UC3656

Preliminary

CMOS IC

UNISONICTECHNOLOGIESCO.,LTD

6 of 7

www.unisonic.com.tw

QW-R502-469.b

APPLICATION INFORMATION

Capacitor Selection

Proper selection of capacitors using the UTC UC3656 plays a critical role for system performance. The source

current of the top MOSFET is square wave of duty cycle in a continuous mode.

The input capacitor is generally used to provide a low impedance loop for the edges of pulsed current drawn by

and be mistaken as loop instability. Increasing the capacitance of the input capacitor can great improve the

oscillation. A typical value is 4.7~ 22μF ceramic (X5R or X7R), POSCAP or Aluminum Polymer. With these

capacitors, better high frequency bypassing is achieved; resistive losses are thus reduced by their low ESR for

higher efficiency. Moreover, the input capacitor RMS current changes with the input voltage and the output voltage.

The maximum RMS current in the input capacitor can be expressed in the following formula:

)

V

V

(

V

V

I

I

IN

O

IN

O

OMAX

RMS

−

=

1

The output capacitor depends upon the suitable ripple voltage. Low ripple voltage corresponds to lower effective

series resistance (ESR).

The output ripple voltage can be calculated by:

)

fC

ESR

(

I

V

OUT

L

OUT

8

1

+

Δ

≅

Δ

The formula as follows shows the output capacitor RMS ripple current:

IN

OUT

IN

OUT

RMS

V

f

L

)

V

V

(

V

I

×

×

−

×

=

3

2

1

For the UTC UC3656, X5R or X7R dielectric formulations have the priority in choosing the input and output

ceramic capacitors. This is because these dielectrics provide the best temperature and voltage characteristics of all

the ceramics for a given value and size.

Inductor Selection

Selecting an optimum inductor is dependent upon the expected ripple current and large value inductors lower

ripple current and small value inductors result in higher ripple current. In addition, the losses associated with the

DCR and its effect on the total converter efficiency should be taken consideration.

The inductor is also chosen to limit the ripple current to some pre-fixed value, approx. 20 ~ 40% (TYP.) of the

full load current at the maximum input voltage.

The inductance value can be calculated in the following equation:

OUT(MAX)

L

I

0.4

~

.

I

×

=

Δ

2

0

)

V

V

(

I

f

V

L

IN

OUT

L

OUT

−

Δ

×

=

1

Adjustable Output Voltage

A resistive divider adjusts the output voltage of the UTC UC3656, which divides a 0.6V reference voltage at FB

pin. The output voltage can be obtained by the equation as follows:

2

1

0.6

(1

)

OUT

R

VV

R

=× +