AND8031/D

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5

60

50

40

30

20

10

Figure 10. NCP100 with PNP

Figure 11. NCP100 without PNP

Gain

Phase

–20

–40

–10

–30

0

Gain

Phase

180

90

0

–90

–180

180

90

0

–90

–180

10

100

1 k

10 k

100 k

Frequency (Hz)

10

100

1 k

10 k

100 k

Frequency (Hz)

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.