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TC105

PFM/PWM Step-Down DC/DC Controller

Microchip Technology 

TC105

4.0 APPLICATIONS

4.1 Circuit Examples

Figure 4-3 shows a TC105 using a PNP switching

transistor (Zetex FZT749) that has an hFE of 180 and

VCESAT of 100 mV at IC = 1A. Other high beta transis-

tors can be used, but the values of RB and CB may

need adjustment if hFE is significantly different from that

of the FZT749.

The circuit of Figure 4-4 utilizes a P-channel MOSFET

switching transistor (Silconix Si9430). This transistor is

a member of the Littlefoot™ family of small outline

MOSFETs.

FIGURE 4-1:

SOFT START CIRCUIT

RSS

470K

VIN

+ CSS

4 – 0.033µF

SHDN/SS

TC105

Shutdown Not Used

+

4–

SHDN/SS

TC105

RSS

470K

CSS

0.033µF

SHDN

OFF ON

(From System

Control Logic)

FIGURE 4-2:

P

VIN

Shutdown Used

EXTERNAL TRANSISTOR CONNECTION

L

Q

D

+

–

CL

VOUT VIN

L

D

+

– CL

VOUT

TC105

EXT

1

+

RB – CB

TC105

EXT

1

P-Channel MOSFET

PNP Bipolar Transistor

DS21349B-page 6

© 2002 Microchip Technology Inc.

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