el7641 Intersil Corporation, el7641 Datasheet - Page 15

• Manufacturer Part Number: el7641
• Description: Tft-lcd Dc/dc With Integrated Amplifiers
• Manufacturer: Intersil Corporation

V
supply of LX switching edges be interrupted, for example
during discontinuous operation at light boost load currents,
then this may affect the performance of V
regulation – depending 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.
The following equation gives the boundary between
discontinuous and continuous boost operation. For
continuous operation (LX switching every clock cycle) we
require that:
I(V
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
OFF
OUT
BOOST
, switching frequency and the V
supplies. It can be appreciated that should a regular
_load) > D*(1-D)*V
ON
/V
BOOST
BOOST
BOOST
OFF
IN
= 5V, F
regulation, the boundary of
) > 51mA
) > 74mA
) > 153mA
BOOST
15
IN
OSC
/(2*L*F
= 1.2MHz and V
– V
BOOST
OSC
IN
ON
)/V
)
current loading,
BOOST
and V
EL7640, EL7641, EL7642
BOOST
OFF
IN
,
=
Start-up Sequence
Figure 23 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
220nF, the soft-start time is ~2ms.
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
DLY
CDLY
capacitor. For C
BOOST
. During the initial slow
CDLY
rises to V
is discharged
February 22, 2006
DLY
OFF
CDLY
FN7415.2
IN
of
-
and
.