TC57
DS21437B-page 4
2002 Microchip Technology Inc.
3.0 DETAILED DESCRIPTION
The TC57 series of precision low dropout regulator
controllers use an external PNP transistor to
accommodate a wide range of output currents. A series
resistor (R
B
) limits the maximum base current drawn
from the PNP transistor. Limiting the base drive both
determines the regulators output current capability, as
well as limits ground current when the device is
operated in dropout. The PNP transistors V
CE(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
h
FE
. The highest guaranteed output current is given by:
EQUATION 3-1:
The transistors 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 h
FE
of 100, and a
V
CE(SAT)
of less than 0.6V at full output current. For
example, the Zetex FZT749 has an h
FE
of 170 at a
collector current of 1A, and a guaranteed V
CE(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 R
B
can be estimated
using:
EQUATION 3-3:
Where:
h
FE
is the current gain of the pass transistor
V
IN
Is the input voltage (in volts)
V
BE
is the base-emitter voltage at the desired
output current (in volts)
I
OUT
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 h
FE
of 170,
and a V
BE
of 0.8V; both specified at a collector current
of 1.0A. Substituting these values into the equation
above results in an R
B
value of 704& (closest standard
value = 680&).
3.3
Pull-Up Resistor and Output
Capacitor
A pull-up resistor (R
BE
, 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& and 47K&. A Tantalum
output capacitor of at least 10礔 must be used to
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
V
OUT
GND
SHDN
V
IN
EXT
C
IN
10礔
(Tantalum)
FZT749
Q1
R
B
680&
C
L
10礔
(Tantalum)
R
L
V
IN
R
BE
33K
1
2
ON
OFF
3
4
5
V
OUT
I
LOAD(MAX)
= 25 mA x h
FE(MIN)
P
D
H I
LOAD(MAX)
x (V
IN(MAX)
V
OUT(MIN)
)
h
FE
(V
IN
V
BE
)
I
OUT
R
B
=
TC57
V
OUT
SHDN
V
IN
EXT
C
IN
10礔
(Tantalum)
FZT749
Q1
R
B
680&
C
OUT
10礔
(Tantalum)
V
IN
= 5V
R
BE
33K&
1
2
ON
OFF
3
4
5
V
OUT
= 3.3V
GND
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