An external setpoint resistor divider from VIN to GND can be used to determine the minimum operating input range of the regulator. The divider must be designed such that the EN pin exceeds the 1.26-V standby threshold when VIN is in the desired operating range. The internal 6-μA current source must be included when determining the resistor values. The shutdown and standby thresholds have 100-mV hysteresis to prevent noise from toggling between modes. When the VIN voltage is below 3.5 VDC during start-up and the operating temperature is below –20°C, the EN pin must have a pullup resistor that provides 2 μA or greater current. The EN pin is internally protected by a 6-V Zener diode through a 1-kΩ resistor. The enabling voltage may exceed the Zener voltage, however the Zener current must be limited to less than 4 mA.

Two dedicated comparators connected to the EN pin are used to detect undervoltage and shutdown conditions. When the EN pin voltage is below 0.45 V, the controller is in a low-current shutdown mode where the VIN current is reduced to 95 μA. For an EN pin voltage greater than 0.45 V but less than 1.26 V, the controller is in standby mode, with all internal circuits operational, but the PWM gate driver signal is blocked. Once the EN pin voltage is greater than 1.26 V, the controller is fully enabled. Two external resistors can be used to program the minimum operational voltage for the power converter as shown in Figure 8-2. When the EN pin voltage falls below the
1.26 V threshold, an internal 100 mV threshold hysteresis prevents noise from toggling the state, so the voltage must be reduced to 1.16 V to transition to standby. Resistance values for R1 and R2 can be determined from Equation 2 and Equation 3.

Equation 2.

Equation 3.

where:

• V_PWR is the desired turnon voltage
• I_DIVIDER is an arbitrary current through R1 and R2

For example, if the LM5002 is to be enabled when V_PWR reaches 16 V, I_DIVIDER could be chosen as 501 μA that sets R1 to 29.4 kΩ and R2 to 2.49 kΩ. The voltage at the EN pin must not exceed 10 V unless the current into the 6 V protection Zener diode is limited below 4 mA. The EN pin voltage must not exceed 14 V at any time. Be sure to check both the power and voltage rating (some 0603 resistors are rated as low as 50 V) for the selected R1 resistor.

Remote configuration of the LM5002's operational modes can be accomplished with open drain device(s) connected to the EN pin as shown in Figure 8-3. A MOSFET or an NPN transistor connected to the EN pin can force the regulator into the low power off state. Adding a PN diode in the drain (or collector) provides the offset to achieve the standby state. The advantage of standby is that the VCC LDO is not disabled and external circuitry powered by VCC remains functional.