EVALA5970D STMicroelectronics, EVALA5970D Datasheet - Page 17

• Manufacturer Part Number: EVALA5970D
• Description: Power Management Modules & Development Tools EVAL BOARD A5970D Auto Grade
• Manufacturer: STMicroelectronics
• Type: DC/DC Switching Converters, Regulators & Controllersr

Specifications of EVALA5970D

• Input Voltage: 4.4 V to 36 V
• Output Voltage: 1.235 V to 35 V
• Board Size: 23 mm x 20 mm
• Product: Power Management Modules
• Supply Current: 1 A
• Dimensions: 23 mm x 20 mm
• For Use With/related Products: AN1330
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

AN1330
5
5.1
Application information
Component selection
●
The input capacitor must be able to withstand the maximum input operating voltage and the
maximum RMS input current.
Since 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 has to 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
Where η is the expected system efficiency, D is the duty cycle and I
This function reaches its maximum value at D = 0.5 and the equivalent RMS current is equal
to I
Equation 18
Where V
internal PDMOS. Considering the range D
I
●
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.
RMS
O
Input capacitor
–
–
–
Output capacitor
divided by 2 (considering η = 1). The maximum and minimum duty cycles are:
going through the input capacitor. Capacitors that can be considered are:
F
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). The drawback is the considerably high cost.
Tantalum capacitors: 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.
is the freewheeling diode forward voltage and V
D
MAX
=
--------------------------------------- -
V
INMIN
V
OUT
I
–
RMS
+
V
V
SW
F
=
I
O
•
MIN
and
D 2
to D
–
----------------- -
MAX
•
η
D
, it is possible to determine the max
2
D
+
MIN
SW
D
-------
η
=
2
the voltage drop across the
-----------------------------------------
V
INMAX
V
Application information
OUT
O
the output DC current.
+
–
V
V
F
SW
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