The ADM1278 is a hot swap controller that allows a circuit board to be removed from or inserted into a live backplane. It also features current, voltage, power, and temperature readback via an integrated 12-bit analog-to-digital converter (ADC), accessed using a PMBus interface. The load current is measured using an internal current sense amplifier that measures the voltage across a sense resistor in the power path via the HS+ and HS− pins. A default current limit of 20 mV is set, but this limit can be adjusted, if required.
The ADM1278 limits the current through the sense resistor by controlling the gate voltage of an external N-channel FET in the power path, via the GATE pin. The sense voltage, and therefore the load current, is maintained below the preset maximum. The ADM1278 protects the external FET by limiting the time that the FET remains on while the current is at its maximum value.
This current-limit time is set by the choice of capacitor connected to the TIMER pin. In addition, a constant power foldback scheme is used to control the power dissipation in the MOSFET during power-up and fault conditions. The level of this power, along with the TIMER regulation time, can be set to ensure that the MOSFET remains within safe operating area (SOA) limits.
In case of a short-circuit event, a fast internal overcurrent detector responds within 320 ns and signals the gate to shut down. A 1500 mA pull-down device ensures a fast FET response.
The ADM1278 features overvoltage (OV) and undervoltage (UV) protection, programmed using external resistor dividers on the UV and OV pins. A PWRGD signal can be used to detect when the output supply is valid, using the PWGIN pin to accurately monitor the output.
Undervoltage and overvoltage
The ADM1278 monitors the supply voltage for undervoltage (UV) and overvoltage (OV) conditions. The UV and OV pins are connected to the input of an internal voltage comparator, and its voltage level is internally compared with a 1 V voltage reference.
An external resistor network divides the supply voltage for monitoring.
An undervoltage event when UV <1 V, and overvoltage OV >1 V.
Choose resistors for determining overvoltage and undervoltage:
OverVoltage: OV = (R2+R14)/R14*1V=13.95V
UnderVoltage: UV = (R3+R12)/R3*1V=9.26V
Setting the current limit (ISET)
The maximum current limit is partially determined by selecting a sense resistor to match the current sense voltage limit on the controller for the desired load current. However, as currents become larger, the sense resistor requirements become smaller, and resolution can be difficult to achieve when selecting the appropriate sense resistor. The ADM1278 provides an adjustable current sense voltage limit to manage this issue. The device allows the user to program the required current sense voltage limit from 5 mV to 25 mV.
The default value of 20 mV is achieved by connecting the ISET pin directly to the VCAP pin. This connection configures the device to use an internal 1 V reference, which equates to 20 mV at the sense inputs.
The VCAP pin has a 2.7 V internal generated voltage.
VCAP: Place a capacitor with a value of 1 μF or greater on this pin to maintain accuracy.
Choose I(TRIP)= 5.4A
V(ISET) = I(TRIP)*RSNS*50 = 1.35V
R18 = 100K
R21 = 100K
Start-up current limit
When powering up in dv/dt mode, the inrush current is typically configured to be in the order of <5 A. The other active current limits (PSET and ISET) may be much higher than this. The startup current limit is intended as an extra level of protection during this initial power-up stage. It helps catch a resistive type fault that causes the inrush to be higher than expected.
The start-up current limit is only active during power-up. It is enabled while PWRGD is deasserted and is disabled when PWRGD is asserted.
The start-up current limit can be programmed via the ISTART pin or via the PMBus register, STRT_UP_IOUT_LIM (Register 0xF6). If both are configured, the lowest current limit is selected as the active current limit. The clamp level in both cases is a 2 mV VSENSE current limit.
Startup_CB set to 5A
V(ISTART) = (Startup_CB * RSNS+0.00088)*50=1.294V
R26 = R(upper) = 100K
R35+R39 = R(lower) = 47K*2
To prevent the start-up current limit from being triggered during a normal dv/dt power-up, set the circuit breaker level above the maximum expected inrush current.
The ISTART pin can be tied to VCAP to disable the start-up current limit. The start-up current limit PMBus register is set to the maximum by default; therefore, it is effectively disabled by default.
Maximum FET power level
Foldback is a method that actively reduces the current limit as the voltage drop across the FET increases. It keeps the power across the FET below the programmed value during power-up, overcurrent, or short-circuit events. This allows a smaller FET to be used, resulting in board size savings and cost savings. The foldback method used is a constant power foldback scheme, meaning power in the FET is held constant, regardless of the VDS of the FET. This simplifies the task of ensuring that the FET is always operating within the SOA limits.
Choose MOSFET Power limit: P(MOSFET-limit) = 25W
The maximum FET power level is configured with a resistor divider on the PSET pin:
V(PSET)= P(MOSFET-limit) *50 * RSNS / 8 = 0.78125
R(upper) = 100K
R(lower) = 41.2K
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.
For temperature sensors operating in noisy environments to mitigate the effects of noise to use a low-pass R-C-R filter with: R = 100 Ω and C = 1 nF.