UD05124

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LINEAR INTEGRATED CIRCUIT

UNISONICTECHNOLOGIESCO.,LTD

6 of 9

www.unisonic.com.tw

QW-R115-038.c



APPLICATION INFORMATION

Output Voltage Setting

The output voltage VOUT is set by using a resistive divider from the output to FB. The FB pin regulated voltage is

0.6V. Thus the output voltage is:

1

0.6

1

2

OUT

R

VV

R



 





Table 1 lists recommended values of R1 and R2 for most used output voltage.

Table 1. Recommended Resistance Values

VOUT

R1

R2

3.3V

453kΩ

100kΩ

2.5V

316kΩ

100kΩ

1.8V

200kΩ

100kΩ

1.5V

150kΩ

100kΩ

1.2V

100kΩ

100kΩ

Place resistors R1 and R2 close to FB pin to prevent stray pickup.

Input Capacitor Selection

The use of the input capacitor is filtering the input voltage ripple and the MOSFETS switching spike voltage.

Because the input current to the step-down converter is discontinuous, the input capacitor is required to supply the

current to the converter to keep the DC input voltage. The capacitor voltage rating should be 1.25 to 1.5 times

greater than the maximum input voltage. The input capacitor ripple current RMS value is calculated as:

()

(1

)

CIN RMS

OUT

OUT

IN

II

D

D

V

D

V



 



Where D is the duty cycle of the power MOSFET.

This function reaches the maximum value at D=0.5 and the equivalent RMS current is equal to IOUT/2.

A low ESR capacitor is required to keep the noise minimum. Ceramic capacitors are better, but tantalum or low

ESR electrolytic capacitors may also suffice.

Output Capacitor Selection

The output capacitor is used to keep the DC output voltage and supply the load transient current. When

operating in constant current mode, the output ripple is determined by four components:

( )

(ESR)

(ESL)

NOISE

( )

(t)

(t)

(t) V

(t)

RIPPLE

RIPPLE C

RIPPLE

RIPPLE

Vt

V

V

V







The following figures show the form of the ripple contributions.

(ESR)

1

××

×

OUT

OUT

RIPPLE

IN

OSC

V

V

VESR

V

FL











(ESL)

IN

×V

RIPPLE

ESL

V

L



()

2

1

8

OUT

OUT

RIPPLE C

IN

OSC

COUT

V

V

V

V

FL





 

 

