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LTC2917/LTC2918

29178f

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

ﬁxed at –5%. Threshold accuracy is guaranteed at ±1.5%

over the entire operating temperature range.

The LTC2917/LTC2918 asserts the ⎯R⎯S⎯T output low when

VM is below the programmed threshold, and for a reset

timeout period (tRST) after VM goes above the threshold.

The reset timeout can be conﬁgured 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 ﬁltering ensures reliable reset

operation without false triggering.

Power-Up

VCC powers the drive circuits for the ⎯R⎯S⎯T pin. Therefore,

as soon as VCC reaches 0.8V during power up, the ⎯R⎯S⎯T

output asserts low.

Until VCC reaches the undervoltage lockout threshold

(guaranteed less than 1.5V), ⎯R⎯S⎯T is held low regardless

of the state of VM.

Once VCC 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 ⎯R⎯S⎯T and the external pull-up resistor

pulls high.

Power-Down

On power-down, once VM drops below its threshold or

VCC drops below the undervoltage lockout, ⎯R⎯S⎯T 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, VCC or left unconnected during normal

operation. Note that when left unconnected, the maximum

leakage allowable from the pin to either GND or VCC 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 VOL and VOH which meet the VIL and VIH of

the TOL pin speciﬁed 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

VCC

Open

3.3V

VCC

GND

2.5V

Open

VCC

1.8V

Open

*1.5V

Open

GND

*1.2V

GND

VCC

*1V

GND

Open

*ADJ (0.5V)

GND

*Require a separate supply for VCC

Table 2. System Voltage Tolerance Settings

TOLERANCE

TOL

–5%

VCC

–10%

Open

–15%

GND

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