参数资料
型号: IR3628MTRPBF
厂商: International Rectifier
英文描述: HIGH FREQUENCY SYNCHRONOUS PWM BUCK CONTROLLER
中文描述: 高频同步PWM降压控制器
文件页数: 2/22页
文件大小: 581K
代理商: IR3628MTRPBF
IR3628MPbF
08/10/2007
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
efficiency and reduce 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 low side MOSFET
(Q2) which sets the current limit set point.
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 a voltage which is given by:
The OCP circuit starts sampling current when the
low gate drive is about 3V. The OCSet pin is
internally clamped (~1.5V) during on time of high
side MOSFET including deadtime to prevent
false trigging, figure 9 shows the OCSet pin
during one switching cycle. As it is shown there
is about 150ns delay to mask the deadtime, since
this node contains switching noises, this delay
also functions as a filter.
Fig. 7: Connection of over current sensing resistor
Fig. 8: 3uA current source for discharging
soft-start capacitor during hiccup
L1
RSET
IR3624
OCSet
IOCSET
VOUT
Hiccup
Control
Q1
Q2
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 8 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.
SS1 / SD 20
28uA
3uA
OCP
)
2
)
--(
I
(R
)
R
(I
V
L
DS(on)
OCSet
=
0
)
I
(R
)
R
(I
V
L
DS(on)
OCSet
=
=
)
3
--(
R
I
R
I
on
DS
OCSet
critical
L
SET
)
(
)
(
=
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 IR3628 current
limit is designed primarily as disaster preventing,
and doesn't operate as a precision current
regulator.
The critical inductor current can be calculated by
setting:
20uA
IR3628
I
OCSet*ROCSet
Blanking time
Deadtime
Clamp voltage
10
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