RT8294 datasheet(9/15 Pages) RICHTEK

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RT8294

DS8294-02

March 2011

www.richtek.com

Application Information

The RT8294 is a synchronous high voltage buck converter

that can support the input voltage range from 4.5V to 23V

and the output current can be up to 2A.

Output Voltage Setting

The resistive voltage divider allows the FB pin to sense

the output voltage as shown in Figure 1.







OUT

= V1

where VFB is the feedback reference voltage 0.923V (typ.).

External Bootstrap Diode

Connect a 10nF low ESR ceramic capacitor between the

BOOT pin and SW pin. This capacitor provides the gate

driver voltage for the high side MOSFET.

It is recommended to add an external bootstrap diode

between an external 5V and BOOT pin for efficiency

improvement when input voltage is lower than 5.5V or duty

ratio is higher than 65% .The bootstrap diode can be a

low cost one such as IN4148 or BAT54. The external 5V

can be a 5V fixed input from system or a 5V output of the

RT8294. Note that the external boot voltage must be lower

than 5.5V

Figure 2. External Bootstrap Diode

Figure 1. Output Voltage Setting

Soft-Start

The RT8294 contains an external soft-start clamp that

gradually raises the output voltage. The soft-start timing

can be programmed by the external capacitor between

SS pin and GND. The chip provides a 6

µA charge current

for the external capacitor. If 0.1

µF capacitor is used to

set the soft-start, the period will be 15.5ms (typ.).

Chip Enable Operation

The EN pin is the chip enable input. Pulling the EN pin

low (<0.4V) will shut down the device. During shutdown

mode, the RT8294 quiescent current drops to lower than

µA. Driving the EN pin high ( > 2.7V, < 5.5V) will turn on

the device again. For external timing control (e.g.RC),

the EN pin can also be externally pulled high by adding a

REN* resistor and CEN* capacitor from the VIN pin

(see Figure 5).

An external MOSFET can be added to implement digital

control on the EN pin when no system voltage above 2.5V

is available, as shown in Figure 3. In this case, a 100k

Ω

pull-up resistor, REN, is connected between VIN and the

EN pin. MOSFET Q1 will be under logic control to pull

down the EN pin.

The output voltage is set by an external resistive voltage

divider according to the following equation :

Figure 3. Enable Control Circuit for Logic Control with

Low Voltage

To prevent enabling circuit when VIN is smaller than the

VOUT target value, a resistive voltage divider can be placed

between the input voltage and ground and connected to

the EN pin to adjust IC lockout threshold, as shown in

Figure 4. For example, if an 8V output voltage is regulated

from a 12V input voltage, the resistor, REN2, can be

selected to set input lockout threshold larger than 8V.

RT8294

GND

VOUT

VIN

GND

BOOT

VOUT

Chip Enable

VIN

RT8294

CSS

COMP

9 (Exposed Pad)

CBOOT

COUT

CIN

REN

100k

BOOT

RT8294

10nF

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