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| 型号： | IR3651STRPBF |
| 厂商： | International Rectifier |
| 文件页数： | 11/19页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM VM 14-SOIC |
| 标准包装： | 2,500 |
| PWM 型： | 电压模式 |
| 输出数： | 1 |
| 频率 - 最大： | 460kHz |
| 占空比： | 80% |
| 电源电压： | 4.5 V ~ 13.2 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： | 0°C ~ 125°C |
| 封装/外壳： | 14-SOIC（0.154"，3.90mm 宽） |
| 包装： | 带卷 (TR) |
| 配用： | IRDC3651-ND - KIT REF DESIGN SYNCH BUCK REG |
IR3651SPBF
Over-Current Protection
The over current protection is performed by
sensing current through the R DS(on) of low side
MOSFET. This method enhances the converter’s
10uA
Hiccup
efficiency and reduce cost by eliminating a
current sense resistor. As shown in figure 6, an
external resistor (R SET ) is connected between
OCSet pin and the drain of low side MOSFET
(Q2) which sets the current limit set point.
The internal current source develops a voltage
SS / SD
3
3uA
across R SET . When the low side MOSFET is
turned on, the inductor current flows through the
Fig. 7: 3uA current source for discharging
soft-start capacitor during hiccup
Q2 and results a voltage which is given by:
The OCP circuit starts sampling current
V OCSet = (I OCSet ? R OCSet ) ? (R DS(on) ? I L )
--( 2 )
approximately 200ns before the low gate drive
turns off. The OCSet pin is internally clamped to
about 1.5V during deadtime to prevent false
IR3651
I OCSET
OCSet R SET
Q1
L1
V OUT
trigging, figure 8 shows the OCSet pin during one
switching cycle.
Q2
Hiccup
Control
Fig. 6: Connection of over current sensing resistor
The critical inductor current can be calculated by
setting:
V OCSet = (I OCSet ? R OCSet ) ? (R DS(on) ? I L ) = 0
I SET = I L ( critical ) =
R OCSet ? I OCSet
R DS ( on )
--( 3 )
I SET = I L ( critical ) = 1 . 5 * I o +
Δ i L
2
Fig. 8: OCset pin during normal condition
Ch1: Inductor point, Ch2:Ldrv, Ch3:OCSet
An over current is detected if the OCSet pin goes
below ground. This trips the OCP comparator
and cycles the soft start function in hiccup mode.
The hiccup is performed by charging and
discharging the soft-start capacitor in certain
slope rate. As shown in figure 7 a 3uA current
source is used to discharge the soft-start
capacitor.
The OCP comparator resets after every soft start
cycles, the converter stays in this mode until the
overload or short circuit is removed. The
converter will automatically recover.
06/08/2011
The value of R SET should be checked in an actual
circuit to ensure that the over current protection
circuit activates as expected. The IR3651 current
limit is designed primarily as disaster preventing,
"no blow up" circuit, and doesn't operate as a
precision current regulator.
11
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