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參數(shù)資料
| 型號： | ISL6520A |
| 廠商： | Intersil Corporation |
| 英文描述： | Adjustable Precision Shunt Regulator 3-SOT-23 -40 to 125 |
| 中文描述： | 單同步降壓脈寬調制（PWM）控制器 |
| 文件頁數(shù)： | 5/11頁 |
| 文件大?。?/td> | 380K |
| 代理商： | ISL6520A |
5
FN9016.3
December 28, 2004
The overcurrent function will trip at a peak inductor current
(I
PEAK)
determined by:
where I
OCSET
is the internal OCSET current source (20
μ
A
typical). The OC trip point varies mainly due to the
MOSFET’s r
DS(ON)
variations. To avoid overcurrent tripping
in the normal operating load range, find the R
OCSET
resistor
from the equation above with:
1. The maximum r
DS(ON)
at the highest junction
temperature.
2. The minimum I
OCSET
from the specification table.
3. Determine I
PEAK
for
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’.
,
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 20
μ
A through R
OCSET
. Once the
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.
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
CC
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
CC
rail, through a voltage limiting protection
device, or other methods, the capacitance on the V
CC
bus
will absorb the current. This situation will allow voltage level
of the V
CC
rail to increase. If the voltage level of the rail is
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.
FIGURE 1. OVERCURRENT OPERATION
TIME (5ms/DIV.)
OUTPUT INDUCTOR
CURRENT
5A/DIV.
I
PEAK
I
x R
DS ON
)
---------------r
=
I
PEAK
I
OUT MAX
)
I
2
(
----------
)
+
>
FIGURE 2. SOFT START INTERVAL
V
OUT
500mV/DIV.
COMP/OCSET
1V/DIV.
TIME (2ms/DIV.)
ISL6520A
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