AME
AME5268
n
Detailed Description (Contd.)
Inductor
The inductor is required to supply constant current to
the load while being driven by the switched input voltage.
A larger value inductor will have a larger physical size,
higher series resistance, and lower saturation current. It
will result in less ripple current that will in turn result in
lower output ripple voltage. Make sure that the peak induc-
tor current is below the maximum switch current limit.
Determine inductance is to allow the peak-to peak ripple
current to be approximately 30% of the maximum switch
current limit. The inductance value can be calculated by:
3A, 28V, 340KHz Synchronous
Rectified Step-Down Converter
At V
IN
= 2V
OUT
, where IC1 = I /2 is the worst-case
LOAD
condition occurs. For simplification, use an input capaci-
tor with a RMS current rating greater than half of the maxi-
mum load current. When using ceramic capacitors, make
sure that they have enough capacitance to provide suffi-
cient charge to prevent excessive voltage ripple at input.
When using electrolytic or tantalum capacitors, a high
quality, small ceramic capacitor, i.e. 0.1µF, should be placed
as close to the IC as possible. The input voltage ripple for
low ESR capacitors can be estimated by:
L
=
V
OUT
V
OUT
×
1
−
f
s
× ∆
I
L
V
IN
∆
V
IN
=
I
LOAD
V
OUT
V
OUT
×
×
1
−
C
1
×
f
s
V
IN
V
IN
Where C1 is the input capacitance value.
Output Capacitor
The output capacitor (C2) is required to maintain the DC
output voltage. Ceramic, tantalum, or low ESR electrolytic
capacitors are recommended. Low ESR capacitors are
preferred to keep the output voltage ripple low. The output
voltage ripple can be estimated by:
Where fs is the switching frequency, V
IN
is the input
voltage, V
OUT
is the output voltage, and
∆I
L
is the peak-to-
peak inductor ripple current. Choose an inductor that will
not saturate under the maximum inductor peak current,
calculated by:
I
LP
=
I
LOAD
+
V
OUT
V
OUT
×
1
−
2
×
f
s
×
L
V
IN
Where I
LOAD
is the load current. The choice of which style
inductor to use mainly depends on the price vs. size re-
quirements and any EMI constraints.
Input Capacitor
The input current to the step-down converter is discon-
tinuous, therefore a capacitor is required to supply the AC
current while maintaining the DC input voltage. Use low
ESR capacitors for the best performance. Ceramic capaci-
tors are preferred, but tantalum or low-ESR electrolytic
capacitors will also be suggested. Choose X5R or X7R
dielectrics when using ceramic capacitors.
Since the input capacitor (C1) absorbs the input switching
current, it requires an adequate ripple current rating. The
RMS current in the input capacitor can be estimated by:
∆
V
OUT
=
V
OUT
V
OUT
1
×
1
−
×
R
ESR
+
f
s
×
L
V
IN
8
×
f
s
×
C
2
Where R is the equivalent series resistance (ESR)
ESR
value of the output capacitor and C2 is the output capaci-
tance value.
When using ceramic capacitors, the impedance at the
switching frequency is dominated by the capacitance which
is the main cause for the output voltage ripple. For simpli-
fication, the output voltage ripple can be estimated by:
∆
V
OUT
=
V
OUT
V
OUT
×
1
−
8
×
f
s
2
×
L
×
C
2
V
IN
When using tantalum or electrolytic capacitors, the ESR
dominates the impedance at the switching frequency. For
simplification, the output ripple can be approximated to:
I
C
1
=
I
LOAD
×
V
OUT
V
OUT
×
1
−
V
IN
V
IN
∆
V
OUT
=
V
OUT
V
OUT
×
1
−
×
R
ESR
f
s
×
L
V
IN
The characteristics of the output capacitor also affect the
stability of the regulation system. The AME5268 can be
optimized for a wide range of capacitance and ESR values.
Rev.B.01
12
AME [ ANALOG MICROELECTRONICS ]
