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LTC6801

6801p

READING EXTERNAL TEMPERATURE PROBES

The LTC6801 includes two channels of ADC input, VTEMP1

and VTEMP2, that are intended to monitor thermistors

(tempco about –4%/°C generally) or diodes (–2.2mV/°C

typical) located within the cell array. Sensors can be

powered directly from VREF as shown in Figure 9 (up to

30μA typical).

The temperature measurement inputs (VTEMP1, VTEMP2) of

the LTC6801 are comparator input channels with a voltage

threshold of one-half VREF. Input voltages above half VREF

are considered good. Voltages below the one-half VREF

threshold are considered a fault condition. The inputs

may be used in combination with resistors, thermistors,

or diodes to sense both an upper and lower temperature

limit. Figure 9, Figure 10 and Figure 11 illustrate some

possibilities. To ignore these inputs simply connect VTEMP1

and VTEMP2 to VREF. A ﬁltering capacitor to V– is recom-

mended to minimize the error caused by the approximately

700k input impedance of the ADC.

For sensors that require higher drive currents, a buffer

ampliﬁer may be used as shown in Figure 10. Power for

the sensor is actually sourced indirectly from the VREG pin

in this case. Probe loads up to about 1mA maximum are

supported in this conﬁguration. Since VREF is shut down

while the LTC6801 is idle between measurement cycles,

the thermistor drive is also shut off and thus power dis-

sipation is minimized. Since VREG remains always-on, the

buffer op amp (LT6003 shown) is selected for its ultralow

current consumption (10μA).

For circuits that include ﬁltering capacitance, note that

only the fastest DC setting (VREG connection) will keep

VREF steady and allow the VTEMP voltages to settle. To use

the lower power DC settings, VREF must be buffered (see

Figure 10), so that a low impedance is presented to the

ADC, with a time constant of no more than about 1ms.

ADVANTAGES OF DELTA-SIGMA ADCs

The LTC6801 employs a delta-sigma analog to digital

converter for voltage measurement. The architecture of

delta-sigma converters can vary considerably, but the

common characteristic is that the input is sampled many

times over the course of a conversion and then ﬁltered or

averaged to produce the digital output code.

Figure 11. Sensing Both Upper and Lower Temperature

Thresholds. This Example Monitors a –20°C to +60°C Window

Detector. The Thermistors Should Be in Close Proximity

100k

500k

NTC

B = 4567

100k

NTC

B = 4250

1150k

1μF

V+

C12

C11

C10

V–

VTEMP1

VTEMP2

VREF

VREG

OV1

OV0

UV1

UV0

HYST

CC1

CC0

SLT

SLTOK

EOUT

SIN

SOUT

EIN

LTC6801

6801 F11

1μF

APPLICATIONS INFORMATION

LTC6801IG Datasheet(PDF) 21 Page - Linear Technology

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