EA4524YH Datasheet, PDF (7/16 Pages) Active-Semi, Inc – 40V/3.5A Buck Converter with Dual Output and Separated Over Current Protection

APPLICATIONS INFORMATION
ACT4524
Rev 1.1, 22-Feb-16
Inductor Selection
The inductor maintains a continuous current to the
output load. This inductor current has a ripple which
is determined by the inductance value.
Higher inductance reduces the peak-to-peak ripple
current. The trade off for high inductance value is
the increase in inductor core size and series
resistance, and the reduction in current handling
capability. In general, select an inductance value L
based on ripple current requirement:
( ) _{L =}
V_OUT
×
V_IN
V_
OUT
(1)
V f I K _{IN SW LOADMAX RIPPLE}
Where V_IN is the input
voltage, V_OUT is the output voltage, f_SW is the
switching frequency, I_LOADMAX is the maximum load
current, and K_RIPPLE is the ripple factor. Typically,
choose K_RIPPLE = 30% to correspond to the peak-to-
peak ripple current being 30% of the maximum load
current.
With a selected inductor value the peak-to-peak
inductor current is estimated as:
( ) _{ILPK _PK}
=
V_OUT
×
V_IN
V_
OUT
L ×V_IN ×f_SW
(2)
The peak inductor current is estimated as:
1
I_LPK
= I_LOADMAX
+
2
I
LPK
_ _PK
(3)
The selected inductor should not saturate at I_LPK.
The maximum output current is calculated as:
I_OUTMAX
= I_LIM ^_
1
2 ^{ILPK _PK}
(4)
L_LIM is the internal current limit.
External High Voltage Bias Diode
It is recommended that an external High Voltage
Bias diode be added when the system has a 5V
fixed input or the power supply generates a 5V
output. This helps improve the efficiency of the
regulator. The High Voltage Bias diode can be a
low cost one such as IN4148 or BAT54.
HSB
ACT4524
SW
5V
22nF
Input Capacitor
The input capacitor needs to be carefully selected
to maintain sufficiently low ripple at the supply input
of the converter. A low ESR capacitor is highly
recommended. Since large current flows in and out
of this capacitor during switching, its ESR also
affects efficiency.
The input capacitance needs to be higher than
10µF. The best choice is the ceramic type,
however, low ESR tantalum or electrolytic types
may also be used provided that the RMS ripple
current rating is higher than 50% of the output
current. The input capacitor should be placed close
to the VIN and GND pins of the IC, with the shortest
traces as possible. In the case of tantalum or
electrolytic types, they can be placed a little bit
away of IC if a paralleled ceramic capacitor is
placed right next to the IC.
Output Capacitor
The output capacitor also needs to have low ESR to
keep low output voltage ripple. The output ripple
voltage is:
V_RIPPLE
 I_OUTMAX K _RIPPLE R_ESR

28

V_IN
f_SW ² LC_OUT
(5)
Where I_OUTMAX is the maximum output current,
K_RIPPLE is the ripple factor, R_ESR is the ESR of the
output capacitor, f_SW is the switching frequency, L is
the inductance, and C_OUT is the output capacitance.
In the case of ceramic output capacitors, R_ESR is very
small and only contributes a very small portion of the
ripple. Therefore, a lower capacitance value can be
used for ceramic type. In the case of tantalum or
electrolytic capacitors, the ripple is dominated by
R_ESR multiplied by the ripple current. In that case, the
output capacitor should be chosen to have
sufficiently low ESR.
For ceramic type output capacitor, typically choose
a capacitance of about 22µF. For tantalum or
electrolytic capacitors, choose a capacitor with less
than 50mΩ ESR. A 330µF or 470µF electrolytic
capacitor is recommended.
Rectifier Diode
Use a low forward voltage drop (Vf<0.5V) Schottky
diode as the rectifier to conduct current when the
High-Side Power Switch is 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.
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