IRF7821 datasheet(11/18 Pages) RICHTEK

11 / 18 page

DS8298-01

November 2011

www.richtek.com

RT8298

is a registered trademark of Richtek Technology Corporation.

Application Information

Output Voltage Setting

The resistive divider allows the FB pin to sense the output

voltage as shown in Figure 1.

Figure 1. Output Voltage Setting

The output voltage is set by an external resistive voltage

divider according to the following equation :

OUT

REF

= V

⎛⎞

⎜⎟

⎝⎠

Where VREF is the feedback reference voltage (0.808V

typ.).

External Bootstrap Diode

Connect a 1

μF 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

RT8298. Note that the external boot voltage must be lower

than 5.5V.

Figure 2. External Bootstrap Diode

RT8298

GND

VOUT

BOOT

RT8298

1µF

Chip Enable Operation

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

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

mode, the RT8298 quiescent current drops to lower than

μA. Driving the EN pin high (2V < EN < 5.5V) will turn on

An external MOSFET can be added to implement digital

control on the EN pin, as shown in Figure 4. In this case,

a 100k

Ω pull-up resistor, REN, is connected between VIN

pin and the EN pin. MOSFET Q2 will be under logic control

to pull down the EN pin.

Figure 3. Enable Timing Control

Figure 4. Digital Enable Control Circuit

RT8298

GND

100k

VIN

REN

the device again. For external timing control, the EN pin

can also be externally pulled high by adding a REN resistor

and CEN capacitor from the VIN pin (see Figure 3).

The chip starts to operate when VIN rises to 4.2V (UVLO

threshold). During the VIN rising period, if an 8V output

voltage is set, VIN is lower than the VOUT target value and

it may cause the chip to shut down. To prevent this

situation, a resistive voltage divider can be placed between

the input voltage and ground and connected to the EN pin

to adjust enable threshold, as shown in Figure 5. For

example, the setting VOUT is 8V and VIN is from 0V to

12V, when VIN is higher than 10V, the chip is triggered to

enable the converter. Assume REN1 = 50k

Ω. Then,

Figure 5. Resistor Divider for Lockout Threshold Setting

RT8298

GND

VIN

REN1

REN2

RT8298

GND

VIN

REN

CEN

EN1

EN_T

EN2

IN_S

EN_T

x V

)

−

where VEN_T is the enable comparator's logic-high reference

threshold voltage (1.7V) and VIN_S is the target turn on

input voltage (10V in this example). According to the

equation, the suggested resistor R EN2 is 10.2k

Ω.

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