Datasheet

TLV2553
SLAS354B – SEPTEMBER 2001 – REVISED SEPTEMBER 2002
25
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PRINCIPLES OF OPERATION
chip-select input (CS
) (continued)
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. Note that the first cycle in the series still requires a transition 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. No conversions is
performed. The internal output buffer keeps the previous conversion cycle data results, provided that all digital
inputs are held above V
CC
– 0.5 V or below 0.5 V. The I/O 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 or 1111b) 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.
The ADC also has an auto power-down mode. This is transparent to users. The ADC gets 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. The 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 input 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.