LM2598 datasheet(34/45 Pages) TI1

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LM2598

SNVS125C – MARCH 1998 – REVISED APRIL 2013

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Figure 45. Maximum Load Current for Inverting

Regulator Circuit

An additional diode is required in this regulator configuration. Diode D1 is used to isolate input voltage ripple or

noise from coupling through the CIN capacitor to the output, under light or no load conditions. Also, this diode

isolation changes the topology to closely 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 1N5400 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 68

μH, 1.5 Amp inductor is the best choice. Capacitor

selection can also be narrowed down to just a few values. Using the values shown in Figure 44 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 LM2598 current limit (approximately 1.5A) are needed for 2 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 relatively high startup currents required by

the inverting topology, the Soft-start feature shown in Figure 44 is recommended.

Also shown in Figure 44 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 46 contains a undervoltage lockout circuit for a buck configuration, while Figure 47 and Figure 48 are for

the inverting types (only the circuitry pertaining to the undervoltage lockout is shown). Figure 46 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 1.5V greater than the zener voltage.

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