LM2597 datasheet(30/34 Pages) NSC

30 / 34 page

Application Information (Continued)

An additional diode is required in this regulator configuration.

Diode D1 is used to isolate input voltage ripple or noise from

coupling through the C

IN capacitor to the output, under light

or no load conditions. Also, this diode isolation changes the

topology to closely resemble a buck configuration thus pro-

viding good closed loop stability. A Schottky diode is recom-

mended for low input voltages, (because of its lower voltage

drop) but for higher input voltages, a 1N4001 diode could be

used.

Because of differences in the operation of the inverting

regulator, the standard design procedure is not used to

select the inductor value. In the majority of designs, a 100

µH, 1 Amp inductor is the best choice. Capacitor selection

can also be narrowed down to just a few values. Using the

values shown in

Figure 29 will provide good results in the

majority of inverting designs.

This type of inverting regulator can require relatively large

amounts of input current when starting up, even with light

loads. Input currents as high as the LM2597 current limit

(approximately 0.8A) are needed for 1 ms or more, until the

output reaches its nominal output voltage. The actual time

depends on the output voltage and the size of the output

capacitor. Input power sources that are current limited or

sources that can not deliver these currents without getting

loaded down, may not work correctly. Because of the rela-

tively high startup currents required by the inverting topology,

the Soft-start feature shown in

Figure 29 is recommended.

Also shown in

Figure 29 are several shutdown methods for

the inverting configuration. With the inverting configuration,

some level shifting is required, because the ground pin of the

regulator is no longer at ground, but is now at the negative

output voltage. The shutdown methods shown accept

ground referenced shutdown signals.

UNDERVOLTAGE LOCKOUT

Some applications require the regulator to remain off until

the input voltage reaches a predetermined voltage.

Figure

31 contains a undervoltage lockout circuit for a buck configu-

ration, while

Figure 32 and Figure 33 are for the inverting

types (only the circuitry pertaining to the undervoltage lock-

out is shown).

Figure 31 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 ex-

ceeds 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 1.5V greater than the zener voltage.

Figure 32 and Figure 33 apply the same feature to an

inverting circuit.

Figure 32 features a constant threshold

voltage for turn on and turn off (zener voltage plus approxi-

mately one volt). If hysteresis is needed, the circuit in

Figure

33 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. Since the SD /SS pin has an

internal 7V zener clamp, R2 is needed to limit the current into

this pin to approximately 1 mA when Q1 is on.

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 and the

switching waveform present at the OUT pin, is shown in

Figure 34. This unregulated negative voltage is approxi-

mately equal to the positive input voltage (minus a few volts),

and can supply up to a 100 mA of output current. There is a

requirement however, that there be a minimum load of sev-

DS012440-49

FIGURE 30. Maximum Load Current for Inverting

Regulator Circuit

DS012440-50

FIGURE 31. Undervoltage Lockout for a Buck

Regulator

DS012440-52

FIGURE 32. Undervoltage Lockout Without

Hysteresis for an Inverting Regulator

DS012440-53

FIGURE 33. Undervoltage Lockout With

Hysteresis for an Inverting Regulator

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