LM5069 hot-swap circuit for 48 V
I’m designing a 48 V input protection / hot-swap stage using the LM5069 and would really appreciate a sanity check on my schematic and component choices.
Application specs
-
Input source: 48 V nominal battery, absolute max 65 V (15Ah)
-
Downstream load: isolated DC-DC module (48V to 12V)
-
Input range: up to 75 V max
-
Max input current: ~1A
-
Environment: industrial, frequent hot-plugging of the battery connector, possible ESD / surge events on the 48 V input
I want to protect against reverse polarity, inrush, short circuit and over-voltage; keep the DC-DC input within its 75 V limit.
Hot Swap Controller - LM5069
MOSFET - SI4058DY-T1-GE3
Questions -
-
Is a 3 A fuse reasonable here for a 1A max load plus inrush events, or should I go lower / higher? Would you recommend a slow-blow / time-lag fuse for this kind of hot-swap circuit, or a fast-acting type to better protect the MOSFET?
-
For a 48 V system with up to 1A continuous load, is a simple series diode (V1PM15HM3_H) acceptable? or should I replace with MOSFET-based ideal diode)
-
Is SI4058DY-T1-GE3 MOSFET a good fit for a 48 V (up to 65 V max) with max load of 1A ?
Any feedback on the overall design with this LM5069 implementation would be very helpful.
2 answers
I looked at the brief description on page 1 and the table on page 3 of the hot swap controller datasheet and noticed a few things:
- The current sense resistor between the Vin and Sense pins is used to determine the output current. The chip limits the load current so that the voltage across the sense resistor does not exceed 55 mV. Your sense resistor is 350 mΩ. (55 mV)/(350 mΩ) = 157 mA.
I assume this only applies during the startup phase. That's your job to check. If so, is that the current you are willing a hot-swapped device to draw immediately? Make sure this is really what you want.
- The chip goes into over-voltage lockout when the OVLO pin exceeds 2.5 V. With your R83 and R86 divider, that will happen at an input voltage of 32 V. That won't allow operation at 48 V, let alone the max valid input of 65 V.
Again, I didn't really read the datasheet, so maybe there are some details not apparent from the chart on page 3. Still, this is something you should look into unless you're sure it's right.
0 comment threads
Inrush, overcurrent, fuse, etc: That hot-swap IC has both current limit (ie for diode and fuse) and MOSFET-power limit, and the time to blow the fuse is longer than timescales of the semiconductors. Peak current while blowing higher than 3A also. The use of the fuse would be to protect battery from shorting into a damaged circuit.
Series Diode: V1PM15 -- could work with careful thermal design. Could be a size larger. 150V is a good choice. Datasheet I-V-temperature graph gives V*I wattage at 1A: 0.5W-1W (with 0.5 being hot), and that'll be similar in a larger package. The 130C/W called out for the "minimal footprint" (with the DO219AD) can be improved with extra copper at the pad, and 4 layer stack with plane in the second layer. But that means effectively enlarging the floorspace it takes up, so then a bigger diode package may be a safer bet without any downside. Also take into account if circuit lives in an unventilated enclosure, or even a controls cabinet, these are above ambient.
TVS: 5.0SMDJ54A -- Strictly speaking, supply_max=65V exceeds Vrwm=54V. Looking at Vbr(1mA)=60V-66V (at 25C and rising with temperature), it would conduct, and for specimens on the low end of the Vbr, might settle into a livable equilibrium after self heating. Is that too aggressive for an industrial application? I'd feel better with 5.0SMDJ58A or 5.0SMDJ60A; Noting that Vcl then gets close to the 100V limit of both the LM5069 and the mosfet.
MOSFET: SI4058DY-T1- -- Should be fine. If you want more margin nothing wrong with using a 150V model. It's exposed to the Vcl of the TVS, just like the LM5069, but at least it would be one less thing that can get close to its limits. Looks plenty fast and easy to drive.
0 comment threads
0 comment threads