TLV2556

SLAS355A – DECEMBER 2001 – REVISED SEPTEMBER 2002

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PRINCIPLES OF OPERATION

chip-select input (CS)

CS enables and disables the device. During normal operation, CS should be low. Although the use of CS is not

necessary to synchronize a data transfer, it can be brought high between conversions to coordinate the data

transfer of several devices sharing the same bus.

When CS is brought high, the serial-data output is immediately brought to the high-impedance state, releasing

its output data line to other devices that may share it. After an internally generated debounce time, I/O CLOCK

is inhibited, thus preventing any further change in the internal state.

When CS is subsequently brought low again, the device is reset. CS must be held low for an internal debounce

time before the reset operation takes effect. After CS is debounced low, I/O CLOCK must remain inactive (low)

for a minimum time before a new I/O cycle can start.

CS can interrupt any ongoing data transfer or any ongoing conversion. When CS is debounced low long enough

before the end of the current conversion cycle, the previous conversion result is saved in the internal output

buffer and shifted out during the next I/O cycle.

When CS is held low continuously for multiple cycles, the first data bit of the newly completed conversion occurs

on DATA OUT on the rising edge of EOC or falling edge of INT. Note that the first cycle in the series still requires

a transition of CS from high to low. When a new conversion is started after the last falling edge of I/O CLOCK,

EOC goes low and the serial output is forced low until EOC goes high again.

When CS is toggled between conversions, the first data bit occurs on DATA OUT on the falling edge of CS. On

each subsequent falling edge of I/O CLOCK after the first data bit appears, the data is changed to the next bit

in the serial conversion result until the required number of bits has been output.

power-down features

When command (D7–D4) 1110b is clocked into the input data register during the first four I/O CLOCK cycles,

the software power-down mode is selected. Software power down is activated on the falling edge of the fourth

I/O CLOCK pulse.

During software power down, all internal circuitry is put in a low-current standby mode. The internal reference

(if being used) is powered down. No conversion is performed. The internal output buffer keeps the previous

logic remains active so the current I/O cycle must be completed even when the power-down mode is selected.

Upon power-on reset and before the first I/O cycle, the converter normally begins in the power-down mode. The

device remains in the software power-down mode until a valid input address (other than command 1110b) is

clocked in. Upon completion of that I/O cycle, a normal conversion is performed with the results being shifted

out during the next I/O cycle. If using the internal reference, care must be taken to allow the reference to power

on completely before a valid conversion can be performed. It requires 1 ms to resume from a software power

down.

The ADC also has an auto power-down mode. This is transparent to users. The ADC goes into auto power down

within 1 I/O CLOCK cycle after the conversion is complete and resumes, with a small delay after an active CS

is sent to the ADC. This mode keeps built-in reference so resumption is fast enough to be used between cycles.

analog MUX

The 11 analog inputs, three internal voltages, and power-down mode are selected by the input multiplexer

according to the input addresses shown in Table 2. The input multiplexer is a break-before-make type to reduce

input-to-input noise rejection resulting from channel switching. Sampling of the analog inputs starts on the falling

edge of the fourth I/O CLOCK and continues for the remaining I/O CLOCK pulses. The sample is held on the

falling edge of the last I/O CLOCK pulse. The three internal test inputs are applied to the multiplexer, then

sampled and converted in the same manner as the external analog inputs. The first conversion after the device

has returned from the power-down state may not read accurately due to internal device settling.