TPS3808
www.ti.com
SBVS050E – MAY 2004 – REVISED OCTOBER 2005
1.2V
3.3V
by the choice of resistor.
shows a fixed
20ms delay time by leaving the C
T
pin open.
shows a ground referenced capacitor
connected to C
T
for a user-defined program time
between 1.25ms and 10s.
SENSE V
DD
V
I/O
V
CORE
SENSE
V
DD
TPS3808G12
RESET
C
T
TPS3808G33
MR
C
T
DSP
RESET
GND
GPIO
GND
GND
The capacitor C
T
should be
≥
100pF nominal value in
order for the TPS3808xxx to recognize that the
capacitor is present. The capacitor value for a given
delay time can be calculated using the following
equation:
C
T
(nF)
+
t
D
(s)*0.5
10
*3
(s)
175
(1)
Figure 12. Using MR to Monitor Multiple System
Voltages
3.3V
V
DD
SENSE
90kΩ
C
T
TP S38 08xx x
The reset delay time is determined by the time it
takes an on-chip precision 220nA current source to
charge the external capacitor to 1.23V. 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 1.23V, RESET is de-asserted. 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
G ND
Figure 13. Using an External MOSFET to Minimize
I
DD
When MR Signal Does Not Go to V
DD
SELECTING THE RESET DELAY TIME
The TPS3808 has three options for setting the
RESET delay time as shown in
shows the configuration for a fixed 300ms typical
delay time by tying C
T
to V
DD
; a resistor from 40kΩ to
200kΩ must be used. Supply current is not affected
3.3V
SENSE V
DD
50kΩ
TPS3808G33
C
T
RESET
3.3V
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
6)
in the
Typical Characteristics section.
3.3V
SENSE V
DD
TPS3808G33
C
T
RESET
C
T
SENSE V
DD
TPS3808G33
C
T
RESET
300ms Delay
(a)
20ms Delay
(b)
Delay (s) = C
T
(nF) + 0.5 x 10
−3
(s)
175
(c)
Figure 14. Configuration Used to Set the RESET Delay Time
9
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