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參數(shù)資料
| 型號(hào)： | ISL6522CR-T |
| 廠商： | INTERSIL CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | Adjustable Precision Shunt Regulator 3-SOT-89 -40 to 125 |
| 中文描述： | SWITCHING CONTROLLER, 1000 kHz SWITCHING FREQ-MAX, PQCC16 |
| 封裝： | 5 X 5 MM, PLASTIC, MO-220-VHHB, QFN-16 |
| 文件頁數(shù)： | 6/14頁 |
| 文件大小： | 473K |
| 代理商： | ISL6522CR-T |
6
FN9030.7
March 4, 2005
Overcurrent Protection
The overcurrent function protects the converter from a
shorted output by using the upper MOSFETs on-resistance,
r
DS(ON)
to 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
OCSET
)
programs the overcurrent trip level. An internal 200
μ
A
(typical) current sink develops a voltage across R
OCSET
that
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 ISL6522 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 ISL6522 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 ISL6522 comes from the V
IN
rail, then the
FIGURE 3. SOFT-START INTERVAL
VOLTAGE
TIME
V
OSC(MIN)
t
0
t
1
t
2
CLAMP ON V
COMP
RELEASED AT
STEADY STATE
V
COMP
V
SOFT START
V
OUT
t
1
C
SS
-----------
V
OSC MIN
)
=
t
SoftStart
t
2
t
1
–
C
SS
-----------
V
IN
------------------------------------------------
V
OSC
=
=
Where:
C
SS
= Soft Start Capacitor
I
SS
= Soft Start Current = 10
μ
A
V
OSC(MIN)
= Bottom of Oscillator = 1.35V
V
IN
= Input Voltage
V
OSC
= Peak to Peak Oscillator Voltage = 1.9V
V
OUTSteadyState
= Steady State Output Voltage
O
S
0A
0V
TIME (20ms/DIV)
5A
10A
15A
2V
4V
FIGURE 4. OVERCURRENT OPERATION
I
PEAK
I
---------------------------------------------------
R
r
DS ON
)
=
I
PEAK
for I
PEAK
I
OUT MAX
)
I
(
)
2
+
>
ISL6522
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