L7986TA datasheet(21/43 Pages) STMICROELECTRONICS

Part #	L7986TA

Description	3 A step-down switching regulator

PDF	43 Pages

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Manufacturer	STMICROELECTRONICS [STMicroelectronics]

Direct Link	http://www.st.com

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21 / 43 page

L7986TA

Application information

Doc ID 022098 Rev 3

21/43

Figure 9.

Type III compensation network

In Figure 10 the Bode diagram of the PWM and LC filter transfer function (GPW0 · GLC(f))

and the open-loop gain (GLOOP(f) = GPW0 · GLC(f) · GTYPEIII(f)) are drawn.

Figure 10.

Open-loop gain: module Bode diagram

The guidelines for positioning the poles and the zeroes and for calculating the component

values can be summarized as follows:

Choose a value for R1, usually between 1 kΩ and 5 kΩ.

Choose a gain (R4/R1) in order to have the required bandwidth (BW), that means:

Equation 24

where K is the feed-forward constant and 1/K is equal to 18.

Calculate C4 by placing the zero at 50% of the output filter double pole frequency (fLC):

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L7986TA Datasheet(PDF) 21 Page - STMicroelectronics