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RT9218B

12V Synchronous Buck PWM DC-DC and Linear Power Controller

Richtek Technology 

RT9218B

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 40μA current 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 =

R DS(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)

IL (10A/Div)

VIN = 5V, VCC = 12V

VOUT = 1.5V

Time (5μs/Div)

OCP

UGATE (10V/Div)

IL (10A/Div)

VIN = 12V, VCC = 12V

VOUT = 1.5V

Time (2.5μs/Div)

DS9218B-09 March 2007

UGATE

(10V/Div)

OPS (200mV/Div)

IL (10A/Div)

VIN = 5V, VCC = 12V

VOUT = 1.5V

Time (5μs/Div)

OCP

OPS

(200mV/Div)

UGATE

(10V/Div)

VIN = 12V, VCC = 12V

VOUT = 1.5V

Time (2.5μs/Div)

IL (10A/Div)

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