Datasheet

LTC2915/LTC2916
9
29156fa
Typically, the user will pick a value of R1 based on accept-
able current draw. Current used by the resistor divider will
be approximately
R
V
I
1
05
=
⎛
⎝
⎜
⎞
⎠
⎟
.
Recommended range of R1 is 1k to 1M. Higher values
of resistance exacerbate the degradation of threshold
accuracy due to leakage currents.
If the nominal value of the supply being monitored is
V
NOM
, then
R2 = R1(2V
NOM
– 1)
Resistor tolerances must be taken into account when
determining the overall accuracy.
Selecting the Reset Timing Capacitor
The reset timeout period can be set to one of two fi xed
internal timers or set with a capacitor in order to accom-
modate a variety of applications. Connecting a capacitor,
C
RT
, between the RT pin and ground sets the reset timeout
period, t
RST
. The following formula approximates the value
of capacitor needed for a particular timeout:
C
RT
= t
RST
• 110 [pF/ms]
For example, using a standard capacitor value of 2.2nF
would give 20ms delay.
Figure 2 shows the reset timeout period as a function of
the value of the timer capacitor.
Leaving RT open with no external capacitor generates a
reset timeout of approximately 400μs. Shorting RT to V
CC
generates a reset timeout of approximately 200ms.
RST Output Characteristics
The DC characteristics of the RST pull-down strength
are shown in the Typical Performance Characteristics
section. RST is an open-drain pin and thus requires an
external pull-up resistor to the logic supply. RST may be
pulled above V
CC
, providing the voltage limits of the pin
are observed.
The open-drain of the RST pin allows for wired-OR con-
nection of several LTC2915/LTC2916s.
As noted in the discussion of power up and power down,
the circuits that drive RST are powered by V
CC
. During a
fault condition, V
CC
of at least 1V ensures that RST pulls
low.
Manual Reset (LTC2916 Only)
The LTC2916 includes the MR pin for applications where
a manual reset is desired. MR is internally pulled up, al-
lowing it to interface with a push button with no external
components required. Asserting MR low when RST is high,
initiates a reset, resulting in RST being asserted low for
the set reset timeout.
Shunt Regulator
The LTC2915 and LTC2916 contain an internal 6.2V shunt
regulator on the V
CC
pin to allow operation from a high
voltage supply. To operate the part from a supply higher
than 5.7V, the V
CC
pin must have a series resistor, R
CC
, to
the supply. This resistor should be sized according to the
following equation:
VV
mA
R
VV
A
SMAX
CC
SMIN() ()
.−
≤≤
−57
5
7
250μ
where V
S(MIN)
and V
S(MAX)
are the operating minimum
and maximum of the supply.
As an example, consider operation from an automobile
battery which might dip as low as 10V or spike to 60V. We
must then pick a resistance between 10.86k and 12k.
APPLICATIONS INFORMATION
Figure 2. Reset Timeout Period vs RT Capacitance
RT PIN CAPACITANCE, C
RT
(nF)
0.001
0.1
RESET TIMEOUT PERIOD, t
RST
(ms)
10
10000
0.10.01 1 10 100 1000
29156 F02
1
100
1000