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参数资料
| 型号： | IRDC3629A |
| 厂商： | International Rectifier |
| 文件页数： | 10/23页 |
| 文件大小： | 0K |
| 描述： | BOARD EVAL SYNC BUCK CONTROLLER |
| 标准包装： | 1 |
| 系列： | * |
IR3629/IR3629A MPbF
Over-Current Protection
28uA
The over current protection is performed by
sensing current through the R DS(on) of the low-
side MOSFET. This method enhances the
20uA
OCP
converter’s efficiency and reduces cost by
eliminating a current sense resistor. As shown in
figure 7, an external resistor (R SET ) is connected
between OCSet pin and the drain of the low-side
SS1 / SD
20
3uA
MOSFET (Q2) which determines the current limit
set point.
Fig. 8: 3uA current source for discharging
The internal current source develops a voltage
across R SET . When the low-side MOSFET is
turned on, the inductor current flows through the
Q2 and results in a voltage which is given by:
V OCSet = (I OCSet ? R OCSet ) ? (R DS(on) ? I L ) --( 2 )
soft-start capacitor during hiccup
The OCP circuit starts sampling current when the
low gate drive is about 3V. The OCSet pin is
internally clamped during deadtime to prevent
false trigging. Figure 9 shows the OCSet pin
during one switching cycle. As shown, there is
IR3624
IR3629/29A
I OCSET
OCSet R SET
Q1
L1
V OUT
about 150ns delay to mask the deadtime. Since
this node contains switching noises, this delay
also functions as a filter.
Hiccup
Control
Q2
Fig. 7: Connection of over current sensing resistor
The critical inductor current can be calculated by
Deadtime
setting:
V OCSet = (I OCSet ? R OCSet ) ? (R DS(on) ? I L ) = 0
R OCSet ? I OCSet
I SET = I L ( critical ) =
--( 3 )
R DS ( on )
I OCSet *R OCSet
Blanking time
Clamp voltage
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 a certain
slope rate. As shown in figure 8, a 3uA current
source is used to discharge the soft-start
capacitor.
The OCP comparator resets after every soft start
cycle. The converter stays in this mode until the
overload or short circuit is removed. The
converter will automatically recover.
11/29/2007
Fig. 9: OCset pin during normal condition
Ch1: Inductor point, Ch2:Ldrv, Ch3:OCSet
The value of R SET should be checked in an actual
circuit to ensure that the over-current protection
circuit activates as expected. The IR3629 current
limit is designed primarily as a disaster
preventing, "no blow up" circuit, and does not
operate as a precision current regulator.
10
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