BQ77904 Datasheet, PDF (30/45 Pages) Texas Instruments – Open Wire Stackable Lithium-ion Battery Protector

bq77904, bq77905
SLUSCM3A – JUNE 2016 – REVISED JUNE 2016
Application Information (continued)
bq77094/5
Upper
device
Ra is used to keep the LD pin pull
down when load detect circuit is
LD ^{not activated}
pin
Ra
(1M)
Vss
pin
Must have block
diode on the upper
device.
RLD
bq77094/5
Bottom
device
LD
pin
Vss
pin
RLD
Diode for the bottom
device is optional.
Use if the LD pin will
be exposed lower
than -30V.
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PACK-
Figure 25. Simplified Circuit: LD Connection On Upper Device When Using For UV Fault Recovery
9.1.1.5 Temperature Protection
The device detects temperature by checking the voltage divided by R_{TS_PU} and R_TS, with the assumption of using
10 KΩ R_{TS_PU} and 103AT NTC for R_TS. System designer should always check the thermistor resistance
characteristic and refer to the temperature protection threshold specification in the Electrical Characteristic table
to determine if a different pull up resistor should be used. If a different temperature trip pint is required, it is
possible to scale the threshold using this equation: Temperature Protection Threshold = R_TS/(R_TS + R_{TS_PU}).
Example: Scale OTC trip points from 50°C to 55°C
The OTC protection can be set to 45°C or 50°C. When the device's OTC threshold is set to 50°C, it is referred to
configure the V_OTC parameter to 29.38% of VTB (typical), with the assumption of R_{TS_PU} = 10KΩ and R_TS =
103AT or similar NTC (which the NTC resistance at 50°C = 4.16KΩ). The V_OTC specification is simply the resistor
divider ratio of R_{TS_PU} and R_TS.
The V_OTC, V_OTD, V_UTC and V_UTD configuration options are fixed in the device. Hence, the actual temperature trip
point can only adjust by using a different B-value NTC and/or using a different R_{TS_PU}.
In this example, the 103AT NTC resistance at 55°C is 3.536 KΩ. By changing the R_{TS_PU} from 10 KΩ to 8.5 KΩ,
we can scale the actual OTC temperature trip point from 50°C to 55°C. Because the R_{TS_PU} value is smaller, this
change affects all the other temperature trip points and scales OTD, UTC and UTD to ~5°C higher as well.
9.1.1.6 Adding Filter To Sense Resistor
Current fault is sense through voltage across sense resistor. Optional RC filters can be added to the sense
resistor to improve stability.
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