ISL6529 Intersil Corporation, ISL6529 Datasheet - Page 7

• Manufacturer Part Number: ISL6529
• Description: Dual Regulator.Synchronous Rectified Buck PWM and Linear Power Controller
• Manufacturer: Intersil Corporation

Undervoltage Protection
The FB and FB2 pins are monitored during converter
operation by two separate undervoltage (UV) comparators. If
the FB voltage drops below 51.5% of the reference voltage
(0.41V), a fault signal is generated. The internal fault logic
shuts down both regulators simultaneously when the fault
signal triggers a restart.
Figure 3 illustrates the protection feature responding to an UV
event on V
51.5% 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 875ms 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
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 4.
0V
0V
FIGURE 3. UNDERVOLTAGE PROTECTION RESPONSE
(0.5V/DIV)
t0
OUT1
INTERNAL SOFT-START FUNCTION
. At time t0, VOUT1 has dropped below
V
OUT2
DELAY INTERVAL
V
DELAY INTERVAL
t1
OUT1
IN
(2.5V)
(1.5V)
(i.e. +3.3V) and the internal
V
TIME
OUT2
7
t2
(2.5V)
t3
t4
ISL6529
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 value chosen for R1,
R4 can be calculated based on the following equation:
If the output voltage desired is 0.8V, simply route VOUT1
back to the FB pin through R1, but do not populate R4.
The linear regulator output voltage is also set by means of
an external resistor divider as shown in Figure 5. The two
resistors used to set the output voltage should not exceed a
parallel equivalent value, referred to as R
restriction is due to the manner of implementation of the soft-
start function. The following relationship must be met:
R4
R
+3.3V
V
C
FB
FIGURE 5. OUTPUT VOLTAGE SELECTION OF THE LINEAR
OUT2
V
FIGURE 4. OUTPUT VOLTAGE SELECTION OF THE PWM
C
OUT2
V
=
OUT2
OUT1
OUT1
IN
=
------------------------------------- -
V
+
R5
--------------------- -
R5
OUT1
R1
+
=
R3
×
+
×
R6
R6
0.8
0.8V
–
0.8V
<
×
5kΩ


L
Q3
C3
1
R1
OUT
+
R4
R5
------- -
R6


R5
R2
R12
Q1
Q2
+3.3V
C2
R6
C1
C4
DRIVE2
UGATE
LGATE
FB2
COMP
FB
FB
, of 5kΩ. This
+12V
12VCC
ISL6529
ISL6529
5VCC
+5V
(EQ. 1)
(EQ. 2)