EL7641 datasheet(15/19 Pages) RENESAS

15 / 19 page

EL7640, EL7641, EL7642

FN7415 Rev 2.00

Page 15 of 19

February 22, 2006

can be appreciated that should a regular supply of LX

switching 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.

The following equation 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)

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 23 shows a detailed start-up sequence waveform. For a

successful power-up, there should be 6 peaks at VCDLY. 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 CCDLY. During the initial slow ramp, the

device checks whether there is a fault condition. If no fault is

found during the initial ramp, CCDLY 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 CDLY capacitor. For CDLY of 220nF, the

soft-start time is ~2ms.

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.

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