ZXSC100 Zetex Semiconductors plc., ZXSC100 Datasheet - Page 11

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

ISSUE 3 - JANUARY 2004
R
Output Power
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
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 I
determined by:
Output power definition
By making the above assumptions for the inductor and
I
where
T
and
T
Note: V
PK
SENSE
ON
DIS
the output power can be determined by:
=
=
I xL
=
(
PK
V
V
PK
OUT
V
IN
OUT
I xL
ISENSE
PK
I
PK
is decided the value of R
−
= output voltage + rectifier diode VF
V
=
IN
PK
)
(V
value must be chosen to ensure that
OUT
2 x (T
−
V
IN
On
)
x I
+
T
PK
OFF
x T
)
DIS
SENSE
can be
11
0A
I
Figure 3 shows the discontinuous inductor current and
the relationship between output power, T
T
Figure 3
Discontinuous inductor current
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
selection of the output capacitors; their capacitance
value, I
selected to meet the load transient requirements. The
capacitors I
current ripple of the solution.
The ESR of the output capacitor can also affect loop
stability and transient performance. The capacitors
selected for the solution, and indicated in the
reference designs, are optimised to provide the best
overall performance.
PK
OFF
.
T
RIPPLE
ON
RIPPLE
and ESR. The capacitance value is
T
DIS
rating must meet or exceed the
T
OFF
S E M I C O N D U C T O R S
ZXSC100
ON
, T
DIS
and