AME5242 Datasheet, PDF (6/15 Pages) Analog Microelectronics – 40V CC/CV Buck Converter

AME
AME5242
n Detailed Description
Under Voltage Lockout (UVLO)
The AME5242 incorporates an under voltage lockout
circuit to keep the device disabled when V_IN (the input
voltage) is below the UVLO rising threshold voltage. Once
the UVLO rising threshold voltage is reached,the device
start-up begins. The device operates until V_IN falls below
the UVLO falling threshold voltage. The typical hyster-
esis in the UVLO comparator is 1V.
Over Voltage Protection
The AME5242 has input and output over-voltage pro-
tections. The thresholds of input and output OVP circuit
include are typicapl 35V and minimum 106% x V_OUT, re-
spectively. Once the input voltage or output voltage is
higher than the threshold, the high-side MOSFET is turned
off. When the input voltage or output voltage drops lower
than the threshold, the high-side MOSFET will be en-
abled again.
Over Current Protection
The AME5242 cycle-by-cycle limits the peak inductor
current to protect embedded switch from dameage. High-
side switch current limiting is implemented by monitor-
ing the current through the high side MOSFET.
Thermal Shutdown
The AME5242 protects itself from overheating with an
internal thermal shutdown circuit. If the junction tempera-
ture exceeds the thermal shutdown trip point, the high-
side MOSFET is turned off. The part is restarted when
the junction temperature drops 20^oC below the thermal
shutdown trip point
Setting the Output Voltage
The output voltage is using a resistive voltage divider
connected from the output voltage to FB. It divides the
output voltage down to the feedback voltage by the ratio:
V_FB
= V_out
×
R₂
R₁ + R₂
6
40V CC/CV Buck Converter
the output voltage is:
V_out
=
0.8 ×
R₁ + R₂
R₂
Inductor Selection
The inductor is required to supply contant current to
the load while being driven by the switched input voltage.
A larger value inductor will have a larger physical size
and higher series resistance. It will result in less ripple
current that will in turn result in lower output ripple volt-
age. Make sure that the peak inductor current is below
the maximum switch current limit. Determine inductance
is to allow the peak-to-peak ripple current to be approxi-
mately 30% of the maximum load current. The induc-
tance value can be calculated by:
L = ^Vout × 1− ^Vout
f_s × ∆I_L
V_in
Where f_S is the switching frequency, V_IN is the input
voltage, V_OUT is the output voltage, and ∆Ι_L is the peak-
to-peak inductor ripple current. Choose an inductor that
will not saturate under the maximum inductor peak cur-
rent, calculated by:
I _LPK
=
I _LOAD
+
V_out
2× f_s × L
×
1 − ^Vout
V_in
Where I_LOAD is the load current. The choice of which
style inductor to use mainly depends on the price vs.
size requirements 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
capacitors are preferred, but tantalum or low-ESR elec-
trolytic capacitors will also be suggested. Choose X5R
or X7R dielectrics when using ceramic capacitors.
Rev. A.02