RT9214
OPS (Over Current Setting, VIN_POR and Shutdown)
1.OCP
Sense the low-side MOSFET’ s RDS(ON) to set over-current trip point.
Connecting a resistor (ROCSET) from this pin to the source of the upper MOSFET and the drain of the lower MOSFET
sets the over-current trip point. ROCSET, an internal 40μAcurrent source, and the lower MOSFET on resistance, RDS(ON)
,
set the converter over-current trip point (IOCSET) according to the following equation :
40uA × ROCSET − 0.4V
IOCSET =
RDS(ON) of the lower MOSFET
OPS pin function is similar to RC charging or discharging circuit, so the over-current trip point is very sensitive to
parasitic capacitance (ex. shut-down MOSFET) and the duty ratio.
Below Figures say those effect. And test conditions are Rocset = 15kΩ (over -current trip point = 20.6A), Low-side
MOSFET is IR3707.
OCP
OCP
UGATE
(10V/Div)
UGATE (10V/Div)
I
(10A/Div)
L
I
(10A/Div)
L
OPS (200mV/Div)
= 5V, VCC = 12V
V
V
= 5V, VCC = 12V
= 1.5V
V
V
IN
OUT
IN
OUT
= 1.5V
Time (5μs/Div)
Time (5μs/Div)
OCP
OCP
OPS
(200mV/Div)
UGATE (10V/Div)
UGATE
(10V/Div)
I
(10A/Div)
L
I
(10A/Div)
L
V
= 12V, VCC = 12V
= 1.5V
IN
V
V
= 12V, VCC = 12V
IN
V
OUT
= 1.5V
OUT
Time (2.5μs/Div)
Time (2.5μs/Div)
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11
DS9214-13 September 2007
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
