ISL6526 Intersil Corporation, ISL6526 Datasheet - Page 7

• Manufacturer Part Number: ISL6526
• Description: Single Synchronous Buck Pulse-Width Modulation (PWM) Controller
• Manufacturer: Intersil Corporation

If the output voltage desired is 0.8V, simply route the output
back to the FB pin through R1, but do not populate R4.
Overcurrent Protection
The overcurrent function protects the converter from a shorted
output by using the upper MOSFET on-resistance, r
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
programs the overcurrent trip level (see Typical Application
diagrams on pages 2 and 3). An internal 20µA (typical) current
sink develops a voltage across R
V
referenced to V
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 over current still been present after the
delay interval, the over current condition would be sensed
and the regulator would be shut down again for another
delay interval of three soft-start cycles. The resulting hiccup
IN
. When the voltage across the upper MOSFET (also
+3.3V
ISL6526
FIGURE 2. OUTPUT VOLTAGE SELECTION
VCC
IN
) exceeds the voltage across R
CPVOUT
BOOT
UGATE
PHASE
LGATE
FB
COMP
C2
D1
C4
C1
7
VIN
R2
OCSET
Q2
Q1
L
OUT
C
R4
OUT
that is referenced to
R1
C3
+
OCSET
R3
DS(ON)
V
OCSET
OUT
, the
, to
)
ISL6526
mode style of protection would continue to repeat
indefinitely.
The overcurrent function will trip at a peak inductor current
(I
where I
typical). The OC trip point varies mainly due to the MOSFET
r
normal operating load range, find the R
the equation above with:
For an equation for the ripple current see the section under
component guidelines titled ‘Output Inductor Selection’.
A small ceramic capacitor should be placed in parallel with
R
presence of switching noise on the input voltage.
Current Sinking
The ISL6526 incorporates 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 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
I
DS(ON)
1. The maximum r
2. The minimum I
3. Determine I
PEAK)
PEAK
OCSET
0V
temperature.
where ∆I is the output inductor ripple current.
FIGURE 3. OVER CURRENT PROTECTION RESPONSE
=
OCSET
determined by:
variations. To avoid overcurrent tripping in the
T0
to smooth the voltage across R
I
---------------------------------------------------- -
OCSET
V
INTERNAL SOFT-START FUNCTION
OUT
r
is the internal OCSET current source (20µA
DS ON
PEAK
x R
(
(2.5V)
OCSET
DS(ON)
Delay Interval
OCSET
for
)
I
from the specification table.
PEAK
at the highest junction
TIME
>
I
OUT MAX
(
OCSET
OCSET
T1
)
+
(
--------- -
resistor from
∆I
2
in the
)
T2
,