AN2495 datasheet(20/36 Pages) STMICROELECTRONICS

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Frequency response and loop compensation

AN2495

20/36

Doc ID 13127 Rev 5

From the L6565 datasheet we know that Icomp = 5 mA (source current) and Rcomp = 15 kΩ.

and can obtain:

Equation 39

R24 = 1.5 kΩ is a good choice. It is good practice to put a 3.3 kΩ resistor (R23 = 3.3 kΩ) in

parallel to the photodiode.

The resistive partition must be set with high precision according to the following equation:

Equation 40

There is no next close commercial value so it is a good idea to put two 47 k

Ω resistors

(R19 = R20 = 47 kΩ) in series.

Now C11 (Ccomp), C13 (CF) and R21 (RF) must be set in order to satisfy frequency response

requirements.

A good choice is to set the pole of G2(s) so as to cancel the low frequency zero of G1(s):

Equation 41

The next close commercial value for C11 = 2.2 nF has been chosen.

Similarly C13 and R21 have been determined by setting the corresponding zero close to the

pole of G1(s) and imposing the open loop gain to cross the 0 dB axis only once at

f = fc = 10 kHz:

Equation 42

In such a way we ideally get a phase margin of 90 degrees and an adequate closed-loop

bandwidth.

out

ref

diode

–

()

–

co mp

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

24V

3.5V

–

5mA

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

4.1k

high

out

re f

–

ref

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

•

23.2k

π R

co mp

•

co mp

•

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

5kHz

com p

2.12nF

≅

⇒

() C

•

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

400 Hz

10nF

⇒

()

ω 2π 10

– rad

•

15k

≅

⇒

⎩

⎪

⎨

⎪

⎧

/sec

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