TPS3808-Q1
www.ti.com
SBVS085H –JANUARY 2007–REVISED JUNE 2012
3.3V
V
SENSE
DD
Ω
90k
CT
TPS3808xxx
GND
Figure 11. Using an External MOSFET to Minimize IDD When MR Signal Does Not Go to VDD
Selecting the Reset Delay Time
The TPS3808 has three options for setting the RESET delay time as shown in Figure 12. Figure 12a shows the
configuration for a fixed 300-ms typical delay time by tying CT to VDD; a resistor from 40 kΩ to 200 kΩ must be
used. Supply current is not affected by the choice of resistor. Figure 12b shows a fixed 20-ms delay time by
leaving the CT pin open. Figure 12c shows a ground referenced capacitor connected to CT for a user-defined
program time between 1.25 ms and 10 s.
The capacitor CT should be ≥100 pF nominal value in order for the TPS3808 to recognize that the capacitor is
present. The capacitor value for a given delay time can be calculated using the following equation:
*3
ƪ
ƫ
CT (nF) + tD (s)*0.5 10 (s) 175
(1)
The reset delay time is determined by the time it takes an on-chip precision 220-nA current source to charge the
external capacitor to 1.23 V. When a RESET is asserted, the capacitor is discharged. When the RESET
conditions are cleared, the internal current source is enabled and begins to charge the external capacitor. When
the voltage on this capacitor reaches 1.23 V, RESET is deasserted. Note that a low-leakage type capacitor such
as a ceramic should be used and that stray capacitance around this pin may cause errors in the reset delay time.
Immunity to SENSE Pin Voltage Transients
The TPS3808 is relatively immune to short negative transients on the SENSE pin. Sensitivity to transients is
dependent on threshold overdrive, as shown in the Maximum Transient Duration at Sense vs Sense Threshold
Overdrive Voltage graph (Figure 5) in the Typical Characteristics section.
3.3V
3.3V
3.3V
SENSE VDD
SENSE VDD
SENSE VDD
Ω
50k
TPS3808G33
TPS3808G33
TPS3808G33
RESET
CT RESET
CT RESET
CT
CT
−3
Delay (s) = C (nF) + 0.5 x 10 (s)
T
300ms Delay
20ms Delay
175
(b)
(a)
(c)
Figure 12. Configuration Used to Set the RESET Delay Time
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