ZXSC100

SEMICO NDUC TORS

ISSUE 3 - JANUARY 2004

10

APPLICATIONS INFORMATION

The following section is a design guide for optimum

converter performance.

Switching transistor selection

The choice of switching transistor has a major impact

on the DC-DC converter efficiency. For optimum

and high gain is required. The majority of losses in the

transistor are, ‘on-state’ and can be calculated by

using the formula below:

P

((I xV

I x V

))xT

(T

T

Q1

AV

CE(SAT)

B

BE(SAT)

ON

ON

OFF)

=

+

+

)(

)

where I

I

2

AV

PK

=

From the calculations above the impact on converter

efficiency can be seen.

External drive transistor selection

For higher power applications an external transistor is

required to provide the additional base drive current

to the main switching transistor. For this, any small

signal PNP transistor is sufficient. Please see reference

designs for recommended part numbers.

Schottky diode selection

As with the switching transistor the Schottky rectifier

diode has a major impact on the DC-DC converter

efficiency. A Schottky diode with a low forward

voltage and fast recovery time should be used for this

application. The majority of losses in the diode are,

‘on-state’ and can be calculated by using the formula

below:

P

Ix V

x T

(T

T

D1

AV

F(MAX)

DIS

On

OFF

=

+

)

where I

I

2

AV

PK

=

The diode should be selected so that the maximum

forward current is greater or equal to the maximum

peak current in the inductor, and the maximum

reverse voltage is greater or equal to the output

voltage.

The Zetex ZHCS1000 meets these needs. A data sheet

for the ZHCS1000 is available on the Zetex web site or

through your local Zetex sales office. Outline

information is included in the characteristics section of

this data sheet.

Inductor selection