AME5287 Datasheet, PDF (8/20 Pages) Analog Microelectronics – Rectified Step-Down Converter

AME
AME5287
3A, 300KHz ~ 2MHz Synchronous
Rectified Step-Down Converter
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.
Output Voltage Programming
The output voltage of the AME5287 is set by a resistive
divider according to the following formula:
V_OUT
=
0.8 × _{}1+
R1
R2
^Volt.
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 ≅ IOMAX}
( ) V_OUT V_IN −V_OUT
V_IN
This formula has a maximum at V _IN=2V _OUT ,
whereI_RMS=I_OUT/2. For simplification, use an input capaci-
tor 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 require-
ment for C_OUT has been met, the RMS current rating gen-
erally far exceeds the I_RIPPLE(P-P) requirement. The output
ripple ∆V_OUT is determined by:
Loop Compensation
The AME5287 employs peak current mode control for
easy use and fast transient response. Peak current mode
control eliminates the double pole effect of the output LC
filter. It greatly simplifies the compensation loop design.
With peak current mode control, the buck power
stagecan be simplified to be a one-pole and one-zero sys-
tem in frequency domain. The pole can be calculated by:
f _P1
=
2π
1
× C_OUT
×
R_L
The zero is a ESR zero due to output capacitor and its
ESR. It can be calculated by:
f_Z1
=
2π
1
× C_OUT × ESR_COUT
∆V_OUT
≅
∆I _{L } ESR +
1
8 fC_OUT

For a fixed output voltage, the output ripple is highestat
maximum input voltage since ∆I_L increases with input volt-
age.
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.
8
Rev. A.01