ZXSC100
Th e in d u cto r va lu e m u s t b e ch o s e n to s a tis fy
perform ance, cost and size requirem ents of the overall
solution. For the reference designs we recom m end an
inductor value of 22µH with a core saturation current
rating greater than the converter peak current value.
Figure 3 shows the discontinuous inductor current and
the relationship between output power, TON, TDIS and
TOFF
.
Inductor selection has a significant im pact on the
converter efficiency. For applications where efficiency
is critical, a 5% im provem ent can be achieved with a
high perform ance inductor. This should be selected
with a core saturation current rating m uch higher than
the peak current of the converter, say 3 tim es greater.
The resultant reduction in core losses brings about the
efficiency im provem ent.
IPK
Peak current definition
0A
TON
The peak current rating is a design param eter 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.
TDIS
TOFF
In general, the IPK value m ust be chosen to ensure that
the switching transistor, Q1, is in full saturation with
m a xim u m o u tp u t p o w e r co n d itio n s , a s s u m in g
worse-case input voltage and transistor gain under all
operating tem perature extrem es.
Figure 3
Discontinuous inductor current
Once IPK is decided the value of RSENSE can be
determ ined by:
Output capacitors
Output capacitors are a critical choice in the overall
perform ance of the solution. They are required to filter
the output and supply load transient currents. There
are three param eters which are param ount in the
selection of the output capacitors; their capacitance
value, IRIPPLE and ESR. The capacitance value is
selected to m eet the load transient requirem ents. The
capacitors IRIPPLE rating m ust m eet or exceed the
current ripple of the solution.
V
ISENSE
RSENSE
=
IPK
Output pow er definition
By m aking the above assum ptions for the inductor and
IPK the output power can be determ ined by:
(VOUT − V ) x IPK x TDIS
The ESR of the output capacitor can also affect loop
stability and transient perform ance. The capacitors
s e le cte d fo r th e s o lu tio n , a n d in d ica te d in th e
reference designs, are optim ised to provide the best
overall perform ance.
IN
Output Power =
2 x (TOn + TOFF
)
where
IPKxL
TON
=
V
IN
and
TDIS
IPKxL
(VOUT − V )
=
IN
Note: VOUT = output voltage + rectifier diode VF
ISSUE 3 - J ANUARY 2004
S E M IC O N D U C T O R S
11
