AD8280 datasheet(17/24 Pages) AD

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AD8280

Rev. C | Page 17 of 24

Also, if the resistor (RTOP) used in the top of the thermistor

bridge circuit is less than 10 kΩ, another resistor (RIN) must be

added in series to the input to the VTx pin (see Figure 46). The

two resistors together must be greater than 10 kΩ (RTOP + RIN >

10 kΩ). This configuration is required only if VTH is not the

AD8280 LDO.

RTOP

THERMISTOR

AD8280

VTx

RIN

VTH

Figure 46. Input Configuration for VTx When Not Using the LDO for VTH

The part can work with both negative temperature coefficient

(NTC) and positive temperature coefficient (PTC) thermistors.

For NTC, the NPTC pin should be tied to logic low (VBOTx

pin); for PTC, the NPTC pin should be tied to logic high (LDO

pin). If the part is set to NTC mode, the OT alarm is tripped

when the voltages at VT1 and VT2 drop below the trip point.

If the part is set to PTC mode, the OT alarm is tripped when

the voltages at VT1 and VT2 rise above the trip point.

NUMBER OF CELLS SELECTION

The part can be configured to work with three, four, five, or six

cells. Table 5 describes how to program the SEL0 and SEL1 pins

to determine the number of cells being monitored. A logic low

represents VBOTx, and a logic high represents the LDO output

voltage. Figure 47 through Figure 49 show how to connect the

cells to the part in a five-cell, four-cell, or three-cell application.

Table 5. SELx Pin Programming

Number of Cells Used

SEL0

SEL1

6 cells

5 cells (VIN5 shorted)

4 cells (VIN4 and VIN5 shorted)

3 cells (VIN3, VIN4, and VIN5 shorted)

VIN6

VIN5

VIN4

VIN3

VIN2

VIN1

VIN0

AD8280

Figure 47. Five-Cell Connections for the AD8280

VIN6

VIN5

VIN4

VIN3

VIN2

VIN1

VIN0

AD8280

Figure 48. Four-Cell Connections for the AD8280

VIN6

VIN5

VIN4

VIN3

VIN2

VIN1

VIN0

AD8280

Figure 49. Three-Cell Connections for the AD8280

THRESHOLD INPUTS

The thresholds (or trip points) are set externally with a voltage

divider providing maximum flexibility. The desired trip point

voltage is connected to the following pins: OV (overvoltage trip

point), UV (undervoltage trip point), and OT (overtemperature

trip point). The +5 V output of either the reference (REF) or the

LDO can be used as the top voltage of the divider. However,

because the reference output is more accurate than the LDO out-

put, the reference output is better suited to power the trip point

setting dividers. If the thermistor dividers used for temperature

sensing are driven from the LDO output, it is recommended that

the LDO be used to drive the OT trip point divider as well for

better temperature drift performance.

Decoupling capacitors (0.1 μF) should be used with the bottom

leg of each divider in addition to a 2.2 μF capacitor at the REF

output, as shown in Figure 45.

The REF pin should be loaded with no more than 25 kΩ of

resistance. Therefore, when using REF to drive three voltage

dividers (OV, UV, and OT), it is recommended that the resis-

tance of each divider total at least 75 kΩ. If driving only two

dividers (OV and UV) with the reference, each divider should

total no less than 50 kΩ.

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