AME5242 Datasheet, PDF (7/15 Pages) Analog Microelectronics – 40V CC/CV Buck Converter

AME
AME5242
Since the input capacitor (C1) absorbs the input switch-
ing current, it requires an adequate ripple current tating.
The RMS current in the input capacitor can be esimated
by:
I_C1 = I _LOAD ×
V_{out × 1−} V_out
V_in
V_in
At V_IN=2V_OUT, where I_C1 = I_LOAD/2 is the worst-case con-
dition occurs. For simplification, use an input capacitor
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:
I_C1
=
I _LOAD
C1× f_s
× ^Vout
V_in
×
1− ^Vout
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
electrolutic capacitors are recommended. Low ESR ca-
pacitors are preferred to keep the output voltage ripple
low. The output voltage ripple can be estimated by:
∆V_out
=
V_{out ×}
f_s × L
1− ^Vout
V_in
×
R_ESR
+
8×
1
f_s ×C2
Where R_ESR is the equivalent series resistance (ESR)
value of the output capacitor and C2 is the output capaci-
tance value.
When using ceramic capacitors, the impandance at the
switching frequency is dominated by the capacitance
which is the main cause for the output voltage ripple. For
simplification, the output voltage ripple can be estimated
by:
∆V_out
=
8×
f
V_out
2
s
×
L
×
C
2
×
1 − ^Vout
V_in
Rev. A.02
40V CC/CV Buck Converter
When using tantalum or electrolytic capacitors, the
ESR dominates the impedance at the switching frequency.
For simplification, the output ripple can be approximated
to:
∆V_out
=
V_{out ×}
f_s × L
1− ^Vout
V_in
× R_ESR
The characteristics of the output capacitor also affect
the stability of the regulation system.
Rectifier Diode
Use a Schottky diode as the rectifier to conduct cur-
rent when the High-Side MOSFET is turned off. The
Schottky diode must have current rating higher than the
maximum output current and a reverse voltage rating
higher than the maximum input voltage.
Compensation Components
AME5242 has current mode control for easy compen-
sation and fast transient response. The system stability
and transient response are controlled through the COMP
pin. COMP is the output of the internal transconductance
error amplifier. A series capacitor-resistor combination
sets a pole-zero combination to govern the characteris-
tics of the control system. The DC gain of the voltage
feedback loop is given by:
A_VDC
=
R_LOAD
× G_CS
×
A_EA
×
V_FB
V_out
Where V_FB is the feedback voltage (0.8V), A_VEA is the
error amplifier voltage gain, G_CS is the current sense
transconducductance and R_LOAD is the load resistor value.
The system has two poles of importance. One is due to
the output capacitor and the load resistor, and the other
is due to the compansation capacitor (C4) and the output
resistor of the error amplifier. These poles are located at:
f _P1
=
2×π
G_EA
×C4×
A_VEA
f_P2
=
2×π
1
×C2×
R_LOAD
7