8.3.1 Undervoltage Lockout

Some applications require the regulator to remain off until the input voltage reaches a predetermined voltage. Figure 25 contains a undervoltage lockout circuit for a buck configuration, while Figure 26 and Figure 27 are for the inverting types (only the circuitry pertaining to the undervoltage lockout is shown). Figure 25 uses a Zener diode to establish the threshold voltage when the switcher begins operating. When the input voltage is less than the Zener voltage, resistors R1 and R2 hold the Shutdown/Soft-Start pin low, keeping the regulator in the shutdown mode. As the input voltage exceeds the Zener voltage, the Zener conducts, pulling the Shutdown/Soft-Start pin high, allowing the regulator to begin switching. The threshold voltage for the undervoltage lockout feature is approximately 1.5 V greater than the Zener voltage.

Figure 25. Undervoltage Lockout for a Buck Regulator

Figure 26 and Figure 27 apply the same feature to an inverting circuit. Figure 26 features a constant threshold voltage for turnon and turnoff (Zener voltage plus approximately 1 V). If hysteresis is needed, the circuit in Figure 27 has a turnon voltage which is different than the turnoff voltage. The amount of hysteresis is approximately equal to the value of the output voltage. Because the SD/SS pin has an internal 7-V Zener clamp, R2 is needed to limit the current into this pin to approximately 1 mA when Q1 is on.

Figure 26. Undervoltage Lockout Without Hysteresis for an Inverting Regulator

Figure 27. Undervoltage Lockout With Hysteresis for an Inverting Regulator

8.3.2 Negative Voltage Charge Pump

Occasionally a low current negative voltage is needed for biasing parts of a circuit. A simple method of generating a negative voltage using a charge pump technique is shown in Figure 28. This unregulated negative voltage is approximately equal to the positive input voltage (minus a few volts), and can supply up to a 600 mA of output current. There is a requirement however, that there be a minimum load of 1.2 A on the regulated positive output for the charge pump to work correctly. Also, resistor R1 is required to limit the charging current of C1 to some value less than the LM2593HV current limit. This method of generating a negative output voltage without an additional inductor can be used with other members of the SIMPLE SWITCHER® family, using either the buck or boost topology.

Figure 28. Charge Pump for Generating a Low-Current, Negative Output Voltage

8.3.3 Shutdown/Soft-Start

This reduction in start-up current is useful in situations where the input power source is limited in the amount of current it can deliver. In some applications, soft-start can be used to replace undervoltage lockout or delayed start-up functions. If a very slow output voltage ramp is desired, the soft-start capacitor can be made much larger. Many seconds or even minutes are possible. If only the shutdown feature is needed, the soft-start capacitor can be eliminated.

Figure 29. Typical Circuit Using Shutdown/Soft-Start and Error Flag Features

Figure 30. Soft-Start, Delay, Error Output

8.4.1 Shutdown Mode

The Shutdown/Soft-start pin provides electrical ON and OFF control for the LM2593HV. When the voltage of this pin is less than 0.6 V, the device is in shutdown mode. The typical standby current in this mode is 90 μA.

8.4.2 Active Mode

When the Shutdown/Soft-start pin is left floating or pull above 2 V, the device starts switching and the output voltage rises until it reaches a normal regulation voltage.