EL7640, EL7641, EL7642
V
supplies. It can be appreciated that should a regular
Start-up Sequence
OFF
supply of LX switching edges be interrupted, for example
during discontinuous operation at light boost load currents,
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.
.
CDLY
then this may affect the performance of V
regulation – depending on their exact loading conditions at
the time.
and V
ON
OFF
When the input voltage is higher than 2.4V, an internal
current source starts to charge C
To optimize V /V
ON OFF
regulation, the boundary of
. During the initial slow
CDLY
discontinuous/continuous operation of the boost converter
can be adjusted, by suitable choice of inductor given V
ramp, the device checks whether there is a fault condition. If
no fault is found during the initial ramp, C is discharged
,
IN
current loading,
CDLY
turns on at the peak of the first
V
, switching frequency and the V
OUT
BOOST
after the first peak. V
ramp.
REF
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:
Initially the boost is not enabled so V
rises to V -
IN
BOOST
through the output diode. Hence, there is a step at
V
V
DIODE
BOOST
during this part of the start-up sequence.
V
soft-starts at the beginning of the third ramp, and is
BOOST
I(V
_load) > D*(1-D)*V /(2*L*F
IN
)
BOOST
OSC
– V )/V
checked at the end of this ramp. The soft-start ramp
depends on the value of the C
capacitor. For C
of
where the duty cycle, D = (V
BOOST
DLY
220nF, the soft-start time is ~2ms.
DLY
IN BOOST
For example, with V = 5V, F
12V we find continuous operation of the boost converter can
be guaranteed for:
= 1.2MHz and V
=
IN OSC BOOST
V
turns on at the start of the fourth peak.
OFF
V
V
is enabled at the beginning of the sixth ramp. V
are checked at end of this ramp.
and
OFF
ON
ON
L = 10µH and I(V
L = 6.8µH and I(V
L = 3.3µH and I(V
) > 51mA
BOOST
) > 74mA
BOOST
BOOST
) > 153mA
FN7415.1
September 26, 2005
15
INTERSIL [ Intersil ]
