TC38C46 datasheet(8/10 Pages) TELCOM

Part #	TC38C46

Description	CMOS CURRENT MODE PWM CONTROLLER

PDF	10 Pages

Scroll/Zoom	100%

Manufacturer	TELCOM [TelCom Semiconductor, Inc]

Direct Link	http://www.telcom-semi.com

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8 / 10 page

4-108

TELCOM SEMICONDUCTOR, INC.

CMOS CURRENT MODE

PWM CONTROLLERS

TC18C46

TC28C46

TC38C46

Figure 6. Oscillator Circuit

0.5V 3.5V

ICHARGE

3.5V

2mA

DISCHARGE

CURRENT

SYNC

VDD

3.5V

PIN 8

PIN 10

TCHG

OUTPUT DEAD TIME (TD)

–

1/FO

Selection of Timing Capacitor and Resistor

(CT & RT)

First determine the frequency of operation FO and

the desired " dead" time", TD (see Fig.6 graph).

We need to choose RT and CT.

During discharge (TD), the discharge current pulls

2 m A out of CT, minus the ICHG that keeps trying to

charge it:

2 m A - ICHG = CT

= 3.0 Volts

∆V ,where ∆V= 3.0 Volts

The current mirror in Fig. 6 , shows:

ICHG = 3.5 V

Also,

ICHG = CT

Where,

and

∆T = TCHG

∆T

∆T= T

CT = TD

(.002 - 3.5)

Knowing TD choosing RT permits calculation of CT .

Substituting and rearranging,

∆V

∆V = 3.5 - 0.5

∆T

TC38C46 Datasheet(PDF) 8 Page - TelCom Semiconductor, Inc

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