RT9205SLASHACS datasheet(10/16 Pages) RICHTEK

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RT9205/A

Preliminary

www.richtek.com

DS9205/A-03 May 2003

Functional Description

The RT9205/A operates at either single 5V power

supply with a bootstrap UGATE driver or a 5V/12V

dual-power supply form the ATX SMPS. The dual-

power supply is recommended for high current

applications, the RT9205/A can deliver higher gate

driving current while operating with ATX SMPS based

on a dual-power supply.

The Bootstrap Operation

In a single power supply system, the UGATE driver of

RT9205/A is powered by an external bootstrap circuit,

as shown in the Fig.3. The boot capacitor, CBOOT,

generates a floating reference at the PHASE pin.

Typically a 0.1

µF CBOOT is enough for most of

MOSFETs used with the RT9205/A. The voltage drop

between BOOT and PHASE is refreshed to a voltage

of VCC – diode drop (VD) while the lower MOSFET

turning on.

Fig.3 Single 5V power Supply Operation

Dual Power Operation

The RT9205/A was designed to supply a regulated

6.0V at VCC pin automatically when BOOT pin is

power supply, the RT9205/A is ideal for higher

current applications due to the higher gate driving

capability, VUGATE = 12V and VLGATE = 6.0V. A RC

(10

Ω/1µF) filter is also recommended at BOOT pin to

prevent the ringing induced from fast power-on, as

shown in Fig.4.

Fig.4 Dual Power Supply Operation

Power On Reset

The Power-On Reset (POR) monitors the supply

voltage (normal +5V) at the VCC pin and the input

voltage at the OCSET pin. The VCC POR level is set

to 4.1V with 0.5V hysteresis and the normal level at

OCSET pin is set to 1.5V (see over-current

protection). The POR function initiates soft-start

operation after all supply voltages exceed their POR

thresholds.

Soft Start

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

from power supply input during powering on. The

soft-start voltage is controlled by an internal digital

counter. It slows down and clamps the ramping of

reference voltage at the input of error amplifier and

the pulse-width of the output driver. The typical soft-

start duration is 2.5mS.

Under Voltage and Over Voltage Protection

The voltage presents at FB pin is monitored and

protected against OC (over current), UV (under

voltage), and OV (over voltage). The UV threshold is

0.56V and OV-threshold is 1.0V. Both UV and OV

detection are with 30

µS delay after triggered. When

OC or UV trigged, a hiccup re-start sequence will be

initialized, as shown in Fig.5. For RT9205, only 3

times of trigger are allowed before latching off. But

for RT9205A, UVP will be kept in hiccup mode.

Hiccup is disabled during soft-start interval.

BOOT

UGATE

PHASE

LGATE

RT9205/A

VCC

0.1

µF

VCC

LGATE

UGATE

BOOT

1uF

RT9205/A

1uF

6.0V

Regulation

12V

VCC

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