ISL97642 Просмотр технического описания (PDF) - Renesas Electronics
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ISL97642 Datasheet PDF : 19 Pages
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ISL97642
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:
IvBOOST_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.
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FN6436 Rev 0.00
June 18, 2007
Page 15 of 19
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