ISL6522A Intersil Corporation, ISL6522A Datasheet - Page 6

• Manufacturer Part Number: ISL6522A
• Description: Buck and Synchronous Rectifier Pulse-Width Modulator (PWM) Controller
• Manufacturer: Intersil Corporation

is reference to V
MOSFET (also referenced to V
across R
sequence. The soft-start function discharges C
10µA current sink and inhibits PWM operation. The soft-start
function recharges C
the error amplifier clamped to the SS voltage. Should an
overload occur while recharging C
inhibits PWM operation while fully charging C
complete its cycle. Figure 4 shows this operation with an
overload condition. Note that the inductor current increases
to over 15A during the C
overcurrent trip. The converter dissipates very little power
with this method. The measured input power for the
conditions of Figure 4 is 2.5W.
The overcurrent function will trip at a peak inductor current
(I
where I
is typical). The OC trip point varies mainly due to the
MOSFETs r
in the normal operating load range, find the R
from the equation above with:
The maximum r
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 ISL6522A 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 ISL6522A 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 V
being down-converted. If there is nowhere for this current to go,
such as to other distributed loads on the V
voltage limiting protection device, or other methods, the
capacitance on the V
situation will cause the voltage level of the V
the voltage level of the rail is boosted to a level that exceeds the
maximum voltage rating of the MOSFETs or the input
I
1. The minimum I
2. Determine
PEAK
PEAK)
OCSET
where ∆I is the output inductor ripple current.
=
OCSET
determined by:
OCSET
to smooth the voltage across R
I
-------------------------------------------------- -
OCSET
DS(ON)
r
DS ON
is the internal OCSET current source (200µA
I
DS(ON)
, the overcurrent function initiates a soft-start
PEAK
IN
•
(
. When the voltage across the upper
R
OCSET
variations. To avoid overcurrent tripping
OCSET
IN
SS
)
for I
bus will absorb the current. This
at the highest junction temperature.
, and PWM operation resumes with
SS
PEAK
from the specification table.
charging interval and causes an
6
IN
>
I
) exceeds the voltage
OUT MAX
SS
IN
, the soft-start function
(
rail, the voltage that is
OCSET
IN
IN
)
rail, through a
+
rail to increase. If
OCSET
(
SS
∆I
SS
) 2 ⁄
in the
to 4V to
with a
,
resistor
ISL6522A
capacitors, damage may occur to these parts. If the bias
voltage for the ISL6522A comes from the V
maximum voltage rating of the ISL6522A may be exceeded and
the IC will experience a catastrophic 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 these failure modes.
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.
Figure 5 shows the critical power components of the
converter. To minimize the voltage overshoot the
interconnecting wires indicated by heavy lines should be part
of ground or power plane in a printed circuit board. The
components shown in Figure 6 should be located as close
together as possible. Please note that the capacitors C
and C
Locate the ISL6522A within three inches of the MOSFETs,
Q1 and Q2. The circuit traces for the MOSFETs’ gate and
source connections from the ISL6522A must be sized to
handle up to 1A peak current.
Figure 6 shows the circuit traces that require additional
layout consideration. Use single point and ground plane
construction for the circuits shown. Minimize any leakage
current paths on the SS PIN and locate the capacitor, C
close to the SS pin because the internal current source is
only 10µA. Provide local V
GND pins. Locate the capacitor, C
to the BOOT and PHASE pins.
FIGURE 5. PRINTED CIRCUIT BOARD POWER AND
ISL6522A
O
each represent numerous physical capacitors.
UGATE
PHASE
LGATE
PGND
GROUND PLANES OR ISLANDS
CC
V
Q2
Q1
RETURN
IN
decoupling between VCC and
D2
BOOT
C
as close as practical
IN
IN
L
rail, then the
O
C
O
V
OUT
IN
SS