NCP431A データシートの表示（PDF） - ON Semiconductor
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NCP431A Datasheet PDF : 16 Pages
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NCP431A, SC431A, NCP431B, NCP432B Series
APPLICATIONS INFORMATION
The NCP431/NCP432 is a programmable precision
reference which is used in a variety of ways. It serves as a
reference voltage in circuits where a non−standard reference
voltage is needed. Other uses include feedback control for
driving an optocoupler in power supplies, voltage monitor,
constant current source, constant current sink and series pass
regulator. In each of these applications, it is critical to
maintain stability of the device at various operating currents
and load capacitances. In some cases the circuit designer can
estimate the stabilization capacitance from the stability
boundary conditions curve provided in Figure 17. However,
these typical curves only provide stability information at
specific cathode voltages and at a specific load condition.
Additional information is needed to determine the
capacitance needed to optimize phase margin or allow for
process variation.
A simplified model of the NCP431/NCP432 is shown in
Figure 32. When tested for stability boundaries, the load
resistance is 150 W. The model reference input consists of an
input transistor and a dc emitter resistance connected to the
device anode. A dependent current source, Gm, develops a
current whose amplitude is determined by the difference
between the 1.78 V internal reference voltage source and the
input transistor emitter voltage. A portion of Gm flows
through compensation capacitance, CP2. The voltage across
CP2 drives the output dependent current source, Go, which
is connected across the device cathode and anode.
Model component values are:
Vref = 1.78 V
Gm = 0.3 + 2.7 exp (−IC/26 mA)
where IC is the device cathode current and Gm is in mhos
Go = 1.25 (Vcp2) mmhos.
Resistor and capacitor typical values are shown on the
model. Process tolerances are ±20% for resistors, ±10% for
capacitors, and ±40% for transconductances.
An examination of the device model reveals the location
of circuit poles and zeroes:
P1 +
1
+
1
+ 7.96 kHz
2pRGMCP1 2p @ 1.0M @ 20 pF
P2 +
1
+
1
+ 60 kHz
2pRP2CP2 2p @ 10M @ 0.265 pF
Z1 +
1
+
1
+ 500 kHz
2pRZ1CP1 2p @ 15.9k @ 20 pF
In addition, there is an external circuit pole defined by the
load:
PL
+
1
2pRLCL
Also, the transfer dc voltage gain of the NCP431 is:
G + GMRGMGoRL
Example 1:
IC=10 mA, RL= 230 W,CL= 0. Define the transfer gain.
The DC gain is:
G + GMRGMGoRL + (2.138)(1.0M)(1.25m)(230)
+ 615 + 56 dB
Loop gain + G 8.25k + 218 + 47 dB
8.25k ) 15k
The resulting transfer function Bode plot is shown in
Figure 33. The asymptotic plot may be expressed as the
following equation:
ǒ Ǔ 1 ) jf
500 kHz
ǒ Ǔǒ Ǔ Av + 615
1 ) jf
1 ) jf
8.0 kHz
60 kHz
The Bode plot shows a unity gain crossover frequency of
approximately 600 kHz. The phase margin, calculated from
the equation, would be 55.9°. This model matches the
Open−Loop Bode Plot of Figure 14. The total loop would
have a unity gain frequency of about 300 kHz with a phase
margin of about 44°.
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