LTC6801 datasheet(21/28 Pages) AD

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LTC6801

Rev. F

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MEASURING VARIOUS CELL COUNTS

The LTC6801 is designed to measure up to 12 cells

depending on the state of the CC pins (See Table 5). When

using an LTC6801 configured for measuring less than 12

cells, for instance choosing to measure 8 cells by tying

both CC1 and CC0 to the VREF pin, the highest cell poten-

tial (in this case C8) must be connected to the V+ pin for

proper operation. Unused cell connection pins (in this

case C9 to C12) may be left floating or may also be tied

to the highest cell potential.

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 pow-

ered 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, therm-

istors, or diodes to sense both an upper and lower tem-

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

some possibilities. To ignore these inputs simply con-

nect VTEMP1 and VTEMP2 to VREF. A filtering capacitor to

V– is recommended 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

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