Application Note 1907
3
AN1907.0
February 10, 2014
Optional PGOOD Delay Circuit
The ISL8216M has a PGOOD comparator which monitors the
voltage on the FB pin. The PGOOD is asserted (open drain) when
the FB pin voltage is within 14% of the reference voltage. The
PGOOD is de-asserted under disable, overcurrent protection
event, or over-temperature event.
As described in Table 1, a PGOOD delay circuit, as well as a
dummy load resistor of 1k
Ω
, 1W rating, are required if all of the
following conditions are met:
• V
OUT
is higher than 12V
• PGOOD signal is utilized
• Module is self enabled/disabled
The place holders for this PGOOD delay circuit (C2, R3 and M1)
and the dummy load resistor (R20) are provided on the
ISL8216MEVAL1Z evaluation board for the user’s convenience,
as shown in Figure 3, where C2 is 1µF, R3 is typically 150k
Ω
, and
M1 is an N-MOSFET. In the case where input voltage V
IN
is not
monotonic, an optional Schottky diode is recommended in
parallel with resistor R3, to ensure that the capacitor C2 has
been fully discharged when PVCC starts to rise from 0V.
For V
OUT
>12V and light load condition, if the optional PGOOD
delay circuit is not used during V
IN
ramp up, PGOOD logic can
malfunction due to low V
OUT
as a result of bootstrap capacitor not
being fully charged. This issue can be solved by adding a delay of
about 250ms with this optional delay circuit.
Resistor Between V
IN
and BOOT
As described in Table 1, a resistor between V
IN
and BOOT is
recommended if ANY of the following conditions are met:
• V
OUT
is higher than 12V and PGOOD signal is not utilized.
• V
OUT
is higher than 12V, PGOOD signal is utilized, and module
is enabled/disabled by an external control signal.
The place holder for this resistor, R23, is provided on the
ISL8216MEVAL1Z evaluation board for the user’s convenience.
Refer to the
ISL8216M
datasheet for the selection of this
resistor. A minimum 0.25W power rating is recommended for
this resistor.
An additional N-MOSFET along with the external enable circuit is
required when R23 is populated. This N-MOSFET is used for
pulling V
OUT
to ground when the external control signal is logic
low. Without this N-MOSFET, a residual voltage can be generated
on V
OUT
capacitors due to leakage from V
IN
, R23, internal
bootstrap diode, internal bootstrap capacitor, and inductor to
V
OUT
capacitor.
Output Capacitors
The 6x22µF ceramic capacitors with voltage rating of 35V have
been installed on the ISL8216MEVAL1Z evaluation board.
However, additional place holder to add more capacitors are
provided on the board for evaluating low ESR tantalum capacitor,
low ESR polymer capacitor, and low ESR aluminum electrolytic
capacitor. A minimum total output capacitance of 120µF with
low ESR is recommended.
PCOMPX Pin Configuration
Control loop is already compensated internally to provide
sufficient stability margins for applications with various types of
output capacitors. For all ceramic output capacitors, short the
PCOMPX pin to VOUT; a 0
Ω
resistor R1 is provided on the
evaluation board. For tantalum capacitors, polymer capacitors, or
aluminum electrolytic capacitors, change R1 to resistance of
lower than 1k
Ω
. The lower the total ESR, the lower the R1 value
should be.
Radiated Emission
The ISL8216M has been evaluated with respect to CISPR 22
radio disturbance limits class B. The radiated emission test was
performed with a simple input filter installed on the
ISL8216MEVAL1Z evaluation board, as shown in Figure 3.
Figure 15 and Figure 16 show the radiated emissions plots with
respect to the CISPR 22 radio disturbance limits class B at
typical operating conditions of 24V
IN
to 5V
OUT
at 4A and 24V
IN
to
12V
OUT
at 4A.
PVCC
PGOOD
R3 150k
C2 1µF
M1
2N7002
D1
OPTIONAL
DIODE
FIGURE 3. OPTIONAL PGOOD DELAY CIRCUIT WITH DIODE
FIGURE 4. OPTIONAL INPUT FILTER FOR IMPROVED EMI
PERFORMANCE
V
IN
2.2µF×4
V
DD
10µF
100nH
INPUT FILTER
V
IN
ISL8216M
