+2.7V to +5.25V, Low-Power, 8-Channel,

Serial 10-Bit ADCs

______________________________________________________________________________________

17

Software Power-Down

Software power-down is activated using bits PD1 and PD0

of the control byte. As shown in Table 5, PD1 and PD0

also specify the clock mode. When software shutdown is

asserted, the ADC operates in the last specified clock

mode until the conversion is complete. Then the ADC

powers down into a low quiescent-current state. In internal

clock mode, the interface remains active and conversion

results may be clocked out after the MAX148/MAX149

enter a software power-down.

The first logical 1 on DIN is interpreted as a start bit

and powers up the MAX148/MAX149. Following

the start bit, the data input word or control byte also

determines clock mode and power-down states. For

example, if the DIN word contains PD1 = 1, then the

chip remains powered up. If PD0 = PD1 = 0, a

power-down resumes after one conversion.

Hardware Power-Down

Pulling SHDN low places the converter in hardware

power-down (Table 6). Unlike software power-down

mode, the conversion is not completed; it stops coin-

cidentally with SHDN being brought low. SHDN also

controls the clock frequency in internal clock mode.

Letting SHDN float sets the internal clock frequency to

1.8MHz. When returning to normal operation with SHDN

Pulling SHDN high sets internal clock frequency to

225kHz. This feature eases the settling-time requirement

for the reference voltage. With an external reference, the

MAX148/MAX149 can be considered fully powered up

within 2µs of actively pulling SHDN high.

Power-Down Sequencing

The MAX148/MAX149 auto power-down modes can

save considerable power when operating at less than

maximum sample rates. Figures 13, 14a, and 14b show

the average supply current as a function of the sam-

pling rate. The following discussion illustrates the vari-

ous power-down sequences.

Lowest Power at up to 500

Conversions/Channel/Second

The following examples show two different power-down

sequences. Other combinations of clock rates, compen-

sation modes, and power-down modes may give lowest

power consumption in other applications.

Figure 14a depicts the MAX149 power consumption for

one or eight channel conversions utilizing full power-

down mode and internal-reference compensation. A

0.01µF bypass capacitor at REFADJ forms an RC filter

with the internal 20k

Ω reference resistor with a 0.2ms

time constant. To achieve full 10-bit accuracy, 8 time

constants or 1.6ms are required after power-up.

Waiting this 1.6ms in FASTPD mode instead of in full

power-up can reduce power consumption by a factor

of 10 or more. This is achieved by using the sequence

shown in Figure 15.

10,000

1

0.1

1

AVERAGE SUPPLY CURRENT

vs. CONVERSION RATE

(USING FASTPD)

1000

100

10

CONVERSION RATE (Hz)

100

1M

10

1k

10k

100k

CODE = 1010101000

8 CHANNELS

1 CHANNEL

Figure 14c. Typical Reference-Buffer Power-Up Delay vs. Time

in Shutdown

2.0

0

0.001

0.01

0.1

1

10

TYPICAL REFERENCE-BUFFER POWER-UP

DELAY vs. TIME IN SHUTDOWN

1.5

1.0

0.5

TIME IN SHUTDOWN (sec)

Figure 14b. MAX149 Supply Current vs. Conversion Rate,

FASTPD