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LTC2938/LTC2939

293839ff

Supply Monitoring

The LTC2938 and LTC2939 are low power, high accuracy

conﬁgurable four (LTC2938) and six (LTC2939) supply

monitoring circuits with reset output and watchdog func-

tions. Both watchdog and reset timeouts are adjustable

using external capacitors. Single-pin conﬁguration selects

one of sixteen input voltage monitor combinations. All four

(LTC2938) or six (LTC2939) voltage inputs must be above

predetermined thresholds for the reset not to be invoked.

The LTC2938/LTC2939 assert the reset during power-up,

power-down and brownout conditions on any one of the

voltage inputs.

Power-Up

The greater of V1 or V2 serves as the internal supply

voltage (VCC). On power-up, VCC powers the drive circuits

for the RST output. This ensures that the RST output will

be low as soon as V1 or V2 reaches 1V. The RST output

remains low until the part is conﬁgured. After conﬁgura-

tion, if any one of the supply monitor inputs is below its

conﬁgured threshold, RST will be at logic low. Once all

the monitor inputs rise above their thresholds, an internal

timer is started and RST is released after the conﬁgured

delay time.

APPLICATIONS INFORMATION

Figure 1. 1.5% Threshold Accuracy Improves System Reliability

Threshold Accuracy

Consider a 5V system with ±5% tolerance. The 5V supply

may vary between 4.75V to 5.25V. System ICs powered by

this supply must operate reliably within this band (and a

little more as subsequently explained). A perfectly accurate

supervisor for this supply generates a reset at exactly

4.75V. However, no supervisor is perfect. The actual reset

threshold of a supervisor varies over a speciﬁed band.

The LTC2938/LTC2939 varies ±1.5% around its nominal

threshold voltage (see Figure 1) over temperature.

The reset threshold band and the power supply tolerance

bands should not overlap. This prevents false or nuisance

resets when the power supply is actually within its speci-

ﬁed tolerance band.

The LTC2938 and LTC2939 have ±1.5% reset threshold

accuracy, so a 5% threshold is typically set to 6.5% below

the nominal input voltage. Therefore, a typical 5V, 5%

threshold is 4.675V.

The threshold is guaranteed to lie in the band between

4.750V and 4.600V over temperature. The powered system

must work reliably down to the low end of the threshold

band, or risk malfunction before a reset signal is properly

issued.

4.75V

4.675V

±1.5%

THRESHOLD

BAND

4.6V

NOMINAL

SUPPLY

VOLTAGE

SUPPLY TOLERANCE

MINIMUM

RELIABLE

SYSTEM

VOLTAGE

IDEAL

SUPERVISOR

THRESHOLD

REGION OF POTENTIAL MALFUNCTION

–5%

–6.5%

–8%

293839 F01

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