ZXSC100
SINGLE CELL DC-DC CONVERTER SOLUTION
ZXSC100
Document number: DS33613 Rev. 6 - 2
6 of 13
www.diodes.com
January 2012
© Diodes Incorporated
ZXSC100X8 Obsolete
Closest
lternative is ZXSC100N8
Application 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
CE(SAT)
and high gain is required. The majority of losses in the transistor are, ‘on-state’ and can be
calculated by using the formula below:
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:
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 Diodes ZHCS1000 meets these
needs. A data sheet for the ZHCS1000 is available on the Diodes web site or through your local Diodes sales office. Outline
information is included in the characteristics section of this data sheet.
The inductor value must be chosen to satisfy performance, cost and size requirements of the overall solution. For the
reference designs we recommend an inductor value of 22μH with a core saturation current rating greater than the converter
peak current value.
Inductor selection has a significant impact on the converter efficiency. For applications where efficiency is critical, a 5%
improvement can be achieved with a high performance inductor. This should be selected with a core saturation current rating
much higher than the peak current of the converter, say 3 times greater. The resultant reduction in core losses brings about
the efficiency improvement.
Peak Current Definition
The peak current rating is a design parameter whose value is dependent upon the overall application. For the reference
designs, a peak current of 1.2A was chosen to ensure that the converter could provide the required output power.
In general, the I
PK
value must be chosen to ensure that the switching transistor, Q1, is in full saturation with maximum output
power conditions, assuming worse-case input voltage and transistor gain under all operating temperature extremes.
Once IPK is decided the value of R
SENSE
can be determined by:
