Application information
8 Application information
8.1
A5975AD
Component selection
Input capacitor
The input capacitor must be able to support the maximum input operating voltage and the maximum RMS input current.
As step-down converters draw current from the input in pulses, the input current is squared and the height of each pulse is equal to the output current. The input capacitor has to absorb all this switching current, which can be up to the load current divided by two (worst case, with duty cycle of 50%). For this reason, the quality of these capacitors must be very high to minimize the power dissipation generated by the internal ESR, thereby improving system reliability and efficiency. The critical parameter is usually the RMS current rating, which must be higher than the RMS input current. The maximum RMS input current (flowing through the input capacitor) is:
Equation 17
I
RMS
= I
O
D –
2
--------------+
D
2
-------
2 where is the expected system efficiency, D is the duty cycle and I
O
is the output DC current. This function reaches its maximum value at D = 0.5 and the equivalent RMS current is equal to I
O
divided by 2 (considering = 1). The maximum and minimum duty cycles are:
Equation 18
D
MAX
=
V
V
INMIN
+
–
V
V
F
SW and
Equation 19
D
MIN
=
V
V
INMAX
+
–
V
V
SW where V
F
is the freewheeling diode forward voltage and V
SW internal PDMOS. Considering the range D
MIN
to D
MAX
the voltage drop across the
, it is possible to determine the max
IRMS going through the input capacitor. Capacitors that can be considered are:
Electrolytic capacitors:
These are widely used due to their low price and their availability in a wide range of RMS current ratings.
The only drawback is that, considering ripple current rating requirements, they are physically larger than other capacitors.
Ceramic capacitors:
If available for the required value and voltage rating, these capacitors usually have a higher
RMS current rating for a given physical dimension (due to very low ESR).
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A5975AD Application information
The drawback is the considerably high cost.
Tantalum capacitors:
Very good, small tantalum capacitors with very low ESR are becoming more available.
However, they can occasionally burn if subjected to very high current during charge.
Therefore, it is better to avoid this type of capacitor for the input filter of the device. They can, however, be subjected to high surge current when connected to the power supply.
Manufacturer
Table 6. List of ceramic capacitors for the A5975AD
Series Capacitor value ( µ ) Rated voltage (V)
TAIYO YUDEN
MURATA
UMK325BJ106MM-T
GRM42-2 X7R 475K 50
10
4.7
50
50
Output capacitor
The output capacitor is very important to meet the output voltage ripple requirement.
Using a small inductor value is useful to reduce the size of the choke but it increases the current ripple. So, to reduce the output voltage ripple, a low ESR capacitor is required.
Nevertheless, the ESR of the output capacitor introduces a zero in the open loop gain, which helps to increase the phase margin of the system. If the zero goes to a very high frequency, its effect is negligible. For this reason, ceramic capacitors and very low ESR capacitors in general should be avoided.
Tantalum and electrolytic capacitors are usually a good choice for this purpose. A list of
some tantalum capacitor manufacturers is provided in Table 7 .
Manufacturer
Sanyo POSCAP
AVX
KEMET
Sprague
(1)
Table 7. Output capacitor selection
Series
TAE
TV
TPS
T494/5
595D
Cap value ( µ F) Rated voltage (V)
47 to 680
68 to 330
100 to 470
100 to 470
220 to 390
2.5 to 10
4 to 6.3
4 to 35
4 to 20
4 to 20
1. POSCAP capacitors have some characteristics which are very similar to tantalum.
ESR (m )
25 to 35
25 to 40
50 to 200
30 to 200
160 to 650
Inductor
The inductor value is very important as it fixes the ripple current flowing through the output
I capacitor. The ripple current is usually fixed at 20 - 40% of I
Omax
Omax
, which is 0.6 - 1.2 A with
= 3 A. The approximate inductor value is obtained using the following formula:
Equation 20
L =
V –
V
I
T
ON where T
ON
V
OUT
is the ON time of the internal switch, given by D ꞏ T. For example, with
= 3.3 V, V
IN
= 12 V and I
O
= 0.9 A, the inductor value is about 12 µH. The peak current through the inductor is given by:
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