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diodes (for example, 1N4001 or 1N5400, and so on) are also not suitable. See Table 8-3 for Schottky and soft

fast-recovery diode selection guide.

8.1.6 Output Voltage Ripple and Transients

The output voltage of a switching power supply contains a sawtooth ripple voltage at the switcher frequency,

typically about 1% of the output voltage, and can also contain short voltage spikes at the peaks of the sawtooth

waveform.

The output ripple voltage is due mainly to the inductor sawtooth ripple current multiplied by the ESR of the output

capacitor (see Section 8.1.2).

The voltage spikes are present because of the fast switching action of the output switch, and the parasitic

inductance of the output filter capacitor. To minimize these voltage spikes, special low inductance capacitors can

be used, and their lead lengths must be kept short. Wiring inductance, stray capacitance, as well as the scope

probe used to evaluate these transients, all contribute to the amplitude of these spikes.

An additional small LC filter (20 μH and 100 μF) can be added to the output (as shown in Figure 7-4) to further

reduce the amount of output ripple and transients. A 10 × reduction in output ripple voltage and transients is

possible with this filter.

8.1.7 Feedback Connection

The LM2576 (fixed voltage versions) feedback pin must be wired to the output voltage point of the switching

power supply. When using the adjustable version, physically locate both output voltage programming resistors

near the LM2576 to avoid picking up unwanted noise. Avoid using resistors greater than 100 kΩ because of the

increased chance of noise pickup.

8.1.8 ON /OFF INPUT

For normal operation, the ON /OFF pin must be grounded or driven with a low-level TTL voltage (typically

below 1.6 V). To put the regulator into standby mode, drive this pin with a high-level TTL or CMOS signal. The

ON /OFF pin can be safely pulled up to +VIN without a resistor in series with it. The ON /OFF pin must not be left

open.

8.1.9 Inverting Regulator

Figure 8-1 shows a LM2576-12 in a buck-boost configuration to generate a negative 12-V output from a positive

input voltage. This circuit bootstraps the ground pin of the regulator to the negative output voltage, then by

grounding the feedback pin, the regulator senses the inverted output voltage and regulates it to −12 V.

For an input voltage of 12 V or more, the maximum available output current in this configuration is approximately

700 mA. At lighter loads, the minimum input voltage required drops to approximately 4.7 V.

The switch currents in this buck-boost configuration are higher than in the standard buck-mode design, thus

lowering the available output current. Also, the start-up input current of the buck-boost converter is higher than

the standard buck-mode regulator, and this can overload an input power source with a current limit less than

5 A. Using a delayed turn-on or an undervoltage lockout circuit (described in Section 8.1.10) can allow the input

voltage to rise to a high enough level before the switcher can be allowed to turn on.

Because of the structural differences between the buck and the buck-boost regulator topologies, the buck

regulator design procedure section can not be used to select the inductor or the output capacitor. The

recommended range of inductor values for the buck-boost design is between 68 μH and 220 μH, and the

output capacitor values must be larger than what is normally required for buck designs. Low input voltages or

high output currents require a large value output capacitor (in the thousands of micro Farads).

The peak inductor current, which is the same as the peak switch current, can be calculated in Equation 6:

Ip≈ ILOADVIN+VOUT

VIN + VINVOUT

VIN+VOUT 12L1fOSC

(6)

where fosc = 52 kHz

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LM2576, LM2576HV

SNVS107G – JUNE 1999 – REVISED MARCH 2023

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