Datasheet

LTC2917/LTC2918
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APPLICATIONS INFORMATION
Supply Monitoring
The LTC2917/LTC2918 are low voltage single supply
monitors with selectable thresholds. Two three-state inputs
select one of nine internally programmed thresholds. For
the LTC2917, a third three-state input selects the tolerance
at which the supply connected to the VM pin is monitored
(–5%, –10%, –15%). The tolerance for the LTC2918 is
fi xed at –5%. Threshold accuracy is guaranteed at ±1.5%
over the entire operating temperature range.
The LTC2917/LTC2918 asserts the RST output low when
VM is below the programmed threshold, and for a reset
timeout period (t
RST
) after VM goes above the threshold.
The reset timeout can be confi gured to use one of two
internal timers with no external components, or an ad-
justed timer programmed by placing an external capacitor
from RT to ground. Glitch fi ltering ensures reliable reset
operation without false triggering.
Power-Up
V
CC
powers the drive circuits for the RST pin. Therefore,
as soon as V
CC
reaches 0.8V during power up, the RST
output asserts low.
Until V
CC
reaches the undervoltage lockout threshold
(guaranteed less than 1.5V), RST is held low regardless
of the state of VM.
Once V
CC
is above the undervoltage lockout threshold
and VM is above the programmed threshold, the reset
timer is started. After the reset timeout, the open drain
pull-down releases RST and the external pull-up resistor
pulls high.
Power-Down
On power-down, once VM drops below its threshold or
V
CC
drops below the undervoltage lockout, RST asserts
logic low.
Monitor Threshold Control
The monitor threshold on the VM pin is controlled by
the SEL1, SEL2 and TOL three-state pins. The SEL1 and
SEL2 pins select one of nine preset nominal voltages
(including one externally adjustable threshold) as shown
in Table 1.
The SEL1 and SEL2 three-state input pins should be con-
nected to GND, V
CC
or left unconnected during normal
operation. Note that when left unconnected, the maximum
leakage allowable from the pin to either GND or V
CC
is
±5μA.
The tolerance at which the monitored supply is measured
is set by the TOL pin (LTC2917 only) as shown in Table 2.
If desired (e.g. for margining purposes), the TOL pin may
be driven by a three-state buffer. That three-state buffer
must have a V
OL
and V
OH
which meet the V
IL
and V
IH
of
the TOL pin specifi ed in the Electrical Characteristics, and
maintain less than 5μA of leakage in the open state.
Table 1. Voltage Threshold Settings
NOMINAL VOLTAGE SEL1 SEL2
12V V
CC
V
CC
5V V
CC
Open
3.3V V
CC
GND
2.5V Open V
CC
1.8V Open Open
*1.5V Open GND
*1.2V GND V
CC
*1V GND Open
*ADJ (0.5V) GND GND
*Require a separate supply for V
CC
Table 2. System Voltage Tolerance Settings
TOLERANCE TOL
–5% V
CC
–10% Open
–15% GND