D

F =

1

D =

LM1771

SNVS446D – JUNE 2006 – REVISED JANUARY 2016

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7.3 Feature Description

7.3.1 Timing Opinion

setting determines the switching frequency for the application. Derivation and calculation of the dependence of

In a PWM buck switcher, Equation 1, Equation 2, and Equation 3 can be manipulated to obtain the switching

frequency. Equation 1 shows the standard duty-cycle equation given by the volts-seconds balance on the

inductor with Equation 2 and Equation 3 defining standard relationships:

(1)

(2)

(3)

Using these equations and solving for duty-cycle for Equation 4:

(4)

Frequency can now be expressed in Equation 5:

(5)

Or simply written as Equation 6:

where

•

(6)

To maintain a set frequency in an application,

see Table 1.

Table 1. Timing for All Variants

Product ID

α (V µs)

LM1771S

0.5 µs

1.65

LM1771T

1.0 µs

3.3

LM1771U

2.0 µs

6.6

Characteristics of the data sheet.

With

α being a constant regardless of the version of the LM1771 used, Equation 6 shows that the only

Table 2. The recommended frequency range of operation is 100 kHz to 1000 kHz.

10

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