ISL6526 Datasheet, PDF (8/15 Pages) Intersil Corporation – Single Synchronous Buck Pulse-Width Modulation PWM Controller

ISL6526, ISL6526A
+3.3V
VCC
ISL6526,
ISL6526A
VIN
CPVOUT
BOOT
UGATE
PHASE
LGATE
D1
C4
Q1
L_OUT
Q2
+
C_OUT
FB
COMP
C1
R1
C3
C2 R2
R4
V_OUT
R3
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 -
3.3V Input” on page 3 and “Typical Application - 5V Input” on
page 3). 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.
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.
8
V_OUT (2.5V)
0V
INTERNAL SOFT-START FUNCTION
DELAY INTERVAL
t0
t1
t2
TIME
FIGURE 3. OVERCURRENT PROTECTION RESPONSE
The overcurrent function will trip at a peak inductor current
(I_PEAK) determined by Equation 3:
I_PEAK
=
I_{--O-----C----S----E----T-----}x_-----R_{-----O----C-----S---E----T--}
r_DS(ON)
(EQ. 3)
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
Equation 3 with:
1. The maximum r_DS(ON) at the highest junction
temperature.
2. The minimum I_OCSET from the specification table.
3. Determine I_PEAK for
I_P
E
A
K
>
I_OUT(M
A
X)
+
(_---Δ_----I_---)
2
,
where ΔI is the output inductor ripple current.
For the ripple current, see Equation 11 in “Output Inductor
Selection” on page 11.
A small ceramic capacitor should be placed in parallel with
R_OCSET to smooth the voltage across R_OCSET in the
presence of switching noise on the input voltage.
Current Sinking
The ISL6526, ISL6526A incorporate a MOSFET shoot-through
protection method which allows a converter to sink current as
well as source current. Care should be exercised when
designing a converter with the ISL6526, ISL6526A when it is
known that the converter may sink current.
When the converter is sinking current, it is behaving as a
boost converter that is regulating its input voltage. This
means that the converter is boosting current into the input
rail of the regulator. If there is nowhere for this current to go,
FN9055.10
November 24, 2008