ADMC326YN datasheet(18/31 Pages) AD

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ADMC326

–18–

REV. A

VREF

3.5V

0.3V

MINIMUM

RAMP

TARGET

RAMP

Figure 15. Current Ramp

ADC Registers

The configuration of all registers of the ADC System is shown

at the end of the data sheet.

AUXILIARY PWM TIMERS

Overview

The ADMC326 provides two variable frequency, variable duty

cycle, 8-bit, auxiliary PWM outputs that are available at the

AUX1 and AUX0 pins when enabled. These auxiliary PWM

outputs can be used to provide switching signals to other cir-

cuits in a typical motor control system such as power factor

corrected front-end converters or other switching power con-

verters. Alternatively, by addition of a suitable filter network,

the auxiliary PWM output signals can be used as simple single-

bit digital-to-analog converters.

The auxiliary PWM system of the ADMC326 can operate in

two different modes: independent mode, or offset mode. The

operating mode of the auxiliary PWM system is controlled by Bit 8

of the MODECTRL register. Setting Bit 8 of the MODECTRL

mode. In this mode, the two auxiliary PWM generators are

completely independent and separate switching frequencies and

duty cycles may be programmed for each auxiliary PWM output.

In this mode, the 8-bit AUXTM0 register sets the switching fre-

quency of the signal at the AUX0 output pin. Similarly, the

8-bit AUXTM1 register sets the switching frequency of the sig-

nal at the AUX1 pin. The fundamental time increment for the

auxiliary PWM outputs is twice the DSP instruction rate (or

2 tCK) and the corresponding switching periods are given by:

TAUX0 = 2

× (AUXTM0 + 1) × t

TAUX1 = 2

× (AUXTM1 + 1) × t

Since the values in both AUXTM0 and AUXTM1 can range

from 0 to 0xFF, the achievable switching frequency of the auxil-

iary PWM signals may range from 39.1 kHz to 10 MHz for a

CLKOUT frequency of 20 MHz.

The on-time of the two auxiliary PWM signals is programmed

by the two 8-bit AUXCH0 and AUXCH1 registers, according to:

TON, AUX0 = 2

× (AUXCH0) × t

TON, AUX1 = 2

× (AUXCH1) × t

so that output duty cycles from 0% to 100% are possible. Duty

cycles of 100% are produced if the on-time value exceeds the

period value. Typical auxiliary PWM waveforms in independent

mode are shown in Figure 16(a).

When Bit 8 of the MODECTRL register is cleared, the auxiliary

PWM channels are placed in offset mode. In offset mode, the

switching frequency of the two signals on the AUX0 and AUX1

pins are identical and controlled by AUXTM0 in a manner

similar to that previously described for independent mode. In

addition, the on times of both the AUX0 and AUX1 signals are

controlled by the AUXCH0 and AUXCH1 registers as before.

However, in this mode the AUXTM1 register defines the offset

time from the rising edge of the signal on the AUX0 pin to that

on the AUX1 pin according to:

TOFFSET = 2

× (AUXTM1 + 1) × t

For correct operation in this mode, the value written to the

AUXTM1 register must be less than the value written to the

AUXTM0 register. Typical auxiliary PWM waveforms in offset

mode are shown in Figure 16(b). Again, duty cycles from 0% to

100% are possible in this mode.

In both operating modes, the resolution of the auxiliary PWM

system is eight bits only at the minimum switching frequency

(AUXTM0 = AUXTM1 = 255 in independent mode, AUXTM0

= 255 in offset mode). Obviously, as the switching frequency is

increased, the resolution is reduced.

Values can be written to the auxiliary PWM registers at any time.

However, new duty cycle values written to the AUXCH0 and

AUXCH1 registers only become effective at the start of the next

cycle. Writing to the AUXTM0 or AUXTM1 registers causes the

internal timers to be reset to 0 and new PWM cycles to begin.

By default following a reset, Bit 8 of the MODECTRL register

is cleared, thus enabling offset mode. In addition, the registers

AUXTM0 and AUXTM1 default to 0xFF, corresponding to the

minimum switching frequency and zero offset. The on-time reg-

isters AUXCH0 and AUXCH1 default to 0x00.

Auxiliary PWM Interface, Registers and Pins

The registers of the auxiliary PWM system are summarized at

the end of the data sheet.

AUX0

AUX1

(AUXTM0 + 1)

(AUXTM1 + 1)

AUXCH1

AUXCH0

(a) Independent Mode

AUX0

AUX1

(AUXTM1 + 1)

(AUXTM0 + 1)

AUXCH0

(AUXTM0 + 1)

AUXCH1

(b) Offset Mode

Figure 16. Typical Auxiliary PWM Signals.

(All Times in Increments of tCK)

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