Datasheet

POWER DOWN
POWER-DOWN TOPOWER-UP RESPONSE
Power-DownDuration(ms)
Power-Up
TimetoRated
Accuracy(µs)
300
250
200
150
100
50
0
0.1 1 10 100
T
A
=+25°C
V
S
PWRD
BUSY
CS
CONV
EXT/INT
DATA
DATACLK
R1
IN
GND
R2
IN
R3
IN
BUF
CAP
REF
GND
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
1
2
3
4
8
7
6
5
0.01µF1µF
1RF7604
+
Power-DownSignal
ADS8513
SLAS486C – JUNE 2007 – REVISED JANUARY 2009 ...................................................................................................................................................... www.ti.com
The ADS8513 has a power-down mode that is activated by taking CONV low and then PWRD high. This mode
powers down all of the analog circuitry including the reference, reducing power dissipation to under 50 µ W. To
exit the power-down mode, CONV is taken high and then PWRD is taken low. Note that a conversion is initiated
if PWRD is taken high while CONV is low.
While in the power-down mode, the voltage on the capacitors connected to CAP and REF begins to leak off. The
voltage on the CAP capacitor leaks off much more rapidly than on the REF capacitor (the REF input of the
ADS8513 becomes high-impedance when PWRD is high; this condition is not true for the CAP input). When
exiting power-down mode, these capacitors must be allowed to recharge and settle to a 16-bit level. Figure 46
shows the amount of time typically required to obtain a valid 16-bit result based on the amount of time spent in
power down (at room temperature). This figure assumes that the total capacitance on the CAP pin is 1.01 µ F.
Figure 47 shows a circuit that can significantly reduce the power-up time if the power-down time is fairly brief (a
few seconds or less). A low on-resistance MOSFET is used to disconnect the capacitance on the CAP pin from
the leakage paths internal to the ADS8513. This MOSFET allows the capacitors to retain the respective charges
for a much longer period of time, reducing the time required to recharge them at power-up. With this circuit, the
power-down time can be extended to tens or hundreds of milliseconds with almost instantaneous power-up.
Figure 46. Power-Down to Power-Up Response
Figure 47. Improved Power-Up Response Circuit
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Product Folder Link(s): ADS8513