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参数资料
| 型号： | TC574902ECTRT |
| 元件分类： | 模拟信号调理 |
| 英文描述： | SPECIALTY ANALOG CIRCUIT, PDSO5 |
| 封装： | SOT-23A, 5 PIN |
| 文件页数： | 5/10页 |
| 文件大小： | 64K |
| 代理商： | TC574902ECTRT |
4
LINEAR REGULATOR CONTROLLER
TC57 Series
TC57-1
10/6/99
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 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.
Transistor Selection
The PNP pass transistor must have satisfactory power
dissipation, current gain, and collector current specifications
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:
ILOAD(MAX) = 25 mA x hFE(MIN)
The transistor’s actual power dissipation (PD) is equal to
the maximum load current times the maximum input/output
voltage differential, or:
PD = ILOAD(MAX) x (VIN(MAX) –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
100 mA. It is packaged in a SOT223 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 2a).
Base-Current Limiting Resistor (Figure 2)
Base current limiting resistor RB can be estimated using:
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 cur-
rent 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. Substi-
tuting these values into the equation above results in an RB
value of 704
(closest standard value = 680).
Pull-Up Resistor and Output Capacitor (Figure 2)
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.
Recommended values for this resistor are between 20 K
and 47 K
. A Tantalum output capacitor of at least 10 F
must be used to guarantee stability. Higher values decrease
output noise and eliminate power-on overshoot, but extend
power-up times. Table 2a lists several capacitor choices.
Input Capacitor (Figure 2)
The addition of an input capacitor further reduces output
noise, and negates the effects of power supply input imped-
ance. A 10
F(min) Tantalum capacitor is recommended.
Shutdown Mode
The TC57 enters a low power shutdown mode when the
shutdown input (SHDN) is low. During shutdown, the regu-
lator is disabled, the output capacitor is discharged through
the load, and supply current to the TC57 decreases to less
than 1
A. Normal operation resumes when SHDN is brought
high. If the shutdown mode is not used, SHDN should be tied
to VIN.
RB =
hFE(VIN – VBE)
IOUT
Equation 1.
Equation 2.
Equation 3.
Figure 2. 3.3V, 1A Regulator Using 5V Supply Input
TC57
VOUT
GND
SHDN
VIN
EXT
CIN
10
F
(Tantalum)
FZT749
Q1
RB
680
COUT
10
F
(Tantalum)
VIN = 5V
RBE
33K
1
2
3
4
5
2
ON OFF
VOUT = 3.3V
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