isl6520acrz-t Intersil Corporation, isl6520acrz-t Datasheet - Page 6

• Manufacturer Part Number: isl6520acrz-t
• Description: Single Synchronous Buck Pulse-width Modulation Pwm Controller Isl6520a
• Manufacturer: Intersil Corporation

The overcurrent function will trip at a peak inductor current
(I
where I
typical). The OC trip point varies mainly due to the
MOSFET’s r
in the normal operating load range, find the R
from the equation above with:
For an equation for the ripple current see the section under
component guidelines titled ‘Output Inductor Selection’.
Soft-Start
The POR function initiates the soft-start sequence after the
overcurrent set point has been sampled. Soft-start clamps the
error amplifier output (COMP pin) and reference input (non-
inverting terminal of the error amp) to the internally generated
soft-start voltage. Figure 2 shows a typical start up interval
where the COMP/OCSET pin has been released from a
grounded (system shutdown) state. Initially, the COMP/OCSET
is used to sample the overcurrent setpoint by disabling the error
amplifier and drawing 20µA through R
overcurrent level has been sampled, the soft-start function is
initiated. The clamp on the error amplifier (COMP/OCSET pin)
initially controls the converter’s output voltage during soft-start.
The oscillator’s triangular waveform is compared to the ramping
error amplifier voltage. This generates PHASE pulses of
increasing width that charge the output capacitor(s). When the
internally generated soft-start voltage exceeds the feedback
(FB pin) voltage, the output voltage is in regulation. This
method provides a rapid and controlled output voltage rise. The
entire startup sequence typically take about 11ms.
I
PEAK
1. The maximum r
2. The minimum I
3. Determine I
PEAK)
temperature.
where ΔI is the output inductor ripple current.
=
OCSET
determined by:
I
---------------------------------------------------- -
OCSET
FIGURE 1. OVERCURRENT OPERATION
DS(ON)
r
is the internal OCSET current source (20μA
DS ON
PEAK
x R
(
OUTPUT INDUCTOR
OCSET
DS(ON)
variations. To avoid overcurrent tripping
OCSET
)
for
CURRENT
5A/DIV.
TIME (5ms/DIV.)
from the specification table.
at the highest junction
I
PEAK
6
>
I
OUT MAX
OCSET
(
. Once the
OCSET
)
+
(
--------- -
ΔI
2
)
resistor
,
(EQ. 3)
ISL6520A
Current Sinking
The ISL6520A 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 ISL6520A 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 it’s input voltage. This
means that the converter is boosting current into the V
rail, which supplies the bias voltage to the ISL6520A. If there
is nowhere for this current to go, such as to other distributed
loads on the V
device, or other methods, the capacitance on the V
will absorb the current. This situation will allow voltage level
of the V
boosted to a level that exceeds the maximum voltage rating
of the ISL6520A, then the IC will experience an irreversible
failure and the converter will no longer be operational.
Ensuring that there is a path for the current to follow other
than the capacitance on the rail will prevent this failure
mode.
Application Guidelines
Layout Considerations
As in any high frequency switching converter, layout is very
important. Switching current from one power device to another
can generate voltage transients across the impedances of the
interconnecting bond wires and circuit traces. These
interconnecting impedances should be minimized by using
wide, short printed circuit traces. The critical components
should be located as close together as possible, using ground
plane construction or single point grounding.
CC
COMP/OCSET
rail to increase. If the voltage level of the rail is
1V/DIV.
FIGURE 2. SOFT-START INTERVAL
CC
rail, through a voltage limiting protection
TIME (2ms/DIV.)
500mV/DIV.
V
OUT
March 28, 2007
CC
FN9016.5
CC
bus