参数资料
型号: ISL6522CV
厂商: Intersil
文件页数: 7/15页
文件大小: 0K
描述: IC REG CTRLR BST PWM VM 14-TSSOP
标准包装: 96
PWM 型: 电压模式
输出数: 1
频率 - 最大: 1MHz
占空比: 100%
电源电压: 10.8 V ~ 13.2 V
降压:
升压:
回扫:
反相:
倍增器:
除法器:
Cuk:
隔离:
工作温度: 0°C ~ 70°C
封装/外壳: 14-TSSOP(0.173",4.40mm 宽)
包装: 管件
ISL6522
VOLTAGE
V OSC(MIN)
V SOFT START
V OUT
V COMP
CLAMP ON V COMP RELEASED AT
STEADY STATE
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
t 0
t 1
t 2
TIME
with this method. The measured input power for the
conditions of Figure 4 is 2.5W.
t 1 = ----------- ? V OSC ( MIN )
C SS V OUT SteadyState
t SoftStart = t 2 – t 1 = ----------- ? ------------------------------------------------ ? ? V OSC
I OCSET ? R OCSET
I PEAK = ---------------------------------------------------
C SS
I SS
I SS V IN
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
FIGURE 3. SOFT-START INTERVAL
4V
2V
0V
15A
10A
5A
0A
TIME (20ms/DIV)
FIGURE 4. OVERCURRENT OPERATION
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
7
The overcurrent function will trip at a peak inductor current
(I PEAK) determined by:
r DS ( ON )
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 I PEAK for I PEAK > I OUT ( MAX ) + ( ? I ) ? 2 ,
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
FN9030.8
March 10, 2006
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