ADMCF340 datasheet(23/40 Pages) AD

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ADMCF340

–23–

AUXILIARY PWM TIMERS

Overview

The ADMCF340 provides two variable frequency, variable duty

cycle, 16-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 circuits in

a typical motor control system such as power factor corrected

front-end converters or other switching power converters. Alterna-

tively, by addition of a suitable filter network, the auxiliary PWM

output signals can be used as simple single-bit digital-to-analog

converters, which is shown in Figure 16. The auxiliary PWM

system of the ADMCF340 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

auxiliary PWM system in the independent 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

16-bit AUXTM0 Register sets the switching frequency of the

signal at the AUX0 output pin. Similarly, the 16-bit AUXTM1

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:

AUXTM

AUX

21 1

=×

() ×

=×

() ×

Since the values in both AUXTM0 and AUXTM1 can range

from 0 to 0xFFFF, the achievable switching frequency of the

auxiliary PWM signals may range from 152.59 Hz to 10 MHz

for a CLKOUT frequency of 20 MHz. The on-time of the two

auxiliary PWM signals is programmed by the two 16-bit AUXCH0

and AUXCH1 Registers, according to:

AUX

AUXCH

AUX

AUXCH

ON,

=×

() ×

=×

() ×

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 17(a). When Bit 8 of the MODECTRL

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:

AUXTM

OFFSET

=×

() ×

21 1

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 17(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 16 bits only at the minimum switching frequency

(AUXTM0 = AUXTM1 = 65535 in independent mode,

AUXTM0 = 65535 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

registers AUXTM0 and AUXTM1 default to 0xFFFF, corre-

sponding to the minimum switching frequency and zero offset.

The on-time registers AUXCH0 and AUXCH1 default to 0x0000.

AUXPWM

R1 = R2 = 13k

C1 = C2 = 10nF

Figure 16. Auxiliary PWM Output Filter

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

Table IX. Fundamental Characteristics of Auxiliary PWM Timer of ADMCF340

Parameter

Test Conditions

Min

Typ

Max

Unit

Resolution

Bits

PWM Frequency

10 MHz CLKIN

0.152

MHz

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