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ADM1278 Datasheet(PDF) 30 Page - Analog Devices |
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ADM1278 Datasheet(HTML) 30 Page - Analog Devices |
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30 / 61 page  ADM1278 Data Sheet ADM1278 FET HEALTH MONTIOR OVER CURRENT FAULT OVER TEMPERATURE FAULT GATE TEMP FAULT RSENSE HS+ HS– ENABLE Figure 58. FAULT Pin Operation ENABLE/ENABLE INPUT The ADM1278 provides a dedicated ENABLE/ENABLE digital input pin. The ADM1278-1 and ADM1278-2 have an active high ENABLE pin whereas the ADM1278-3 has an active low ENABLE pin. The ENABLE/ENABLE pin allows the ADM1278 to remain off by using a hardware signal, even when the voltage on the UV pin is greater than 1.0 V and the voltage on the OV pin is less than 1.0 V. Although the UV pin can be used to provide a digital enable signal, using the ENABLE/ENABLE pin for this purpose means that the ability to monitor for undervoltage conditions is not lost. In addition to the conditions for the UV and OV pins, the ADM1278 ENABLE/ENABLE input pin must be asserted for the device to begin a power-up sequence. CURRENT SENSE OUTPUT (CSOUT) The ADM1278 provides a CSOUT pin voltage output that is proportional to the VSENSE_HS voltage. CSOUT = VSENSE_HS × 350 The CSOUT voltage is an analog representation of the main system current flowing through RSENSE. A resistor divider can be added to CSOUT to clamp the voltage output to any downstream devices, provided the maximum load conditions described in Table 2 are not exceeded. The response time of the CSOUT pin to a change in VSENSE voltage is very fast; therefore, it can be used when fast response time is required, for example, power throttling. The CSOUT response time to a 10 mV step in VSENSE voltage is typically 10 µs. REMOTE TEMPERATURE SENSING The ADM1278 provides the capability to measure temperature at a remote location with a single discrete NPN or PNP transistor. The temperature measurements can be read back over the PMBus interface. Warning and fault thresholds can also be set on the temperature measurement. Exceeding a fault threshold causes the controller to turn off the pass MOSFET, deassert the PWRGD pin, and assert the FAULT pin. The external transistor is typically placed close to the main pass MOSFETs to provide an additional level of protection. The controller can then monitor and respond to an elevated MOSFET operating temperature. It is not possible to measure temperature at more than one location on the board. Place the transistor close to the MOSFET for best accuracy. If the transistor is placed on the opposite side of the PCB, use multiple vias to ensure the optimum transfer of heat from the MOSFET to the transistor. Temperature Measurement Method A simple method of measuring temperature is to exploit the negative temperature coefficient of a diode by measuring the base-emitter voltage (VBE) of a transistor operated at constant current. However, this technique requires calibration to null the effect of the absolute value of VBE, which varies from device to device. The technique used in the ADM1278 is to measure the change in VBE when the device is operated at three different currents. The use of a third current allows automatic cancellation of resistances in series with the external temperature sensor. The temperature sensor takes control of the ADC for 64 µs (typical) every 6 ms. It takes 12 ms to obtain a new temperature measurement from the ADC. Remote Sensing Diode The ADM1278 is designed to work with discrete transistors. The transistor can be either a PNP or NPN connected as a diode (base shorted to the collector). If an NPN transistor is used, the collector and base are connected to the TEMP pin and the emitter to PGND. If a PNP transistor is used, the collector and base are connected to PGND and the emitter to TEMP. The best accuracy is obtained by choosing devices according to the following criteria: • Base-emitter voltage greater than 0.25 V at 6 µA, at the highest operating temperature. • Base-emitter voltage less than 0.95 V at 100 µA, at the lowest operating temperature. • Base resistance less than 100 Ω. • Small variation in hFE (50 to 150) that indicates tight control of VBE characteristics. Transistors, such as the 2N3904, 2N3906, or equivalent in SOT-23 packages are suitable devices to use. Noise Filtering For temperature sensors operating in noisy environments, the industry standard practice has been to place a capacitor across the temperature pins to mitigate the effects of noise. However, large capacitances affect the accuracy of the temperature measurement, leading to a recommended maximum capacitor value of 1000 pF. Although this capacitor reduces the noise, it does not eliminate it, making it difficult to use the sensor in a very noisy environment. The ADM1278 has a major advantage over other devices for eliminating the effects of noise on the external sensor. The Rev. A | Page 30 of 61 |
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