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ISL97642

TFT-LCD DC/DC with Integrated Amplifiers

Intersil 

ISL97642

edges be interrupted (for example during discontinuous

operation at light boost load currents), then this may affect

the performance of VON and VOFF regulation – depending

on their exact loading conditions at the time.

To optimize VON/VOFF regulation, the boundary of

discontinuous/continuous operation of the boost converter

can be adjusted by suitable choice of inductor given VIN,

VOUT, switching frequency and the VBOOST current loading,

to be in continuous operation.

Equation 15 gives the boundary between discontinuous and

continuous boost operation. For continuous operation (LX

switching every clock cycle) we require that:

I(vBOOST_load > D • 1 – D • VIN ⁄ 2 • L • fOSC)

(EQ. 15)

where the duty cycle, D = (VBOOST – VIN)/VBOOST

For example, with VIN = 5V, fOSC = 1.2MHz and VBOOST =

12V, we find continuous operation of the boost converter can

be guaranteed for:

L = 10µH and I(VBOOST) > 51mA

L = 6.8µH and I(VBOOST) > 74mA

L = 3.3µH and I(VBOOST) > 153mA

Start-up Sequence

Figure 22 shows a detailed start-up sequence waveform. For

a successful power-up, there should be 6 peaks at VCDEL.

When a fault is detected, the device will latch off until either

EN is toggled or the input supply is recycled.

When the input voltage is higher than 2.4V, an internal

current source starts to charge CCDEL. During the initial slow

ramp, the device checks whether there is a fault condition. If

no fault is found during the initial ramp, CCDEL is discharged

after the first peak. VREF turns on at the peak of the first

ramp.

Initially, the boost is not enabled so VBOOST rises to VIN-

VDIODE through the output diode. Hence, there is a step at

VBOOST during this part of the start-up sequence.

VBOOST soft-starts at the beginning of the third ramp, and is

checked at the end of this ramp. The soft-start ramp

depends on the value of the CDEL capacitor. For CDEL of

100nF, the soft-start time is ~7ms.

VOFF turns on at the start of the fourth peak.

VON is enabled at the beginning of the sixth ramp. VOFF and

VON are checked at end of this ramp.

15

FN6436.0

June 18, 2007

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