LTC2909CDDB-3.3(RevA) データシートの表示（PDF） - Linear Technology

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LTC2909CDDB-3.3
(Rev.:RevA)

Precision Triple/Dual Input UV, OV and Negative Voltage Monitor

Linear Technology 

LTC2909

APPLICATIO S I FOR ATIO

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:

RP5

=

RP4

VOV – VUV

VUV

( ) RP6 = RP4

2VUV – 1

VOV

VUV

For monitoring a negative supply with a given RN4:

RN5

=

RN4

VUV – VOV

1– VUV

( ) RN6 = RN4

1– 2VUV

1– VOV

1– VUV

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

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

VMON

RP6

RP5

RP4

ADJ1

+

–

ADJ2

+

–

0.5V +–

2909 F03

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

12

REF

ADJ1

+

RN4

RN5

RN6

VMON

–

ADJ2

+

–

0.5V +–

2909 F04

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

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.

2909fa

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