ZXSC100 Zetex Semiconductors plc., ZXSC100 Datasheet - Page 10

• Manufacturer Part Number: ZXSC100
• Description: Single Cell Dc-dc Converter Solution
• Manufacturer: Zetex Semiconductors plc.

P
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
performance, a bipolar transistor with low V
and high gain is required. The majority of losses in the
transistor are, ‘on-state’ and can be calculated by
using the formula below:
where I
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.
Inductor selection
ZXSC100
Q1
S E M I C O N D U C T O R S
=
((I xV
AV
AV
=
I
2
PK
CE(SAT)
(T
) (
ON
+
+
I x V
B
T
OFF)
BE(SAT)
)
))xT
ON
CE(SAT)
10
P
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:
where I
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.
D1
=
I
AV
AV
x V
(T
=
On
I
F(MAX)
2
PK
+
T
OFF
x T
)
DIS
ISSUE 3 - JANUARY 2004