ISL97642 Intersil Corporation, ISL97642 Datasheet - Page 15

• Manufacturer Part Number: ISL97642
• Description: TFT-LCD DC/DC
• Manufacturer: Intersil Corporation

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edges be interrupted (for example during discontinuous
operation at light boost load currents), then this may affect
the performance of V
on their exact loading conditions at the time.
To optimize V
discontinuous/continuous operation of the boost converter
can be adjusted by suitable choice of inductor given V
V
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:
where the duty cycle, D = (V
For example, with V
12V, we find continuous operation of the boost converter can
be guaranteed for:
L = 10µH and I(V
L = 6.8µH and I(V
L = 3.3µH and I(V
I v
(
OUT
BOOST_load
, switching frequency and the V
ON
>
D 1 D V
/V
BOOST
BOOST
BOOST
•
OFF
IN
ON
–
= 5V, f
regulation, the boundary of
) > 51mA
and V
•
) > 74mA
) > 153mA
IN
BOOST
15
OSC
⁄
OFF
2 L f
•
= 1.2MHz and V
regulation – depending
•
– V
BOOST
OSC
IN
)/V
)
current loading,
BOOST
BOOST
(EQ. 15)
IN
,
=
ISL97642
Start-up Sequence
Figure 22 shows a detailed start-up sequence waveform. For
a successful power-up, there should be 6 peaks at V
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 C
ramp, the device checks whether there is a fault condition. If
no fault is found during the initial ramp, C
after the first peak. V
ramp.
Initially, the boost is not enabled so V
V
V
V
checked at the end of this ramp. The soft-start ramp
depends on the value of the C
100nF, the soft-start time is ~7ms.
V
V
V
DIODE
BOOST
BOOST
OFF
ON
ON
is enabled at the beginning of the sixth ramp. V
are checked at end of this ramp.
turns on at the start of the fourth peak.
through the output diode. Hence, there is a step at
during this part of the start-up sequence.
soft-starts at the beginning of the third ramp, and is
REF
turns on at the peak of the first
DEL
CDEL
capacitor. For C
BOOST
. During the initial slow
CDEL
rises to V
is discharged
June 18, 2007
DEL
OFF
CDEL
FN6436.0
IN
of
and
-
.