Datasheet
C
T
(nF) +
ƪ
t
D
(s)*0.5 10
*3
(s)
ƫ
175
(1)
1.2V 3.3V
TPS3808G12 TPS3808G33 DSP
SENSE V
DD
V
DD
SENSE
V
I/O
V
CORE
GPIO
GNDGNDGND
RESET MR
C
T
C
T
RESET
IMMUNITY TO SENSE PIN VOLTAGE
3.3V
TPS3808xxx
V
DD
SENSE
MR
90kW
GND
SELECTING THE RESET DELAY TIME
Delay (s) = C
T
(nF) + 0.5 x 10
−3
(s)
20ms Delay
300ms Delay
(c)
(b)
(a)
175
3.3V
TPS3808G33
V
DD
SENSE
C
T
RESET
3.3V
TPS3808G33
V
DD
SENSE
C
T
C
T
RESET
3.3V
TPS3808G33
V
DD
SENSE
C
T
50k
Ω
RESET
TPS3808
www.ti.com
.......................................................................................................................................................... SBVS050J – MAY 2004 – REVISED AUGUST 2008
by the choice of resistor. Figure 14 b shows a fixed
20ms delay time by leaving the C
T
pin open.
Figure 14 c shows a ground referenced capacitor
connected to C
T
for a user-defined program time
between 1.25ms and 10s.
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:
The reset delay time is determined by the time it
Figure 12. Using MR to Monitor Multiple System
takes an on-chip precision 220nA current source to
Voltages
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.
TRANSIENTS
The TPS3808 is relatively immune to short negative
Figure 13. Using an External MOSFET to Minimize
transients on the SENSE pin. Sensitivity to transients
I
DD
When MR Signal Does Not Go to V
DD
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.
The TPS3808 has three options for setting the
RESET delay time as shown in Figure 14 . Figure 14 a
shows the configuration for a fixed 300ms typical
delay time by tying C
T
to V
DD
; a resistor from 40k Ω to
200k Ω must be used. Supply current is not affected
Figure 14. Configuration Used to Set the RESET Delay Time
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