Datasheet

LM2595

SNVS122B –MAY 1999–REVISED APRIL 2013

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DELAYED STARTUP

The circuit in Figure 34 uses the the ON /OFF pin to provide a time delay between the time the input voltage is

applied and the time the output voltage comes up (only the circuitry pertaining to the delayed start up is shown).

As the input voltage rises, the charging of capacitor C1 pulls the ON /OFF pin high, keeping the regulator off.

Once the input voltage reaches its final value and the capacitor stops charging, and resistor R

pulls the ON

/OFF pin low, thus allowing the circuit to start switching. Resistor R

is included to limit the maximum voltage

applied to the ON /OFF pin (maximum of 25V), reduces power supply noise sensitivity, and also limits the

capacitor, C1, discharge current. When high input ripple voltage exists, avoid long delay time, because this ripple

can be coupled into the ON /OFF pin and cause problems.

This delayed startup feature is useful in situations where the input power source is limited in the amount of

current it can deliver. It allows the input voltage to rise to a higher voltage before the regulator starts operating.

Buck regulators require less input current at higher input voltages.

UNDERVOLTAGE LOCKOUT

Some applications require the regulator to remain off until the input voltage reaches a predetermined voltage. An

undervoltage lockout feature applied to a buck regulator is shown in Figure 35, while Figure 36 and Figure 37

applies the same feature to an inverting circuit. The circuit in Figure 36 features a constant threshold voltage for

turn on and turn off (zener voltage plus approximately one volt). If hysteresis is needed, the circuit in Figure 37

has a turn ON voltage which is different than the turn OFF voltage. The amount of hysteresis is approximately

equal to the value of the output voltage. If zener voltages greater than 25V are used, an additional 47 kΩ resistor

is needed from the ON /OFF pin to the ground pin to stay within the 25V maximum limit of the ON /OFF pin.

INVERTING REGULATOR

The circuit in Figure 38 converts a positive input voltage to a negative output voltage with a common ground. The

circuit operates by bootstrapping the regulator's ground pin to the negative output voltage, then grounding the

feedback pin, the regulator senses the inverted output voltage and regulates it.

This circuit has an ON/OFF threshold of approximately 13V.

Figure 36. Undervoltage Lockout for Inverting Regulator

This example uses the LM2595-5.0 to generate a −5V output, but other output voltages are possible by selecting

other output voltage versions, including the adjustable version.

Since this regulator topology can produce an output voltage that is either greater than or less than the input

voltage, the maximum output current greatly depends on both the input and output voltage. The curve shown in

Figure 39 provides a guide as to the amount of output load current possible for the different input and output

voltage conditions.

The maximum voltage appearing across the regulator is the absolute sum of the input and output voltage, and

this must be limited to a maximum of 40V. For example, when converting +20V to −12V, the regulator would see

32V between the input pin and ground pin. The LM2595 has a maximum input voltage spec of 40V.

Additional diodes are required in this regulator configuration. Diode D1 is used to isolate input voltage ripple or

noise from coupling through the C

capacitor to the output, under light or no load conditions. Also, this diode

isolation changes the topology to closley resemble a buck configuration thus providing good closed loop stability.

A Schottky diode is recommended for low input voltages, (because of its lower voltage drop) but for higher input

voltages, a fast recovery diode could be used.

Product Folder Links: LM2595