Sense Resistor

A low value sense resistor is required to set the peak

current. Power in this resistor is negligible due to the

of recommended sense resistors:

Output power calculation

By making the above assumptions for inductance and

peak current the output power can be determined by:

= (Watts)

where

I

I

2

X

(T

T

(T

T

AV

PK

ON

DIS

ON

OFF

=

+

+

)

)

and

T

Ix L

V

ON

PK

IN

=

and

T

Ix L

V- V

DIS

PK

OUT

IN

=

and

≅ 1.7µs (internally set by ZXSC410)

and

= efficiency i.e. 100% = 1

Operating frequency can be derived by:

F

1

TT

ON

OFF

=

+

Output capacitors

Output capacitors are a critical choice in the overall

performance of the solution. They are required to filter

the output and supply load transient currents. There

are three parameters which are paramount in the

and ESR. The capacitance value is selected to meet the

load transient requirements.

rating must meet or exceed the current ripple of the

solution.

The ESR of the output capacitor can also affect loop

stability and transient performance. The capacitors

selected for the solutions, and indicated in the

reference designs, are optimised to provide the best

overall performance.

Input capacitors

The input capacitor is chosen for its voltage and RMS

current rating. The use of low ESR electrolitic or

tantalum capacitors is recommended. Capacitor

values for optimum performance are suggested in the

reference design section

Also note that the ESR of the input capcitor is

effectively in series with the input and hence

ESR.

ZXSC410

ZXSC420

SEMICO NDUC TORS

ISSUE 2 - May 2003

6

Manufacture

Series

Range

Size

Tolerance

URL

Cyntec

RL1220

0.022 - 10

0805

±5%

http://www.cyntec.com

IRC

LR1206

0.010 - 1.0

1206

±5%

http://www.ictt.com