Power in a PCB assembly may be distributed through several hot-swap or soft-switch ICs in order to partition the loads and isolate faults locally. Measuring the current through each branch of this distribution system is cumbersome and may be unnecessary, if only the total combined load current is of interest. In that case, the current-sense outputs (ISENSE) pins of several MAX15090B/C devices can be connected in parallel to a single, combined summing resistor to ground. The details of this application circuit are described.
The MAX15090B/C device provides a fully integrated power protection and hot-swap solution for supplies from 2.7V to 18V, up to a 12A load current. Additionally, the device features a precision current-sense amplifier output that allows measurement of the load.
Because the current-sense circuit is based on a sense FET and current mirror, the signals from several MAX15090B/C devices can be easily combined into a single summing resistor, providing convenient measurement of the total load current, thus eliminating the need for an external summing amplifier or mathematical summing of ADC results in firmware. This saves BOM components and reduces cost.
This application note describes how to implement this circuit so that it always functions correctly, even if some of the MAX15090B/C devices are disabled or are unpowered.
Basic Circuit Description and Limitations
The ISENSE output pin of the MAX15090B/C produces a current that is proportional to the current flowing through the pass FET with attenuation of 220µA/A. The ISENSE output pins of several MAX15090B/C devices can be connected into a single summing resistor, provided that all of the devices are powered from the same VCC supply and are all enabled at the same time, as shown in Figure 1.
If the devices are enabled and disabled independently, each ISENSE pin must be separated from the others by a series diode, because ISENSE becomes low impedance when the output is disabled.
Selecting the RISENSE Resistor Value
The full-scale voltage across the common summing resistor must remain below the "headroom" of the ISENSE output, which is 2.5V maximum. Therefore, the common summing resistor should be scaled according to the number of MAX15090B/C devices:
RISENSE = 2.5V/(ITOTAL×220×10-6)
For example, consider a system with four MAX15090B/C devices, each of which supplies up to 12A load. If the full-scale ISENSE current is 12A x 220µA/A = 2.64mA for each device, then four devices at full scale would source up to 10.56mA, and the maximum allowable combined ISENSE resistor value would be 236?.
Using Different VCC Supplies
If there is any possibility of one or more of the MAX15090B/C devices being powered while others are not powered, as shown in Figure 2, care must be taken to prevent a powered MAX15090B/C device from overdriving an unpowered device's ISENSE pin. This could cause damage by exceeding the ISENSE pin's absolute maximum rating of VREG + 0.3V.
This can be prevented by simply adding a small-signal Schottky diode in series with each of the ISENSE outputs. A BAT54C common-cathode dual Schottky diode, or similar diode array, can be used.
Remember that these series Schottky diodes reduce the available output compliance (headroom) by their forward voltage (VF), which is typically 0.5V, so the ISENSE resistor (RISENSE) must be determined using a 2V maximum voltage:
RISENSE = 2.0V/(ITOTAL×220×10-6)
To demonstrate the accuracy and practicality of this application circuit, an example similar to Figure 2 was constructed with two MAX15090B devices (instead of four) connected to a common ISENSE resistor.
A 500? resistor was used at the combined ISENSE node. Each ISENSE output was connected in series with a BAT54 Schottky diode to allow operation from independent supplies. This solution provides a full-scale current-reporting range of just over 18A.
ITOTAL = (2.5V - VF)/(RISENSE×gM) = (2.5V-0.5V)/(500O×220µA) ˜ 18.2A
The circuit was operated up to a maximum of 7A load on each MAX15090B device. The test results are shown in Figure 3.
The current-reporting accuracy remains excellent over the full range of system load current, regardless of the distribution of load between the MAX15090B devices.
Related Maxim Devices
Maxim has similar hot-swap IC devices that could also utilize this application circuit technique. These devices are listed in Table 1.
|Device||Maximum Current (A)||Current-Sense Gain (µA/A)|