RT9205APS datasheet(10/15 Pages) RICHTEK

10 / 15 page

RT9205/A

Preliminary

DS9205/A-08 March 2007

www.richtek.com

Application Information

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 Figure 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.

Figure 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 powered by

a 12V. In a system with ATX 5V/12V 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 Figure 4.

Figure 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

(ov er v oltage). The UV threshold is 0.56V and

OV-threshold is 1.0V. Both UV and OV detection are with

µs delay after triggered. When OC or UV trigged, a

hiccup re-start sequence will be initialized, as shown in

Figure 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.

VCC

LGATE

PHASE

UGATE

BOOT

VCC

1uF

RT9205/A

0.1uF

VCC

LGATE

UGATE

BOOT

1uF

RT9205/A

1uF

6.0V

Regulation

12V

VCC

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