RT8101APSP datasheet(9/15 Pages) RICHTEK

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RT8101/A

Preliminary

DS8101/A-01 March 2007

www.richtek.com

Application Information

Power On Reset

The RT8101/A automatically initializes upon applying of

input power VCC. The power on reset function (POR)

continually monitors the input bias supply voltage at the

VCC pin. The POR trip level is typically 9.6V at VCC

rising.

VIN Detection

After POR the RT8101/A continuously generates a 10kHz

pulse train with 1

μs pulse width to turn on the upper

MOSFET for detecting the existence of VIN. RT8101/A

keeps monitoring PHASE pin voltage during the detection

period.

As soon as the PHASE voltage crosses 1.5V two times,

VIN existence is recognized and the RT8101/A initiates

its soft start cycle as described in next section.

Figure 1

Soft Start

A built-in soft-start is used to prevent surge current from

VIN to VOUT during power on. After the existence of VIN is

detected, soft-start (SS) begins automatically. The

feedback voltage (VFB) is clamped by internal linear ramping

up SS voltage, causing PWM pulse width increasing

slowly and thus inducing little surge current. Soft-start

completes when SS voltage exceeds internal reference

voltage (0.8V), the time duration is about 3.2ms.

Over Current Protection

The RT8101/A senses the current flowing through lower

MOSFET for over current protection (OCP) by sensing

the PHASE pin voltage as shown in the Functional Block

Diagram.

VIN POR_H

PHASE

UGATE

PHASE_M

1.5V

Internal Counter will count (VPHASE > 1.5V)

two times (rising & falling) to recognize when

VIN is ready.

1st 2nd PHASE

waveform

A 30

μA current source flows through the internal resistor

21.6k

Ω to PHASE pin causing 0.65V voltage drop across

the resistor. OCP is triggered if the voltage at PHASE pin

(drop of lower MOSFET VDS) is lower than

−0.25V when

low side MOSFET conducting. Accordingly inductor

current threshold for OCP is a function of conducting

resistance of lower MOSFET RDS(ON) as :

OCSET

DS(ON)

30 A 21.6k-0.4V

μ ×

If MOSFET with RDS(ON) = 10m

Ω is used, the OCP

threshold current is about 25A. Once OCP is triggered,

the RT8101/A enters hiccup mode and re-soft starts again.

The RT8101/A shuts down after OCP hiccups twice.

OCP

Time (2.5ms/Div)

UGATE

(10V/Div)

IOUT

(10A/Div)

OCP

Time (2.5ms/Div)

UGATE

(10V/Div)

IOUT

(10A/Div)

Figure 3. Power On then Shorted

Figure 4. Shorted then Power On

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