SC4517A
POWER MANAGEMENT
Application Information (Cont.)
the additional current needed by the load. The ESR and
ESL of the output capacitor, the loop parasitic inductance
between the output capacitor and the load combined
with inductor ripple current are all major contributors to
the output voltage ripple. Surface mount ceramic
capacitors are recommended.
Input Capacitor Selection
The input capacitor selection is based on its ripple current
level, required capacitance and voltage rating. This
capacitor must be able to provide the ripple current by
the switching actions. For the continuous conduction
mode, the RMS value of the input capacitor current
I
CIN(RMS)
can be calculated from:
I
CIN (RMS )
=
I
OMAX
⋅
V
O
⋅
( V
I
−
V
O
)
V
2I
T
W
=
1
⋅
D
max
f
s
Where:
f
s
= the switching frequency and
Dmax = maximum duty ratio, 0.9 for the SC4517A.
The required minimum capacitance for the boost
capacitor will be:
C
boost
=
I
B
⋅
T
W
V
D
Where:
I
B
= the boost current and
V
D
= discharge ripple voltage.
With f
s
= 1.2MHz, V
D
= 0.5V and I
B
=0.045A, the required
minimum capacitance for the boost capacitor is:
C
boost
=
I
B
1
0.045
1
⋅ ⋅
D
max
=
⋅
⋅
0.9
=
67.5nF
V
D
f
s
0.5 1.2M
This current gives the capacitor’s power loss through its
R
CIN(ESR)
as follows:
P
CIN
=
I
2 CIN(RMS )
•
R
CIN(ESR )
The input ripple voltage mainly depends on the input
capacitor’s ESR and its capacitance for a given load, input
voltage and output voltage. Assuming that the input
current of the converter is constant, the required input
capacitance for a given voltage ripple can be calculated
by:
D
⋅
(1
−
D)
C
IN
=
I
OMAX
⋅
fs
⋅
(
∆
V
I
−
I
OMAX
⋅
R
CIN(ESR )
)
The internal driver of the switch requires a minimum 2.7V
to fully turn on that switch to reduce its conduction loss.
If the output voltage is less than 2.7V, the boost capacitor
can be connected to either the input side or an
independent supply with a decoupling capacitor. But the
Pin BST should not see a voltage higher than its maximum
rating.
Freewheeling Diode Selection
This diode conducts during the switch’s off-time. The diode
should have enough current capability for full load and
short circuit conditions without any thermal concerns.
Its maximum repetitive reverse block voltage has to be
higher than the input voltage of the SC4517A. A low
forward conduction drop is also required to increase the
overall efficiency. The freewheeling diode should be
turned on and off fast with minimum reverse recovery
because the SC4517A is designed for high frequency
applications. SS13 Schottky rectifier is recommended
for certain applications. The average current of the diode,
I
D
_
AVG
can be calculated by:
I
D _ AVG
=
I
O max
⋅
(1
−
D)
Where:
∆V
I
= the given input voltage ripple.
Because the input capacitor is exposed to the large surge
current, attention is needed for the input capacitor. If
tantalum capacitors are used at the input side of the
converter, one needs to ensure that the RMS and surge
ratings are not exceeded. For generic tantalum
capacitors, it is suggested to derate their voltage ratings
at a ratio of about two to protect these input capacitors.
Boost Capacitor and its Supply Source Selection
The boost capacitor selection is based on its discharge
ripple voltage, worst case conduction time and boost
current. The worst case conduction time T
w
can be
estimated as follows:
2006 Semtech Corp.
9
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