Enable Threshold

Enable / UVLO

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.

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. This step is a 12 V buck UVLO, VIN_ON at 9 V.
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. The 5 V rail is the same idea with different trips.
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. 100 nF makes \(\tau\) obvious.
4
Leakage shifts the DC trips.
Pin current out of EN pulls the tap down, so VIN_ON moves up. High-Z dividers make this the whole error budget. 500 nA into a 20 µA string is that case.
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 uses the knobs on this page. VIN ramps up, dwells, ramps down. .meas catches EN rise and fall.

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.