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參數(shù)資料
| 型號： | RC4391N |
| 廠商： | RAYTHEON SEMICONDUCTOR |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | Inverting and Step-Down Switching Regulator |
| 中文描述： | 0.375 A SWITCHING REGULATOR, PDIP8 |
| 文件頁數(shù)： | 11/22頁 |
| 文件大?。?/td> | 153K |
| 代理商： | RC4391N |
RC4391
PRODUCT SPECIFICATION
11
Device Shutdown
The entire device may be shut down to an extremely low cur-
rent non-operating condition by disconnecting the ground
(pin 4). This can be easily done by putting an NPN transistor
in series with ground pin and switching it with an external
signal. This switch will not affect the efficiency of operation,
but will add to and increase the reference voltage by an
amount equal to the saturation voltage of the transistor used.
A mechanical switch can also be used in series between
circuit ground and pin 4, without introducing any reference
offset.
Power Transistor Interfaces
The most important consideration in selecting an external
power transistor is the saturation voltage at I
C
= I
MAX
.
The lower the saturation voltage is, the better the efficiency
will be. Also, a higher beta transistor requires less base drive
and therefore less power will be.
Also, a higher beta transistor requires less base drive and
therefore less power will be consumed in driving it, improv-
ing efficiency losses in the interface. The part numbers given
in the following applications are recommended, but other
types may be more appropriate depending on voltage and
power levels.
When troubleshooting external power transistor circuits,
ensure that clean, sharp-edged waveforms are driving the
interface and power transistors. Monitor these waveforms
with an oscilloscop—disconnect the inductor, and tie the
V
FB
input (pin 8) high through a 10K resistor. This will
cause the regulator to pulse at maximum duty cycle without
drawing excessive inductor currents. Check for expected on
time and off time, and look for slow rise times that might
cause the power transistor to enter its linear operating region.
The following external power transistor circuits may demand
some adjustment to resistor values to satisfy various power
levels and input/output voltages. C
X
and L
X
values must be
selected according to the design equations (pages 2-213 and
2-214).
Inverting Medium Power Application
Figure 8 is a schematic of an inverting medium power supply
(250mW to 1W) using an external PNP switch transistor.
Supply voltage is applied to the IC via R3: when the internal
switch transistor is turned on current through R4 is also
drawn through R3; creating a voltage drop from base to
emitter of the external switch transistor. This drop turns on
the external transistor.
Voltage pulses on the supply lead (pin 6) do not affect circuit
operation because the internal reference and bias circuitry
have good supply rejection capabilities. A power Schottky
diode is used for higher efficiency.
Inverting High Power Application
For higher power applications (500mW to 5W), refer to
Figure 9. This circuit uses an extra external transistor to pro-
vide well controlled drive current in the correct phase to the
power switch transistor. The value of R3 sets the drive
current to the switch by making the interface transistor act as
a current source. R4 and R5 must be selected such that the
RC time constant of R4 and the base capacitance of Q2 do
not slow the response time (and affect duty cycle), but not so
low in value that excess power is consumed and efficiency
suffers. The resistor values chosen should be proportional to
the supply voltage (values shown are for +5V).
Step-Down Power Applications
Figures 16 and 17 show medium and high power interfaces
modified to perform step-down functioning. The design
Figure 14. Inverting Medium Power Application
65-2476
+V
4391
s
Cx
GND
3
4
Lx
5
5
0.1
μ
F
R2
62 k
7
6
V
REF
FB
V
150 pF
+5V
R3
1k
220
μ
H
Q1
2N3635
R4
50
Motorola
MBR030
R1
1.2 M
100
μ
F
-24V
C1
Cx
C
F
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