AME5283 Datasheet, PDF (8/20 Pages) Analog Microelectronics – Rectified Step-Down Converter

AME
AME5283
2A, 300KHz ~ 2MHz Synchronous
Rectified Step-Down Converter
Inductor Selection
For most applications, the value of the inductor will fall
in the range of 2.2µH to 4.7µH. Its value is chosen based
on the desired ripple current. Large value inductors lower
ripple current and small value inductors result in higher
ripple currents. Higher V_IN or V_OUT also increase the ripple
current ∆I_L:
∆I _L
=
f
1
×
L ^VOUT
¹
−
V_OUT
V_IN

A reasonable inductor current ripple is usually set as 1/
3 to 1/5 of maximum out current. The DC current rating
of the inductor should be at least equal to the maximum
load current plus half the ripple current to prevent core
saturation. For better efficiency, choose a low DCR in-
ductor.
Capacitor Selection
In continuous mode, the source current of the top
MOSFET is a square wave of duty cycle V_OUT/V_IN. To
prevent large voltage transients, a low ESR input capaci-
tor sized for maximum RMS current must be used. The
maximum RMS capacitor current is given by:
C_IN requires I_RMS ≅ I_OMAX
( ) V_OUT V_IN −V_OUT
V_IN
This formula has a maximum at V_IN=2V_OUT, where
I_RMS=I_OUT/2. For simplification, use an input capacitor with
a RMS current rating greater than half of the maximum
load current.
The selection of C_OUT is driven by the required effective
series resistance (ESR). Typically, once the ESR re-
quirement for C_OUT has been met, the RMS current rating
generally far exceeds the I_RIPPLE(P-P) requirement. The
output ripple V_OUT is determined by:
∆V_OUT
≅
∆I _{L } ESR +
1
8 fC_OUT

For a fixed output voltage, the output ripple is highest
at maximum input voltage since ∆I_L increases with input
voltage.
When choosing the input and output ceramic capaci-
tors, choose the X5R or X7R dielectric formulations. These
dielectrics have the best temperature and voltage char-
acteristics of all the ceramics for given value and size
Output Voltage Programming
The AME5283 output voltage of the AME5283 is set by
a resistive divider according to the following formula:
V_OUT
=
0.8 × _{}1+
R1
R2
^Volt.
Some standard value of R1, R2 for most commonly
used output voltage values are listed in Table 1.
V_OUT(V)
1.1
1.2
1.5
1.8
2.5
3.3
R1(KΩ)
7.5
10
17.4
30
51
75
R2(KΩ)
20
20
20
24
24
24
8
Rev. A.03