ISL6534 Datasheet, PDF (14/26 Pages) Intersil Corporation – Dual PWM with Linear

ISL6534
Good”. After all 3 Soft-Start pins complete their ramp up with
no faults (no short detected on either switcher), the power is
considered “Good”, and the output pin goes high-impedance
(to be pulled up to a logic high level with an external pull-up
resistor). Figure 10 shows a DDR example, with a fast SS1,
a slower SS3, VOUT3 and the PGOOD output. The PGOOD
waits for the last of the SS signals (SS3 here) to reach their
ramp-done trip point before it goes high.
(DDR MODE; SS2/EN2 = 5V)
4: PGOOD
1: SS1/EN1
2: SS3/EN3
3: VOUT3
FIGURE 10. SS1, SS3, VOUT3, PGOOD
Note that if any of the SS/EN pins is held low, PGOOD will
not go high; thus, if one of the three outputs is not used, and
the PGOOD function is desired, then the SS/EN should be
allowed to charge high, and the other pins of the unused
regulator should be tied so as not to cause a fault or
shutdown. Options for OUT1 include: tying FB1 to COMP1,
or tying FB1 to VCC, and leaving COMP1 open. VOUT2 is a
little more difficult; Tie REFIN, FB2, COMP2 to GND; or tie
FB2 to COMP2, and tie REFIN to a voltage well under 3V (to
avoid the short-circuit shutdown). In all of these cases, leave
the LGATE and UGATE pins open; tie BOOT pin to VCC12.
Once the power is “Good”, PGOOD will pull low if any of the
3 SS/EN pins is pulled low. Also, if a short is detected on
either switcher, then the PGOOD will pull low, for as long as
the condition is there. Note that if OUT1 or OUT2 has a short
detected which stays there for 1-2 clock pulses, all three
regulators will shut down, and wait for a power-down and up
cycle to reset (either VCC or VCC12 (or both) must power
down and up). If the short-circuit is not there long enough to
shut down, it may still cause PGOOD to go low momentarily.
If this causes a system issue, a filter capacitor could be tried;
it should be at least several nF to be effective.
Note that this is not a full-feature PGOOD; it is not directly
monitoring if the VOUT1 or VOUT2 drops below a set UV
level; it only checks for the simple short-circuit condition, via
the COMP pins. And it is not monitoring VOUT3 at all. So it is
a good indication that all three outputs have ramped up, but
it is less useful as a monitor from that point on.
Since PGOOD is an open-drain pull-down device, it usually
requires an external pull-up resistor; however, if the pin is not
used, no resistor is necessary. A value in the range of 1kΩ to
10kΩ is typical.
POR
Both the VCC (5V) and VCC12 (12V) are monitored for
Power-On-Reset, as shown in the Specification Table. The
two POR outputs are logically gated together, such that both
have to be above their rising trip points to enable the SS/EN
ramps to start (if they are not held low) and then enable each
output. Either POR output can go below its falling trip point to
disable all outputs, and then back to restart the enable
operation.
Shunt Regulator
The ISL6534 must have both a 12V (for VCC12) and a 5V
power supply (for VCC); both must be above their respective
POR rising trip points to enable the outputs to start
switching. The shunt regulator (nominal 5.8V) was designed
for those systems that do not have a 5V supply available; the
range of the shunt (5.6V to 6.0V) was designed not to
overlap the usual 4.5V to 5.5V range of typical power
supplies. An external resistor between VCC12 and VCC is
required; a typical value of 150Ω is the recommended
starting value (it may change due to other factors, such as
VCC12 voltage, VBS voltages, oscillator frequency, etc.).
Note that the dissipation of the resistor is approximately
1/4W; it needs to be sized accordingly. For example,
12V - 5.8V = 6.2V across the 150Ω resistor is 41mA;
P = IV = 0.256W. Several low-power resistors in parallel can
also be used. See Figure 11.
Note that in either case, both VCC and VCC12 pins have
small decoupling capacitors (typically 1.0 to 10.0µF); they
should each be located near their pin, with a via to the GND
plane.
(VCC) = 5.8V
VCC PIN
VCC
VCC12
OPTIONAL R FOR
SHUNT REGULATOR;
VCC = 5.8V
VCC12 PIN
VCC12
NON-SHUNT
MODE; SEPARATE
5V AND 12V
VCC PIN
VCC12 PIN
FIGURE 11. SHUNT REGULATOR AND DECOUPLING
CAPACITORS FOR VCC AND VCC12 PINS
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FN9134.1