E-004 — Output protection and fault-shutdown strategy concept

Hypothesis

Since there is no MCU, overcurrent and overvoltage protection must be implemented with discrete logic. The LM5156 has a built-in cycle-by-cycle current limit, but we need a latching shutdown for hard faults and a way to signal the fault state to the user (LED or similar).

Alternatives considered

Option A — LM5156 UVLO pin as fault latch Pull the UVLO pin low via an LM339 comparator when Vout exceeds a threshold. The LM5156 shuts down and stays off until power is cycled.

• Pros: no extra ICs, uses existing comparator bank (LM339 has 4 channels)
• Cons: UVLO threshold interacts with startup — needs careful resistor divider design; hard to distinguish OVP from normal UVLO

Option B — SR latch from 74HC logic Use one 74HC00 NAND gate pair as an SR latch. Comparator trips SET, power cycle clears RESET.

• Pros: clean separation of fault latch from UVLO; easy to add an LED driver from the latch Q output
• Cons: one more IC; requires a weak pull-down on the RESET line to clear on power-up

Option C — Self-resetting PTC + no latch PTC fuse in series with output; temperature-based reset. No latching.

• Pros: dead simple, zero ICs
• Cons: does not protect against OVP, only OCP; reset time unpredictable; not suitable for lab supply (user expects immediate restart after fault removal)

Block diagram (Option B preferred)

graph LR
    Vout --> OVP["LM339 OVP comparator"]
    Isense --> OCP["LM339 OCP comparator"]
    OVP -- SET --> LATCH["74HC00 SR latch"]
    OCP -- SET --> LATCH
    PWRON -- RESET --> LATCH
    LATCH -- Q --> EN["LM5156 EN pin"]
    LATCH -- Q --> LED["Fault LED"]

Open questions

• Does the LM5156 EN pin source enough current to drive a LED directly, or do we need a buffer transistor?
• What is the correct OVP threshold — Vout_set + 10% or Vout_set + 5%? Needs tolerance stack-up analysis.
• For the highest output (56 V), what comparator reference voltage is practical with a single 5 V rail?

Next step

daniel@esharp.se to check LM5156 EN pin sink current spec in datasheet and do a tolerance stack-up for the 56 V OVP threshold.