ISL78022 Intersil Corporation, ISL78022 Datasheet - Page 15

• Manufacturer Part Number: ISL78022
• Description: (ISL78020 / ISL78022) Automotive Grade TFT-LCD DC/DC
• Manufacturer: Intersil Corporation

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which LDO regulators generate the V
It 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 V
(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:
where the duty cycle, D = (V
For example, with V
V
converter can be guaranteed for:
L = 10µH and I(V
L = 6.8µH and I(V
L = 3.3µH and I(V
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
Component Selection for Start-up Sequencing and
Fault Protection
The C
to stabilize the V
I V
(
OUT
BOOST
DIODE
BOOST
BOOST
OFF
ON
ON
BOOST_load
is enabled at the beginning of the sixth ramp. V
are checked at end of this ramp.
, switching frequency and the V
turns on at the start of the fourth peak.
REF
through the output diode. Hence, there is a step at
soft-starts at the beginning of the third ramp, and is
= 12V we find continuous operation of the boost
during this part of the start-up sequence.
capacitor is typically set at 220nF and is required
ON
)
>
/V
REF
D
BOOST
BOOST
BOOST
OFF
•
IN
1 (
REF
output. The range of C
= 5V, f
–
regulation, the boundary of
) > 51mA
D ) V
) > 74mA
) > 153mA
turns on at the peak of the first
•
BOOST
15
OSC
DEL
IN
CDEL
⁄
(
= 1.2MHz and
2 L f
capacitor. For C
ON
•
– V
BOOST
BOOST
ON
. During the initial slow
•
and V
IN
and V
CDEL
OSC
)/V
REF
current loading,
BOOST
rises to V
OFF
)
OFF
is discharged
is from
regulation
DEL
ISL78020, ISL78022
supplies.
OFF
(EQ. 17)
CDEL
IN
IN
of
-
and
,
.
22nF to 1µF and should not be more than five times the
capacitor on C
The C
range from 22nF minimum to several microfarads (only
limited by the leakage in the capacitor reaching µA levels).
C
Figure 22). Note that with 100nF on C
will be typically 23ms and the use of a larger/smaller value
will vary this time proportionally (e.g. 1µF will give a fault
time-out period of typically 230ms).
Fault Sequencing
The ISL78020 and ISL78022 have an advanced fault
detection system, which protects the IC from both adjacent
pin shorts during operation and shorts on the output
supplies. A high quality layout/design of the PCB, in respect
of grounding quality and decoupling is necessary to avoid
falsely triggering the fault detection scheme (especially
during start-up). The user is directed to the layout guidelines
and component selection sections to avoid problems during
initial evaluation and prototype PCB generation.
V
The V
switching the voltage on COM between ground, DRN and
SRC, under control of the start-up sequence and the CTL pin.
During the start-up sequence, COM is held at ground via an
NDMOS FET, with ~1k impedance. Once the start-up
sequence has completed, CTL is enabled and acts as a
multiplexer control such that if CTL is low, COM connects to
DRN through a 5Ω internal MOSFET, and if CTL is high,
COM connects to SRC via a 30Ω MOSFET.
The slew rate of start-up of the switch control circuit is mainly
restricted by the load capacitance at COM pin as in
Equation 18:
Where V
circuit, R
including the internal MOSFET r
and the resistor inserted, R
switch control circuit, and C
switch control circuit.
In the “Typical Application Circuit” on page 18, R
C
Where: R
Op Amps
The ISL78020 and ISL78022 have 1 and 5 amplifiers
respectively. The op amps are typically used to drive the
V
ΔV
------- -
Δt
DEL
ON
8
DRN
give the bias to DRN based on Equation 19:
=
-Slice Circuit
ON
DEL
----------------------------------- -
(
should be at least 1/5 of the value of C
=
R
i
g
i
V
------------------------------------------------------------ -
-slice Circuit functions as a three way multiplexer,
||
10
is the resistance between COM and DRN or SRC
ON
is the supply voltage applied to the switch control
capacitor is typically 100nF and has a usable
R
V
L
g
can be adjusted to adjust the slew rate.
) C
•
DEL
•
R
R
9
8
L
+
+
to ensure correct start-up operation.
A
R
VDD
9
•
R
L
L
8
is the load resistance of the
is the load capacitance of the
DS(ON)
DEL
, the trace resistance
the fault time-out
REF
December 6, 2007
8
(see
, R
9
(EQ. 18)
(EQ. 19)
FN6386.2
and