ame5268 AME, Inc., ame5268 Datasheet - Page 12

• Manufacturer Part Number: ame5268
• Description: 3a, 28v, 340khz Synchronous Rectified Step-down Converter
• Manufacturer: AME, Inc.

n Detailed Description (Contd.)
12
AME5268
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:
voltage, V
peak inductor ripple current. Choose an inductor that will
not saturate under the maximum inductor peak current,
calculated by:
inductor to use mainly depends on the price vs. size re-
quirements and any EMI constraints.
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:
Input Capacitor
Where I
The input current to the step-down converter is discon-
Inductor
The inductor is required to supply constant current to
L
I
I
Where fs is the switching frequency, V
LP
C
1
f
I
V
I
s
LOAD
LOAD
LOAD
OUT
OUT
I
AME
L
is the load current. The choice of which style
is the output voltage, and I
2
V
V
V
1
OUT
OUT
IN
f
s
V
V
OUT
L
IN
1
1
V
V
OUT
IN
V
V
OUT
IN
L
is the peak-to-
IN
is the input
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. 0.1 F, should be placed
as close to the IC as possible. The input voltage ripple for
low ESR capacitors can be estimated by:
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:
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:
dominates the impedance at the switching frequency. For
simplification, the output ripple can be approximated to:
stability of the regulation system. The AME5268 can be
optimized for a wide range of capacitance and ESR values.
The characteristics of the output capacitor also affect the
When using tantalum or electrolytic capacitors, the ESR
Output Capacitor
The output capacitor (C2) is required to maintain the DC
At V
Where R
Where C1 is the input capacitance value.
Rectified Step-Down Converter
V
V
V
V
3A, 28V, 340KHz Synchronous
IN
OUT
OUT
OUT
IN
= 2V
C
I
ESR
1
LOAD
8
V
f
V
f
s
s
OUT
OUT
OUT
is the equivalent series resistance (ESR)
f
f
L
s
L
s
, where IC1 = I
V
2
OUT
V
V
L
OUT
1
1
IN
C
V
V
V
V
OUT
2
OUT
IN
IN
1
1
V
V
LOAD
OUT
R
IN
V
R
ESR
V
/2 is the worst-case
ESR
OUT
IN
8
f
s
1
Rev. A.01
C
2