ISL6527 Datasheet, PDF (8/16 Pages) Intersil Corporation – Single Synchronous Buck Pulse-Width Modulation PWM Controller

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 R₁
affects the values of the rest of the compensation
components, it is advisable to keep its value less than 5kΩ. R₄
can be calculated based on Equation 3:
R₄
=
------^R-----¹----^×-----^V----^R-----^E---^F--------
V_OUT1 – V_REF
(EQ. 3)
If the output voltage desired is V_REF, simply route the output
back to the FB pin through R₁, but do not populate R₄.
+3.3V
VCC
VIN
CPVOUT
BOOT
ISL6527,
ISL6527A
UGATE
PHASE
LGATE
D1
C4
Q1
LOUT
Q2
+
COUT
FB
COMP
C1
R1
C3
C2 R2
R4
VOUT
R3
REF_IN
VREF
FIGURE 2. OUTPUT VOLTAGE SELECTION
Overcurrent Protection
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
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
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.
VOUT (2.5V)
0V
INTERNAL SOFT-START FUNCTION
DELAY INTERVAL
t0
t1
t2
TIME
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
delay interval of three soft-start cycles. The resulting hiccup
mode style of protection would continue to repeat
indefinitely.
The overcurrent function will trip at a peak inductor current
(I_PEAK) determined by Equation 4:
I_PEAK
=
I_{--O-----C----S----E----T-----}x_-----R_{-----O----C-----S---E----T--}
r_DS(ON)
(EQ. 4)
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
page 7.
3. Determine I_PEAK for
I_PEAK
>
I_OUT(MAX)
+
(_---Δ_----I_---)
2
,
where ΔI is the output inductor ripple current.
8
FN9056.10
November 18, 2008