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| TC57_13 Datasheet, PDF (4/16 Pages) Microchip Technology – Line Regulator Controller | |||
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TC57
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 (RB) limits the maximum base current drawn
from the PNP transistor. Limiting the base drive both
determines the regulatorâs output current capability, as
well as limits ground current when the device is
operated in dropout. The PNP transistorâs VCE(SAT) is
the only factor limiting dropout voltage.
FIGURE 3-1:
TEST CIRCUIT
VIN
FZT749
Q1
VOUT
CIN
10μF
(Tantalum)
OFF
ON
RBE
33K
RB
680Ω
4
EXT
2 VIN
VOUT 3
TC573002ECT
5
SHDN
GND
1
CL
RL
10μF
(Tantalum)
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:
ILOAD(MAX) = 25 mA x hFE(MIN)
The transistorâs actual power dissipation (PD) is equal
to the maximum load current times the maximum input/
output voltage differential, or:
EQUATION 3-2:
PD ï» ILOAD(MAX) x (VIN(MAX) â VOUT(MIN))
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 can be estimated
using:
EQUATION 3-3:
RB =
hFE (VIN â VBE)
IOUT
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 704ï (closest standard
value = 680ï).
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 20Kï and 47Kï. A Tantalum
output capacitor of at least 10ïF 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:
VIN = 5V
3.3V, 1A REGULATOR
USING 5V SUPPLY INPUT
FZT749
Q1
VOUT = 3.3V
CIN
10μF
(Tantalum)
OFF ON
RBE
33KΩ
4
RB
680Ω
EXT
2 VIN
VOUT 3
1
TC57
GND
5 SHDN
COUT
10μF
(Tantalum)
DS21437C-page 4
ï£ 2001-2012 Microchip Technology Inc.
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