TC57 datasheet(4/16 Pages) MICROCHIP

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TC57

DS21437B-page 4

3.0

DETAILED DESCRIPTION

The TC57 series of precision low dropout regulator

controllers

use

external

PNP

transistor

accommodate a wide range of output currents. A series

resistor (RB) limits the maximum base current drawn

from the PNP transistor. Limiting the base drive both

determines the regulator’s output current capability, as

well as limits ground current when the device is

operated in dropout. The PNP transistor’s VCE(SAT) is

the only factor limiting dropout voltage.

FIGURE 3-1:

TEST CIRCUIT

3.1

Transistor Selection

The PNP pass transistor must have satisfactory power

dissipation, current gain, and collector current specifi-

cations to suit the application at hand. The maximum

output current the circuit can deliver is influenced by

hFE. The highest guaranteed output current is given by:

EQUATION 3-1:

The transistor’s actual power dissipation (PD) is equal

to the maximum load current times the maximum input/

output voltage differential, or:

EQUATION 3-2:

The ideal transistor has a minimum hFE of 100, and a

VCE(SAT) of less than 0.6V at full output current. For

example, the Zetex FZT749 has an hFE of 170 at a

collector current of 1A, and a guaranteed VCE(SAT) of

0.3V at a base current of 100mA. It is packaged in a

SOT-223 and is recommended for use with the TC57.

Other transistors are also suitable, depending on the

required input and output voltages and output current

(Table 3-1).

3.2

Base-Current Limiting Resistor

Base current limiting resistor RB canbeestimated

using:

EQUATION 3-3:

Where:

hFE is the current gain of the pass transistor

VIN Is the input voltage (in volts)

VBE is the base-emitter voltage at the desired

output current (in volts)

IOUT is the output current (in Amps)

For example, assume a desired continuous output

current of 1.0A, an input voltage of 5V, and an FZT749

pass transistor. The FZT749 has a typical hFE of 170,

and a VBE of 0.8V; both specified at a collector current

of 1.0A. Substituting these values into the equation

above results in an RB value of 704Ω (closest standard

value = 680

Ω).

3.3

Pull-Up Resistor and Output

Capacitor

A pull-up resistor (RBE, installed between the base and

emitter of the pass transistor) facilitates rapid turn-off of

the pass transistor in the event of a sudden decrease in

load (Figure 3-2). Recommended values for this

resistor are between 20K

Ω and 47KΩ. A Tantalum

output capacitor of at least 10

µFmust beusedto

guarantee stability. Higher values decrease output

noise and eliminate power-on overshoot, but extend

power-up times. Table 3-1 lists several capacitor

choices.

FIGURE 3-2:

3.3V, 1A REGULATOR

USING 5V SUPPLY INPUT

TC573002ECT

VOUT

GND

SHDN

VIN

EXT

CIN

10µF

(Tantalum)

FZT749

680

Ω

µF

(Tantalum)

VIN

RBE

33K

OFF

VOUT

ILOAD(MAX) =25mA x hFE(MIN)

PD ≈ ILOAD(MAX) x(VIN(MAX) –VOUT(MIN))

hFE (VIN –VBE)

IOUT

RB =

TC57

VOUT

SHDN

VIN

EXT

CIN

10µF

(Tantalum)

FZT749

680

Ω

COUT

µF

(Tantalum)

VIN = 5V

RBE

33K

Ω

OFF

VOUT = 3.3V

GND

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