ADMC330 datasheet(10/20 Pages) AD

10 / 20 page

ADMC330

–10–

REV. 0

PWMSYNC

VIL

COMPARATOR

OUTPUT

VCMAX

TCRST

tVIL

TPWM – TCRST

Figure 6. Analog Input Block Operation

ADC Resolution

Because the operation of the ADC is intrinsically linked to the

PMW block, the effective resolution of the ADC is a function of

the PMW switching frequency. The effective ADC resolution is

determined by the rate at which the counter timer is clocked.

For a CLKIN period of tCK and a PWM period of TPWM, the

maximum count of the ADC is given by

Max Count

T PWM

tCK

For an assumed CLKIN frequency of 10 MHz, the effective

resolution of the ADC block is tabulated for various PWM

switching frequencies in Table II.

Table II. ADC Resolution Examples

PWM Frequency

Effective Resolution

(kHz)

Max Count

(Bits)

2.5

3980

≈12

2480

>11

1230

>10

535

380

External Timing Capacitor

In order to maximize the useful input voltage range and effective

resolution of the ADC, it is necessary to carefully select the

value of the external timing capacitor. For a given capacitance

value, CNOM, the peak ramp voltage is given by:

C max =

CONST

PWM – T CRST

()

NOM

where ICONST is the nominal current source value of 10.5

µA and

TCRST is the PWMSYNC pulsewidth. In selecting the capacitor

value, however, it is necessary to take into account the tolerance

of the capacitor and the variation of the current source value.

To ensure that the full input range of the ADC is utilized, it is

necessary to select the capacitor so that at the maximum capaci-

tance value and the minimum current source output, the ramp

voltage will charge to at least 3.5 V.

As a result, assuming

±10% variations in both the capacitance

and current source, the nominal capacitance value required at a

given PWM period is:

CNOM =

(0.9

× I

CONST )(T PWM – T CRST )

(1.1)(3.5)

The largest standard value capacitor that is less than this calcu-

lated value is chosen. Table III shows the appropriate standard

capacitor value to use for various PWM switching frequencies

assuming

±10% variations in both the current source and ca-

pacitor tolerances. If required, more precise control of the ramp

voltage is possible by using higher precision capacitor compo-

nents, an external current source and/or series or parallel timing

capacitor combinations.

Table III. Timing Capacitor Selection

PWM Frequency

Timing Capacitor

(kHz)

(pF)

2.5–3.0

820

3.0–3.6

680

3.6–4.3

560

4.3–5.2

470

5.2–6.2

390

6.2–7.3

330

7.3–9.0

270

9.0–10.9

220

10.9–13.2

180

13.2–15.8

150

15.8–19.6

120

19.6–23.4

100

23.4–28.2

AUXILIARY PWM TIMERS OVERVIEW

The two auxiliary PWM timers can be used to produce analog

signal outputs when configured as PWM DACs. This allows the

ADMC330 to generate a reference for power factor correction

and supply an analog reference for other systems in the applica-

tion. They can also be used as supplementary PWM outputs for

other control circuits.

The PWM timers generate two fixed frequency edge-based

variable duty cycle PWM signals. The PWM frequency is

1/256 times HCLK, or 39 kHz. The duty cycle is based on a

user-supplied 8-bit value loaded into the AUX0 and AUX1

registers.

The timer output can range from 0% to 99.6%, where the num-

ber written to the register represents the high time. The values

are updated as soon as new values are written in the registers: if

the value is smaller than the present counter value the output

goes low, otherwise it stays high.

RESET, the AUX0 and AUX1 registers are cleared to zero

and remain at zero until a new value is written.

PWM DAC Equation

The PWM output must be filtered in order to produce a low

frequency analog signal between 0 V to 4.98 V dc. For example,

a 2-pole filter with a 1.2 kHz cut off frequency will sufficiently

attenuate the PWM carrier. Figure 7 shows how the filter would

be applied.

ADMC330 Datasheet(PDF) 10 Page - Analog Devices

ADMC330 Datasheet(HTML) 10 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Datasheet Download

Link URL