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FN7493.3
April 24, 2009
After C
DLY
reaches the 4th peak, the internal N-FET is
turned-on and produces an initial current output of
IDELB_ON1 (~50µA). This current allows the user to control
the turn-on inrush current into the A
VDD_delay
supply
capacitors by a suitable choice of C4. This capacitor can
provide extra delay and also filter out any noise coupled into
the gate of M0, avoiding spurious turn-on, however, C4 must
not be so large that it prevents DELB reaching 0.6V by the
end of the start-up sequence on C
DLY
, else a fault time-out
ramp on C
DLY
will start. A value of 22nF is typically required
for C4. The 0.6V threshold is used by the chip's fault
detection system and if V(DELB) is still above 0.6V at the
end of the power sequencing then a fault time-out ramp will
be initiated on C
DLY
.
When the voltage at DELB falls below ~0.6V it's current is
increased to IDELB_ON2 (~1.4mA) to firmly pull the DELB
voltage to ground.
If the maximum V
GS
voltage of M0 is less than the A
VDD
voltage being used, then a resistor may be inserted between
the DELB pin and the gate of M0 such that it's potential
divider action with R4 ensures the gate/source stays below
VGS(M0)max. This additional resistor allows much larger
values of C4 to be used, and hence longer A
VDD
delay,
without affecting the fault protection on DELB.
Component Selection for Start-up Sequencing and
Fault Protection
The C
REF
capacitor is typically set at 220nF and is required
to stabilize the V
REF
output. The range of C
REF
is from
22nF to 1µF and should not be more than five times the
capacitor on C
DEL
to ensure correct start-up operation.
The C
DEL
capacitor is typically 220nF and has a usable
range from 47nF minimum to several microfarads – only
limited by the leakage in the capacitor reaching µA levels.
C
DEL
should be at least 1/5 of the value of C
REF
(see
above). Note with 220nF on C
DEL
the fault time-out will be
typically 50ms 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).
Over-Temperature Protection
An internal temperature sensor continuously monitors the
die temperature. In the event that the die temperature
exceeds the thermal trip point of +150°C, the device will shut
down. Operation with die temperatures between +125°C and
+150°C can be tolerated for short periods of time, however,
in order to maximize the operating life of the IC, it is
recommended that the effective continuous operating
junction temperature of the die should not exceed +125°C.
Layout Recommendation
The device’s performance including efficiency, output noise,
transient response and control loop stability is dramatically
affected by the PCB layout. PCB layout is critical, especially
at high switching frequency.
There are some general guidelines for layout:
1. Place the external power components (the input
capacitors, output capacitors, boost inductor and output
diodes, etc.) in close proximity to the device. Traces to
these components should be kept as short and wide as
possible to minimize parasitic inductance and resistance.
2. Place V
REF
and V
DC
bypass capacitors close to the pins.
3. Reduce the loop with large AC amplitudes and fast slew
rate.
4. The feedback network should sense the output voltage
directly from the point of load, and be as far away from LX
node as possible.
5. The power ground (PGND) and signal ground (SGND)
pins should be connected at only one point.
6. The exposed die plate, on the underneath of the
package, should be soldered to an equivalent area of
metal on the PCB. This contact area should have multiple
via connections to the back of the PCB as well as
connections to intermediate PCB layers, if available, to
maximize thermal dissipation away from the IC.”
7. To minimize the thermal resistance of the package when
soldered to a multi-layer PCB, the amount of copper track
and ground plane area connected to the exposed die
plate should be maximized and spread out as far as
possible from the IC. The bottom and top PCB areas
especially should be maximized to allow thermal
dissipation to the surrounding air.
8. Minimize feedback input track lengths to avoid switching
noise pick-up.
A demo board is available to illustrate the proper layout
implementation.
ISL97651
