AND8031 Datasheet, PDF (5/12 Pages) ON Semiconductor – Isolated Precision Regulation of a Single 1.8 Volt Output from a Universal Line Input

AND8031/D
60
180
50
Gain
90
40
0
Phase
30
–90
20
–180
10
10
100
1k
10 k
Frequency (Hz)
100 k
Figure 10. NCP100 with PNP
0
–10
–20
–30
–40
10
Gain
Phase
180
90
0
–90
–180
100
1k
10 k
Frequency (Hz)
Figure 11. NCP100 without PNP
100 k
The values for the components in Figure 7 are as follows.
Rosc = 12 Ohms, R1 = 12 k Ohms, R2 is adjustable
dependant upon U1, R3 and R4 = 100 Ohms, C2 = 0.01 uF,
and C3 = 1.0 uF.
The open loop gain and phase test injects the oscillator
signal across Rosc. The reference voltage is measured
between Rosc and R1 to ground. The test voltage is
measured at the collector of Q1 for the PNP circuit or at the
cathode of U1 for the traditional circuit. Vout is set to 4.0
volts and R2 is adjusted so the DC test voltage is at the center
of its range. R2 is the only component adjusted since it is
neglected for AC analysis.
There are negligible differences for the open loop gain and
phase of the NCP100 with C3 compared to the TLV431
without C3 in the PNP circuit. Therefore there is no penalty
with the added C3 in the NCP100 circuit.
If the PNP transistor is removed, the TLV431 open loop
responses with and without C3 are very similar with a single
pole roll–off. The pole appears to be near 4 Hz.
The NCP100 without the PNP has flat gain until a pole at
1.3 kHz due to R1 and C2. The plot remains at
–20 dB/decade and –90 degree phase margin until well
beyond 100 kHz.
The overall gain of the feedback network can be limited
due to R1, R3 and C2 in Figure 7 . Varying these components
will modify the maximum gain of the system.
Since the NCP100 provides a single pole roll–off, the
MC33363B compensation pin will not provide this function.
The feedback pin of the MC33363B is connected directly to
the reference pin. This will keep the output of the error amp
low. A diode is in series with the output of the op–amp
allowing the compensation pin to directly control the
feedback to the oscillator ramp. The compensation pin is
connected to the feedback pin via a 2.7 k Ohm resistor. The
collector of the optocoupler is also connected to the
compensation pin and the emitter is grounded. This
completes the design of the system. See Figure 14 for the
board layout of the schematic in Figure 1 and Table 1 for the
component values.
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