AN2317 datasheet(23/27 Pages) STMICROELECTRONICS

23 / 27 page

AN2317 - Application

External Circuit Design

23/27

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Capacitor filter (C3, Filter 2)

Capacitor C3 is used as a filter. Considering load RL, the size of C3 must satisfy the

requirements expressed in

Equation 36:

Equation 36

In fact, considering that the charge stored in the capacitor is:

Equation 37

where,

IL = the load current, and

T = the AC sine wave period, and

the output ripple voltage is expressed as:

Equation 38

then the capacitor C value can be calculated by using a fixed voltage ripple value:

Equation 39

then, fixing our ripple to

ΔV=200mV we can calculate C value accordingly.

For the purposes of this application, C is calculated as follows:

Equation 40

The STPM01 power supply (VCC) configuration range is from 3.3V to 6V. While it

seems to be enough to change the D3 diode (see

Equation 34) from the previously

selected ZMM SOD 80*5.1V G Zener Diode, if the output current is too high, then the

C2 value must be reduced.

Note:

Usually it is not necessary to use resistor R1 in the circuit.

LC15

∼

()T

LTQ

ΔV

----

ΔV

--------

10mA

200mV 50Hz

⋅

--------------------------------------

1000

μF

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