RT7296C

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is a registered trademark of Richtek Technology Corporation.

www.richtek.com

DS7296C-00

May

2015

10

Application Information

The RT7296C is a high voltage buck converter that can

support the input voltage range from 4.5V to 17V and

the input voltage range from 4.5V to 17V and the output

current can be up to 3A.

Output Voltage Selection

The resistive voltage divider allows the FB pin to sense

a fraction of the output voltage as shown in Figure 1.

RT7296C

GND

FB

R1

R2

R5

Figure 1. Output Voltage Setting

For adjustable voltage mode, the output voltage is set

by an external resistive voltage divider according to the

following equation :

OUT

FB

R1

V

V

1

R2









typ.). Table 2 lists the recommended resistors value for

common output voltages.

Table 2. Recommended Resistors Value

R1 (k

)

R2 (k

)

R5 (k

)

1.0

20.5

84.5

140

3.3

40.2

13

24

5.0

40.2

7.68

24

External Bootstrap Diode

Connect a 100nF 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, as shown as

Figure 2, 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 (PVCC) of the RT7296C.

SW

BOOT

5V

RT7296C

100nF

Figure 2. External Bootstrap Diode

External Soft-Start Capacitor

RT7296C provides external soft-start function. The

soft-start function is used to prevent large inrush

current while converter is being powered-up. The

soft-start timing can be programmed by the external

provides a 11

0.8V) can be calculated by the following formula :

SS

SS

SS

C

(nF) 1.3

t

(ms) =

I

( A)





Inductor Selection

The inductor value and operating frequency determine

the ripple current according to a specific input and

output voltage. The ripple current

higher VIN and decreases with higher inductance.

OUT

OUT

L

IN

VV

I1

f L

V





 

 







 

Having a lower ripple current reduces not only the ESR

losses in the output capacitors but also the output

voltage ripple. High frequency with small ripple current

can achieve highest efficiency operation. However, it

requires a large inductor to achieve this goal.

For the ripple current selection, the value of

guarantee that the ripple current stays below the

specified maximum, the inductor value should be

chosen according to the following equation :

OUT

OUT

L(MAX)

IN(MAX)

VV

L1

f

I

V



 





 



 





 







 

