TC572502 Microchip Technology, TC572502 Datasheet - Page 4

• Manufacturer Part Number: TC572502
• Description: (TC572502/3002/3302) Line Regulator Controller
• Manufacturer: Microchip Technology

TC57
DS21437B-page 4
3.0
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
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 V
the only factor limiting dropout voltage.
FIGURE 3-1:
3.1
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
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 h
V
example, the Zetex FZT749 has an h
collector current of 1A, and a guaranteed V
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).
FE
(Tantalum)
CE(SAT)
. The highest guaranteed output current is given by:
V
10µF
IN
C
P
IN
D
DETAILED DESCRIPTION
Transistor Selection
of less than 0.6V at full output current. For
≈ I
B
I
OFF
) limits the maximum base current drawn
LOAD(MAX)
LOAD(MAX)
ON
R
33K
BE
2
5
FZT749
EXT
V IN
SHDN
TC573002ECT
TEST CIRCUIT
Q1
= 25 mA x h
x (V
4
R
680Ω
B
GND
1
IN(MAX)
V
OUT
3
– V
FE(MIN)
FE
OUT(MIN)
FE
C
10µF
(Tantalum)
L
of 100, and a
of 170 at a
CE(SAT)
CE(SAT)
)
R
V
OUT
L
of
is
3.2
Base current limiting resistor R
using:
EQUATION 3-3:
Where:
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
and a V
of 1.0A. Substituting these values into the equation
above results in an R
value = 680Ω).
3.3
A pull-up resistor (R
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 20KΩ and 47KΩ. A Tantalum
output capacitor of at least 10µF 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:
(Tantalum)
V
h
V
V
output current (in volts)
I
IN
OUT
10µF
FE
IN I
BE
= 5V
C
BE
IN
is the current gain of the pass transistor
Base-Current Limiting Resistor
s the input voltage (in volts)
is the base-emitter voltage at the desired
Pull-Up Resistor and Output
Capacitor
is the output current (in Amps)
of 0.8V; both specified at a collector current
OFF
ON
R
B
=
BE
B
2
1
5
33KΩ
, installed between the base and
h
3.3V, 1A REGULATOR
USING 5V SUPPLY INPUT
R
value of 704Ω (closest standard
SHDN
V
GND
FE
BE
IN
©
FZT749
2002 Microchip Technology Inc.
4
TC57
(V
EXT
Q1
I
OUT
IN
R
680Ω
V
B
– V
OUT
B
BE
3
can be estimated
)
V
C
10µF
(Tantalum)
FE
OUT
OUT
of 170,
= 3.3V