RT9218GS datasheet(12/18 Pages) RICHTEK

12 / 18 page

RT9218

DS9218-08 March 2007

www.richtek.com

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 40

μA current source, and the lower MOSFET on resistance, RDS(ON),

set the converter over-current trip point (IOCSET) according to the following equation :

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 = 15k

Ω (over -current trip point = 20.6A), Low-side

MOSFET is IR3707.

OCP

Time (5

μs/Div)

UGATE

(10V/Div)

IL (10A/Div)

OPS (200mV/Div)

VIN = 5V, VCC = 12V

VOUT = 1.5V

OCP

Time (2.5

μs/Div)

UGATE

(10V/Div)

IL (10A/Div)

VIN = 12V, VCC = 12V

VOUT = 1.5V

OPS

(200mV/Div)

OCP

Time (2.5

μs/Div)

UGATE (10V/Div)

IL (10A/Div)

VIN = 12V, VCC = 12V

VOUT = 1.5V

OCP

Time (5

μs/Div)

UGATE (10V/Div)

IL (10A/Div)

VIN = 5V, VCC = 12V

VOUT = 1.5V

MOSFET

lower

the

0.4V

40uA

DS(ON)

OCSET

−

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