately 2 seconds, U1 shuts off. re used to limit current. C3 is used
to dump noise.
DRV1 and DRV2 of U1 are used to drive Q1. DRV1 and DRV2 are controlled by build-in PWM IC.
Q1 is switched which has two build-in IGBTs. The working principle of circuit of T1, the same to circuit of
Power.
The voltage signal on negative pole of D1 sensed through R7/R8 comes to Pin11 of U1 VSEN (Lamp
Current Detection & Control). The CCFL current is detected through R12/R21 and reaches a regulated value.
The CCFL current detected at resistor R12/R21 is converted to a voltage level and input to the ISEN1/2
Once the CCFLs are ignited and current is sensed through resistor R12/R21, performs the loop compensation
function. The voltage at IC pin12 controls the drive duty cycle of the power MOSFETs to regulate the CCFL
current.
2.1.2 Output Circuit and Protection Circuit operations:(fig.3)
The transformer (T1) secondary winding leakage inductance and output capacitance(C22) forms a
lower pass filter,which converts the square-wave driving signal into a sinusoidal output voltage signal for
CCFL.
The over-voltage protection feature is implemented by using an extermal capacitor devider(C20/C21) to
sense the output voltage.The divide-down voltage signal is sent to the IC Pin11(VSEN) ,thus regulating the
output voltage at the transformer secondary.
If a CCFL is removed, fails or damaged during normal operation, CCFL current is no longer sensed and
the voltage on ISEN1/ISEN2 drops. Once the voltage at the ISEN1/ISEN2 pin is less than the lamp" on"
threshold, the shutdown timer is activated. The IC maintains the output voltage for approximately
2-3seconds and once the timer reaches a threshold of approximately 3V, the IC will shutdown. To restart the
IC, either toggle the SST_CMP pin or recycle the power on the VDDA pin.
Fig.3
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Service Manual