Datasheet LTC4070 (Analog Devices) - 10

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APPLICATIONS INFORMATION
The voltage at the NTC pin depends on the ratio of the NTC conditioning. Reviewing the Typical Applications with thermistor value, RNTC, and a bias resistor, RNOM. Choose a 12V wall adapter in Figure 1; the input resistor, RIN, RNOM equal to the value of the thermistor at 25°C. RNOM should be increased to 165Ω such that the maximum is 10k for a Vishay NTHS0402N02N1002F thermistor with shunt current does not exceed 50mA at the lowest pos- a B25/85 value of 3490. RNOM must be connected from sible float voltage due to NTC conditioning, VFLOAT_MIN NTCBIAS to NTC. The ratio of the NTC pin voltage to the = 3.8V. NTCBIAS voltage is: R
Thermal Considerations
NTC (R At maximum shunt current, the LTC4070 may dissipate NTC + RNOM ) up to 205mW. The thermal dissipation of the package When the thermistor temperature rises, the resistance should be taken into account when operating at maximum drops; and the resistor divider between RNOM and the shunt current so as not to exceed the absolute maximum thermistor lowers the voltage at the NTC pin. junction temperature of the device. With qJA of 40°C/W, in An NTC thermistor with higher B the MSOP package, at maximum shunt current of 50mA 25/85 values may also be used with the LTC4070. However the temperature trip the junction temperature rise is about 8°C above ambi- points are shifted due to the higher negative temperature ent. With qJA of 76°C/W in the DFN package, at maximum coefficient of the thermistor. To correct for this difference shunt current of 50mA the junction temperature rise is add a resistor, R about 16°C above ambient. FIX, in series with the higher B25/85 value thermistor to shift the ratio,
Operation with an External PFET to Boost Shunt Current
RFIX +RNTC Recommended External Shunt PFETS lists recommended (R FIX + RNTC + RNOM ) devices to increase the maximum shunt current. Due to up to the internal resistive divider tap points: NTC the requirement for low capacitance on the DRV pin node, TH1 through NTC it is recommended that only low gate charge and high TH4. For a 100k thermistor with a B25/85 value of 3950 NTHS0402N01N1003F, at 70°C (with R threshold PFET devices be used. Also it is recommended NOM = 100k) choose R that careful PCB layout be used to keep leakage at the FIX = 3.92kΩ. The temperature trip points are found by looking up the thermistor R/T values plus DRV pin to a minimum as the IDRV(SINK) current is typi- R cally 3µA. FIX that correspond to the ratios for NTCTH1 = 36.5%, NTCTH2 = 29.0%, NTCTH3 = 22.8%, and NTCTH4 = 17.8%. Refer to device manufacturers data sheets for maxi- Selecting RFIX = 3.92k results in trip points of 39.9°C, mum continuous power dissipation and thermal resis- 49.4°C, 59.2°C and 69.6°C. tance when selecting an external PFET for a particular Another technique may be used without adding an addi- application. tional component. Instead decrease RNOM to adjust the
Table 2. Recommended External Shunt PFETS
NTCTH thresholds for a given R/T thermistor profile. For
DEVICE VENDOR QGS VTH(MIN) RDS(ON)
example, if RNOM = 88.7k (with the same 100k thermis- FDN352AP Fairchild 0.50nC –0.8V 0.33 tor) then the temperature trip points are 41.0°C, 49.8°C, Si3467DV Vishay 1.7nC –1.0V 0.073 58.5°C, and 67.3°C. Si3469DV Vishay 3.8nC –1.0V 0.041 When using the NTC features of the LTC4070 it is impor- DMP2130LDM Diodes Inc. 2.0nC –0.6V 0.094 tant to keep in mind that the maximum shunt current DMP3015LSS Diodes Inc. 7.2nC –1.0V 0.014 increases as the float voltage, VFLOAT_EFF drops with NTC Rev. D 10 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts