參數(shù)資料
型號(hào)： TC573302ECTTR
廠商： Microchip Technology
文件頁(yè)數(shù)： 4/16頁(yè)
文件大?。?/td> 462K
描述： IC REG CTRLR SGL 3.3V SOT23A-5
標(biāo)準(zhǔn)包裝： 3,000
類(lèi)型： 正，固定式
輸出數(shù)： 1
輸出電壓： 3.3V
電流 - 電源： 50µA
輸入電壓： 2.7 V ~ 8 V
工作溫度： -40°C ~ 85°C
封裝/外殼： SC-74A，SOT-753
供應(yīng)商設(shè)備封裝： SOT-23-5
包裝： 帶卷 (TR)
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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