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LTC2909

2909fa

For a positive-monitoring application, RP2 is then chosen

by:

RP2 = RP1(2VTRIP – 1)

For a negative-monitoring application:

RN2 = RN1(1 – 2VTRIP)

Note that the value VTRIP should be negative for a nega-

tive application.

The LTC2909 can also be used to monitor a single supply

for both UV and OV. This may be accomplished with three

resistors, instead of the four required for two independent

supplies. Conﬁgurations are shown in Figures 3 and 4.

RP4 or RN4 may be chosen as is RP1 above.

For a given RP4, monitoring a positive supply:

64 21

()

–

For monitoring a negative supply with a given RN4:

()

–

For example, consider monitoring a –5V supply at ±10%. For

this supply application: VOV = –5.575V and VUV = –4.425V.

Suppose we wish to consume about 5μA in the

divider, so

RN4 = 100k. We then ﬁnd RN5 = 21.0k, RN6 = 1.18M (nearest

1% standard values have been chosen). Suggested values

of resistors for 5% monitoring are shown in Table 3.

VCC Monitoring/UVLO

The LTC2909 contains an accurate third 10% undervoltage

monitor on the VCC pin. This monitor is ﬁxed at a nominal

11.5% below the VCC speciﬁed in the part number. The

standard part (LTC2909-2.5) is conﬁgured to monitor a

2.5V supply (UVLO threshold of 2.213V), but versions

to monitor 3.3V and 5.0V (UVLO of 2.921V and 4.425V,

respectively) are available.

For applications that do not need VCC monitoring, the

2.5V version should be used, and the UVLO will simply

guarantee that the VCC is above the minimum required for

proper threshold and timer accuracy before the timeout

begins.

Setting the Reset Timeout

The reset timeout of the LTC2909 may be conﬁgured

in one of three ways: internal 200ms, programmed by

external capacitor and no timeout (comparator mode).

The mode of the timer is determined by the connection

of the TMR pin.

In externally-controlled mode, the TMR pin is connected

by a capacitor to ground. The value of that capacitor allows

for selection of a timeout ranging from about 400μs to 10

seconds. See the following section for details.

APPLICATIO S I FOR ATIO

–

0.5V

2909 F03

ADJ1

ADJ2

VMON

RP5

RP6

RP4

Figure 3. Setting UV and OV Trip Point for a Positive Supply

–

0.5V

2909 F04

ADJ2

ADJ1

REF

VMON

RN5

RN4

RN6

Figure 4. Setting UV and OV Trip Point for a Negative Supply

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