ISL6527 Intersil Corporation, ISL6527 Datasheet - Page 7

• Manufacturer Part Number: ISL6527
• Description: Single Synchronous Buck Pulse-Width Modulation (PWM) Controller
• Manufacturer: Intersil Corporation

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 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
IN
0V
. When the voltage across the upper MOSFET (also
FIGURE 3. OVERCURRENT PROTECTION RESPONSE
T0
VOUT (2.5V)
INTERNAL SOFT-START FUNCTION
IN
) exceeds the voltage across R
DELAY INTERVAL
TIME
7
OCSET
that is referenced to
T1
OCSET
DS(ON)
OCSET
T2
, the
, to
)
ISL6527
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
operating load range, find the R
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 ISL6527 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 ISL6527 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,
such as to other distributed loads on the rail or through a
voltage limiting protection device, the capacitance on this rail
will absorb the current. This situation will allow the voltage
level of the input rail to increase. If the voltage level of the rail
is boosted to a level that exceeds the maximum voltage
rating of any components attached to the input rail, then
those components may experience an irreversible failure or
experience stress that may shorten their lifespan. Ensuring
that there is a path for the current to flow other than the
capacitance on the rail will prevent this failure mode.
External Reference
The ISL6527 allows the designer to determine the reference
voltage that is used. This allows the ISL6527 to be used in
many specialized applications, such as the V
voltage in a DDR Memory power supply, which must track
the V
this voltage does not exceed 1.5V.
I
DS(ON)
1. The maximum r
2. The minimum I
3. Determine I
PEAK)
PEAK
OCSET
temperature.
where ∆I is the output inductor ripple current.
DDQ
OCSET
=
determined by:
variations. To avoid overcurrent tripping in the normal
to smooth the voltage across R
I
---------------------------------------------------- -
OCSET
voltage by 50%. Care must be taken to insure that
r
is the internal OCSET current source (20µA
DS ON
PEAK
x R
(
OCSET
DS(ON)
OCSET
for
)
I
PEAK
from the specification table.
at the highest junction
OCSET
>
I
OUT MAX
(
resistor from the
OCSET
)
+
TT
(
--------- -
∆I
2
in the
termination
)
,