ISL6528 Datasheet, PDF (6/13 Pages) Intersil Corporation – Dual Regulator - Standard Buck PWM and Linear Power Controller

ISL6528
+5V (VCC)
+3.3V (UPPER FET DRAIN)
0V
(1V/DIV)
V_OUT2 (2.5V)
V_OUT1 (1.5V)
0V
(0.5V/DIV)
T0 T1
T2
TIME
FIGURE 1. SOFT-START INTERVAL
shuts down both regulators simultaneously when the fault
signal triggers a restart.
Figure 2 illustrates the protection feature responding to an
UV event on V_OUT1. At time T0, VOUT1 has dropped below
52.5% of the nominal output voltage. Both outputs are
quickly shut down and the internal soft-start function begins
producing soft-start ramps. The delay interval, T0 to T3,
seen by the output is equivalent to three soft-start cycles.
After a short delay interval of 10.5ms, the fourth internal soft-
start cycle initiates a normal soft-start ramp of the output, at
time T3. Both outputs are brought back into regulation by
time T4, as long as the UV event has cleared.
Had the cause of the UV still been present after the delay
interval, the UV protection circuitry becomes active
approximately 875µs into the soft-start interval. A fault signal
could then be generated and the outputs once again
shutdown. The resulting hiccup mode style of protection
would continue to repeat indefinitely.
Output Voltage Selection
The output voltage of the PWM converter can be
programmed to any level between V_IN (i.e. +3.3V) and the
internal reference, 0.8V. An external resistor divider is used
to scale the output voltage relative to the reference voltage
and feed it back to the inverting input of the error amplifier,
see Figure 3. However, since the value of R1 affects the
values of the rest of the compensation components, it is
advisable to keep its value less than 5kΩ. Depending on the
6
V_OUT2 (2.5V)
V_OUT1 (1.5V)
0V
(0.5V/DIV)
Delay Interval
V_OUT2 (2.5V)
Internal Soft-Start Function
Delay Interval
0V
T0
T1
T2
T3
T4
TIME
FIGURE 2. UNDERVOLTAGE PROTECTION RESPONSE
value chosen for R1, R4 can be calculated based on the
following equation:
R4 = ------R-----1-----^×----0----.-8----V--------
V_OUT1 – 0.8V
(EQ. 1)
If the output voltage desired is 0.8V, simply route VOUT1
back to the FB pin through R1, but do not populate R4.
+5V
V_OUT1
C_OUT1 ⁺
D2
+3.3V
BOOT
VCC
C4
L_OUT
Q1
UGATE
ISL6528
D1
R1
R3
C3
FB
C2
COMP
R2 C1
R4
FIGURE 3. OUTPUT VOLTAGE SELECTION OF THE PWM
FN9038.3
December 28, 2004