RT8202A 데이터 시트보기 (PDF) - Richtek Technology

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RT8202A

Single Synchronous Buck Controller

Richtek Technology 

RT8202/A/B

Output Voltage Setting (FB)

The output voltage can be adjusted from 0.75V to 3.3V by

setting the feedback resistor R1 and R2 (Figure 7). Choose

R2 to be approximately 10kΩ, and solve for R1 using the

equation:

VOUT

= VFB



1



R1

R2



where VFB is 0.75V.

Note that in order for the device to regulate in a controlled

manner, the ripple content at the feedback pin, VFB, should

be approximately 15mV at minimum VBAT, and worst case

no smaller than 10mV. If Vripple at minimum VBAT is less

than 15mV, the above component values should be

revisited in order to improve this. Quite often a small

capacitor, C1, is required in parallel with the top feedback

resistor, R1, in order to ensure that VFB is large enough.

The value of C1 can be calculated as follows, where R2 is

the bottom feedback resistor.

Firstly calculating the value of Z1 required :

  Z1 =

R2 

0.015

Vripple_VBAT(MIN)  0.015



Secondly calculating the value of C1 required to achieve

this :

  1  1

C1 =

Z1 R1

2    fSW_VBAT(MIN)

F

Finally using the equation as follows to verify the value of

VFB :

VFB_VBAT(MIN) = Vripple_VBAT(MIN)















 R2+





1

R1



2





R2

1

 fSW_VBAT(MIN)









C1





V

where Vripple_VBAT(MIN) is the output ripple voltage in

minimum VBAT;

fsw_VBAT(MIN) is the switching frequency in minimum VBAT;

VFB_VBAT(MIN) is the ripple voltage into FB pin in minimum

VBAT.

Copyright ©2014 Richtek Technology Corporation. All rights reserved.

www.richtek.com

14

VIN

UGATE

PHASE

BOOT

VOUT

FB

GND

VOUT

Z1

R1 C1

C2

R2

Figure 7. Setting The Output Voltage

For application that output voltage is higher than 3.3V,

user can also use a voltage divider to keep VOUT pin

voltage within 0.75V to 2.8V as shown in Figure 8. For

this case, TON can be determined as below :

If

R TON

<

2M

then

TON

=

3.85p



RTON 

VIN

VOUT_FB

 0.5

If RTON 

2M

then

TON

=

3.55p 

RTON  VOUT_FB

VIN  0.4

Where RTON is TON set resistor and the VOUT_FB is the

output signal of resistor divider. Since the switching

frequency is

FS

=

VOUT

VIN  TON

For a given switching frequency, we can obtain the RTON

as below

If RTON < 2M then

R TON

=

VOUT 

VIN

0.5



VOUT

VOUT_FB



FS

1

 3.85p

If RTON  2M then

R TON

=

VOUT 

VIN

0.4



VOUT

VOUT_FB



FS

1

 3.55p

VIN

RTON

VIN

UGATE

PHASE

BOOT

VOUT

VOUT

R3

VOUT_FB

R1

C2

R4

FB

R2

GND

Figure 8. Output Voltage Setting for VOUT > 3.3V

Application

is a registered trademark of Richtek Technology Corporation.

DS8202/A/B-07 April 2014

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