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参数资料
| 型号： | ISL9443IRZ-T |
| 厂商： | Intersil |
| 文件页数： | 18/23页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM CM 32-QFN |
| 标准包装： | 6,000 |
| PWM 型： | 电流模式 |
| 输出数： | 3 |
| 频率 - 最大： | 1.32MHz |
| 电源电压： | 4.5 V ~ 26 V |
| 降压： | 是 |
| 升压： | 无 |
| 回扫： | 无 |
| 反相： | 无 |
| 倍增器： | 无 |
| 除法器： | 无 |
| Cuk： | 无 |
| 隔离： | 无 |
| 工作温度： | -40°C ~ 85°C |
| 封装/外壳： | * |
| 包装： | * |
ISL9443
R OCSET = ---------------------------------------
( I OC ) ( r DS ( ON ) )
is asserted. When UVLO is asserted, PGOOD is valid and will be
de-asserted.
Overcurrent Protection
All the PWM controllers use the lower MOSFET's on-resistance,
r DS(ON) , to monitor the current in the converter. The sensed
voltage drop is compared with a threshold set by a resistor
connected from the OCSETx pin to ground.
( 7 ) ( R CS )
(EQ. 7)
Where I OC is the desired overcurrent protection threshold, and
R CS is a value of the current sense resistor connected to the
ISENx pin.
If an overcurrent is detected for 2 consecutive clock cycles, the IC
enters a hiccup mode by turning off the gate drivers and entering
soft-start. The IC will cycle 5 times through soft-start before trying
Feedback Loop Compensation
To reduce the number of external components and to simplify the
process of determining compensation components, all PWM
controllers have internally compensated error amplifiers. To make
internal compensation possible, several design measures were
taken.
Firstly, the ramp signal applied to the PWM comparator is
proportional to the input voltage provided at the VIN pin. This
keeps the modulator gain constant with varying input voltages.
Secondly, the load current proportional signal is derived from the
voltage drop across the lower MOSFET during the PWM time
interval and is subtracted from the amplified error signal on the
comparator input. This creates an internal current control loop.
The resistor connected to the ISEN pin sets the gain in the current
feedback loop. The following expression estimates the required
value of the current sense resistor depending on the maximum
operating load current and the value of the MOSFET’s r DS(ON) .
( I MAX ) ( r DS ( ON ) )
to restart. The IC will continue to cycle through soft-start until the
overcurrent condition is removed. Hiccup mode is active during
soft-start so care must be taken to ensure that the peak inductor
30 μ A
R CS ≥ --------------------------------------------
(EQ. 8)
F PO = ------------------------------
2 π ? R O ? C O
F Z = ------------------------------ = 10kHz
2 π ? R 2 ? C 1
F P = ------------------------------ = 600kHz
2 π ? R 1 ? C 2
current does not exceed the overcurrent threshold during
soft-start.
Because of the nature of this current sensing technique, and to
accommodate a wide range of r DS(ON) variations, the value of the
overcurrent threshold should represent an overload current about
150% to 180% of the maximum operating current. If more
accurate current protection is desired, place a current sense
resistor in series with the lower MOSFET source.
When OCP is triggered the EN/SS1 or TK/SSx pins are pulled to
ground by internal MOSFET. For PWM rails configured to track
another voltage rail the TK/SSx pin rises up much faster than the
internal minimum soft-start ramp. The voltage reference will
then be clamped to the internal minimum soft-start ramp. Thus,
smooth soft-start hiccup is achieved even with the tracking
function.
Overvoltage Protection
All switching controllers within the ISL9443 have fixed
overvoltage set points. The overvoltage set point is set at 118%
of the nominal output voltage, the output voltage set by the
feedback resistors. In the case of an overvoltage event, the IC will
attempt to bring the output voltage back into regulation by
keeping the upper MOSFET turned off and modulating the lower
MOSFET for 2 consecutive PWM cycles. If the overvoltage
condition has not been corrected in 2 cycles and the output
voltage is above 118% of the nominal output voltage, the
ISL9443 will turn off both the upper MOSFET and the lower
MOSFET. The ISL9443 will enter hiccup mode until the output
voltage return to 110% of the nominal output voltage.
Over-Temperature Protection
The IC incorporates an over-temperature protection circuit that
shuts the IC down when a die temperature of +150°C is
reached. Normal operation resumes when the die temperatures
drops below +130°C through the initiation of a full soft-start
cycle. When all the three channels are disabled, thermal
protection is inactive. This helps achieve a very low shutdown
current of 33μA .
18
Choosing R CS to provide 30μA of current to the current sample
and hold circuitry is recommended but values down to 2μA and
up to 100μA can be used. A higher sampling current will help to
stabilize the loop.
Due to the current loop feedback, the modulator has a single
pole response with -20dB slope at a frequency determined by the
load.
1
(EQ. 9)
Where R O is load resistance and C O is load capacitance. For this
type of modulator, a Type 2 compensation circuit is usually
sufficient.
Figure 22 shows a Type 2 amplifier and its response along with
the responses of the current mode modulator and the converter.
The Type 2 amplifier, in addition to the pole at origin, has a
zero-pole pair that causes a flat gain region at frequencies
between the zero and the pole.
1
(EQ. 10)
1
(EQ. 11)
Zero frequency, amplifier high-frequency gain and modulator
gain are chosen to satisfy most typical applications. The
crossover frequency will appear at the point where the modulator
attenuation equals the amplifier high frequency gain. The only
task that the system designer has to complete is to specify the
output filter capacitors to position the load main pole
somewhere within one decade lower than the amplifier zero
frequency. With this type of compensation, plenty of phase
margin is easily achieved due to zero-pole pair phase ‘boost’.
FN7663.1
February 24, 2012
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