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ISL6527, ISL6527A
Output Voltage Selection
The output voltage can be programmed to any level between
V IN and the supplied external reference. 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 2. However, since the value of R 1
affects the values of the rest of the compensation
components, it is advisable to keep its value less than 5k Ω . R 4
is quickly shutdown and the internal soft-start function begins
producing soft-start ramps. The delay interval seen by the
output is equivalent to three soft-start cycles. The fourth
internal soft-start cycle initiates a normal soft-start ramp of the
output, at time t1. The output is brought back into regulation
by time t2, as long as the overcurrent event has cleared.
R 4 = ----------------------------------------
can be calculated based on Equation 3:
R 1 × V REF
V OUT1 – V REF
(EQ. 3)
VOUT (2.5V)
If the output voltage desired is V REF , simply route the output
back to the FB pin through R 1 , but do not populate R 4 .
+3.3V
0V
VCC
CPVOUT
VIN
INTERNAL SOFT-START FUNCTION
D1
BOOT
ISL6527,
UGATE
PHASE
C4
Q1
LOUT
VOUT
DELAY INTERVAL
ISL6527A
LGATE
Q2
+
COUT
FB
t0
TIME
t1
t2
COMP
C2
C1
R2
R1
C3
R4
R3
FIGURE 3. OVERCURRENT PROTECTION RESPONSE
Had the cause of the overcurrent still been present after the
delay interval, the overcurrent condition would be sensed
and the regulator would be shut down again for another
REF_IN
VREF
delay interval of three soft-start cycles. The resulting hiccup
mode style of protection would continue to repeat
indefinitely.
FIGURE 2. OUTPUT VOLTAGE SELECTION
The overcurrent function will trip at a peak inductor current
Overcurrent Protection
(I PEAK) determined by Equation 4:
I PEAK = -----------------------------------------------------
The overcurrent function protects the converter from a shorted
output by using the upper MOSFET ON-resistance, r DS(ON) ,
to monitor the current. This method enhances the converter’s
efficiency and reduces cost by eliminating a current sensing
I OCSET x R OCSET
r DS ( ON )
(EQ. 4)
3. Determine I PEAK for I PEAK > I OUT ( MAX ) + ---------- ,
resistor.
The overcurrent function cycles the soft-start function in a
hiccup mode to provide fault protection. A resistor (R OCSET )
programs the overcurrent trip level (see Typical Application
diagrams on page 3 and page 4). An internal 20μA (typical)
current sink develops a voltage across R OCSET that is
referenced to V IN . When the voltage across the upper
MOSFET (also referenced to V IN ) exceeds the voltage across
R OCSET , the overcurrent function initiates a soft-start
sequence.
Figure 3 illustrates the protection feature responding to an
overcurrent event. At time t0, an overcurrent condition is
sensed across the upper MOSFET. As a result, the regulator
8
where I OCSET is the internal OCSET current source (20μA
typical). The OC trip point varies mainly due to the MOSFET
r DS(ON) variations. To avoid overcurrent tripping in the normal
operating load range, find the R OCSET resistor from the
Equation 4 with:
1. The maximum r DS(ON) at the highest junction
temperature.
2. The minimum I OCSET from the “Specification Table” on
( Δ I )
2
where Δ I is the output inductor ripple current.
FN9056.10
November 18, 2008
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