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參數(shù)資料
| 型號(hào)： | ISL6522ACRZ |
| 廠商： | INTERSIL CORP |
| 元件分類(lèi)： | 穩(wěn)壓器 |
| 英文描述： | Adjustable Precision Shunt Regulator 5-SOT-23 -40 to 85 |
| 中文描述： | SWITCHING CONTROLLER, 1000 kHz SWITCHING FREQ-MAX, PQCC16 |
| 封裝： | 5 X 5 MM, ROHS COMPLIANT, PLASTIC, MO-220VHHB, QFN-16 |
| 文件頁(yè)數(shù)： | 7/13頁(yè) |
| 文件大?。?/td> | 441K |
| 代理商： | ISL6522ACRZ |
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7
FN9122.2
April 13, 2005
is reference 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. The soft-start function discharges C
SS
with a
10
μ
A current sink and inhibits PWM operation. The soft-start
function recharges C
SS
, and PWM operation resumes with
the error amplifier clamped to the SS voltage. Should an
overload occur while recharging C
SS
, the soft-start function
inhibits PWM operation while fully charging C
SS
to 4V to
complete its cycle. Figure 4 shows this operation with an
overload condition. Note that the inductor current increases
to over 15A during the C
SS
charging interval and causes an
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
PEAK)
determined by:
where I
OCSET
is the internal OCSET current source (200
μ
A
is typical). The OC trip point varies mainly due to the
MOSFETs r
DS(ON)
variations. To avoid overcurrent tripping
in the normal operating load range, find the R
OCSET
resistor
from the equation above with:
The maximum r
DS(ON)
at the highest junction temperature.
1. The minimum I
OCSET
from the specification table.
2. Determine
where
I is the output inductor ripple current.
,
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
OCSET
to smooth the voltage across R
OCSET
in the
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
IN
rail, the voltage
that is being down-converted. If there is nowhere for this current
to go, such as to other distributed loads on the V
IN
rail, through
a voltage limiting protection device, or other methods, the
capacitance on the V
IN
bus will absorb the current. This
situation will cause the voltage level of the V
IN
rail to increase. If
the voltage level of the rail is boosted to a level that exceeds the
maximum voltage rating of the MOSFETs or the input
capacitors, damage may occur to these parts. If the bias
voltage for the ISL6522A comes from the V
IN
rail, then the
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
IN
and C
O
each represent numerous physical capacitors.
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
SS
close to the SS pin because the internal current source is
only 10
μ
A. Provide local V
CC
decoupling between VCC and
GND pins. Locate the capacitor, C
BOOT
as close as practical
to the BOOT and PHASE pins.
I
PEAK
I
---------------------------------------------------
R
r
DS ON
)
=
I
PEAK
for I
PEAK
I
OUT MAX
)
I
(
)
2
+
>
PGND
L
O
C
O
LGATE
UGATE
PHASE
Q1
Q2
D2
FIGURE 5. PRINTED CIRCUIT BOARD POWER AND
GROUND PLANES OR ISLANDS
V
IN
V
OUT
RETURN
ISL6522A
C
IN
L
ISL6522A
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