Enable / UVLO

Enable threshold

Key point: a two-resistor string scales the pin’s rising and falling thresholds onto VIN. Filter C delays both edges — so the transient must go up and down or you never see the fall.

Along the way

1
EN is a comparator, not a linear FB pin. Datasheet quotes \(V_{\mathrm{TH,r}}\) (enable) and \(V_{\mathrm{TH,f}}\) (disable). Hysteresis lives in those two voltages.
2
One ratio, two VIN trips. \(V_{\mathrm{IN,ON}}=V_{\mathrm{TH,r}}/\beta\), \(V_{\mathrm{IN,OFF}}=V_{\mathrm{TH,f}}/\beta\). You pick VIN_ON; VIN_OFF follows. A 2-resistor divider cannot set them independently.
3
Cfilt is a time constant on both edges. \(\tau=(R_{\mathrm{TOP}}\parallel R_{\mathrm{BOT}})C\). Rise lag and fall lag. The dwell is forced to \(\ge 5\tau\) so EN actually settles high before VIN comes down.
4
Leakage shifts the DC trips. Pin current out of EN pulls the tap down — VIN_ON moves up. High-Z dividers make this the whole error budget.
5
Need VIN_ON and VIN_OFF decoupled? That’s a third resistor or a monitor IC — LTC2965 calculator.
6
Regulator FB is a different tool. Loop held at \(V_{\mathrm{REF}}\) → Feedback divider.

LTspice download is one .asc from the schematic in this folder: live .param values, inline VIN PWL (up / dwell / down), no sidecar .txt. .meas reports rise and fall crossings on V(en).

VIN_ON target (V) VIN_max (V) \(V_{\mathrm{TH,rise}}\) (V) \(V_{\mathrm{TH,fall}}\) (V) \(V_{\mathrm{TH}}\) tol (%) Resistor series Resistor tol (%) I_DIV at VTH_rise (µA) Cfilt (nF) EN leakage (nA) VIN rise time (ms) VIN fall time (ms) Min dwell (ms)

Pin string

DC trips vs spec

RTOP—

RBOT—

VIN_ON (DC)—

VIN_OFF (DC)—

VIN hyst—

τ—

\[ \beta = R_{\mathrm{BOT}}/(R_{\mathrm{TOP}}+R_{\mathrm{BOT}}),\quad V_{\mathrm{IN,ON}}=V_{\mathrm{TH,r}}/\beta,\quad V_{\mathrm{IN,OFF}}=V_{\mathrm{TH,f}}/\beta \]

rise cross @— fall cross @—

Try it: set Cfilt to 0 nF — crossings sit on the DC VIN_ON / VIN_OFF lines. Then jump to 100 nF: rise lags, and if dwell is too short EN never settles, so the fall crossing is garbage. The tool lengthens dwell to \(5\tau\) so both edges stay visible. Then add 500 nA leakage and watch VIN_ON climb.