Shorted Input Protection

Question

I try to understand that Shorted Input Protection diode. I didn't understand clearly from datasheet. How can I find suitable diode for this protection circuit. What is use for? If I have a 28V 10A power supply, I need to choose 10A diode? Please help me for determining.

Answer 1

I didn't understand clearly from datasheet. ... What is use for?

It's basically for battery-backed-up (doesn't have to be a battery i.e. can be an extra/auxiliary supply) systems:

^{simulate this circuit – Schematic created using CircuitLab}

AUX above is an auxiliary voltage source to supply the load when the input is gone (e.g. power outage). If the input is present and if nominal output voltage (VOUT) is greater than AUX voltage then the load will be supplied from the regulator. Otherwise AUX will supply the system.

This is a relatively detailed schematic with the IC's internals:

^{simulate this circuit}

Now if AUX is something like an always-present power source (battery or a free-running supply) also if the BIAS is tied to AUX (i.e. BIAS is taken from the output directly) then you need to be careful. Because,

• When the input is not present (i.e. floating), since the internal logic will be suppled from the AUX directly, the IC (the internal logic) will consume a few milliamps through the AUX.
• When the input is shorted to ground (this may happen in some systems in order to keep the system grounded when not running) then this means that the M1 (high-side internal MOSFET) will have its drain (therefore body diode cathode) connected to ground. So a large current will flow through D1 (AUX blocking diode), internal inductor and M1's body diode. This will possibly kill the high-side MOSFET and even maybe the inductor.

To prevent this, you must place a diode in series with the input:

^{simulate this circuit}

So when the input is shorted to ground this series diode (D2 above) will block current flow. When the input is present the diode will allow current flow so the system will run normally.

If I have a 28V 10A power supply, I need to choose 10A diode? Please help me for determining.

You don't have to use one if there's no chance your input voltage gets shorted to ground. But if you want to use, you should consider:-

1. Total/maximum input current as it'll flow through this diode (D2 above). Make sure the average current rating is higher enough than the maximum possible input current (flows when the input voltage is at minimum). Since the whole input currents will flow through this diode, it would be beneficial to choose one with lowest forward drop to decrease the losses.
2. Possible peak currents drawn from the input. This happens during a step load change such as 0% to 100%. So the diode should handle these peak currents.
3. Maximum AUX voltage and maximum output voltage. In case of a short-to-ground event on input side the reverse standoff voltage across the D2 will be one diode drop less than the AUX voltage. So the reverse voltage rating of D2 must be higher enough than the AUX and the output voltages.

Answer 2

The use of this diode is well explained in the datasheet, but I will try to summarize in my own words:

This diode protects the LTM8053 against reverse current. Reverse current happens when Vin is shorted while Vout is still present or when the input voltage is (suddendly) reversed.

The diode must be rated for the maximum forward current.

Answer 3

You need a suitably rated Schottky Barrier Diode, to minimize the power dissipation. Something like the MBR1045 (45V 10A) might work, but check data sheets to match your use case.