RT8288A datasheet(9/14 Pages) RICHTEK

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RT8288A

DS8288A-03

June 2012

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

Application Information

The IC is a synchronous high voltage buck converter that

can support the input voltage range from 4.5V to 21V and

the output current can be up to 4A.

Output Voltage Setting

The output voltage is set by an external resistive divider

according to the following equation :

⎛⎞

⎜⎟

⎝⎠

OUT

= V

Figure 1. Output Voltage Setting

where VFB is the feedback reference voltage 0.808V

(typical).

The resistive divider allows the FB pin to sense a fraction

of the output voltage as shown in Figure 1.

RT8288A

GND

VOUT

External Bootstrap Diode

Connect a 100nF low ESR ceramic capacitor between

the BOOT pin and SW pin as shown in Figure 2. 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

IC. Note that the external boot voltage must be lower than

5.5V.

Figure 2. External Bootstrap Diode

Soft-Start

The IC contains an internal soft-start function to prevent

large inrush current and output voltage overshoot when

the converter starts up. Soft-Start automatically begins

once the chip is enabled. During soft-start, the internal

soft-start capacitor becomes charged and generates a

linear ramping up voltage across the capacitor. This voltage

clamps the voltage at the internal reference, causing the

duty pulse width to increase slowly and in turn reduce the

output surge current. Finally, the internal 1V reference

takes over the loop control once the internal ramping-up

voltage becomes higher than 1V. The typical soft-start

time for this IC is set at 2ms.

Under Voltage Lockout Threshold

The IC includes an input Under Voltage Lockout Protection

(UVLO). If the input voltage exceeds the UVLO rising

threshold voltage (4.2V), the converter resets and prepares

the PWM for operation. If the input voltage falls below the

UVLO falling threshold voltage (3.8V) during normal

operation, the device stops switching. The UVLO rising

and falling threshold voltage includes a hysteresis to

prevent noise caused reset.

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 IC quiescent current drops to lower than 1

μA.

Driving the EN pin high (>2V, <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, as can be seen from

the Figure 5.

An external MOSFET can be added to implement digital

control on the EN pin when front age system voltage below

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.

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.

RT8288A

BOOT

100nF

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