LM2575

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SLVS569F – JANUARY 2005 – REVISED AUGUST 2015

Typical Application (continued)

The type of inductor chosen can have advantages and disadvantages. If high performance or quality is a

concern, then more-expensive toroid core inductors are the best choice, as the magnetic flux is contained

completely within the core, resulting in less EMI and noise in nearby sensitive circuits. Inexpensive bobbin core

inductors, however, generate more EMI as the open core does confine the flux within the core. Multiple switching

regulators located in proximity to each other are particularly susceptible to mutual coupling of magnetic fluxes

from each other’s open cores. In these situations, closed magnetic structures (such as a toroid, pot core, or E-

core) are more appropriate.

Regardless of the type and value of inductor used, the inductor never should carry more than its rated current.

Doing so may cause the inductor to saturate, in which case the inductance quickly drops, and the inductor looks

like a low-value resistor (from the DC resistance of the windings). As a result, switching current rises dramatically

(until limited by the current-by-current limiting feature of the LM2575) and can result in overheating of the

inductor and the IC itself.

NOTE

Different types of inductors have different saturation characteristics.

8.2.2.5 Output Voltage Ripple and Transients

As with any switching power supply, the output of the LM2575 has a sawtooth-ripple voltage at the switching

frequency. Typically about 1% of the output voltage, this ripple is due mainly to the inductor sawtooth-ripple

current and the ESR of the output capacitor (see note on COUT). Furthermore, the output also may contain small

voltage spikes at the peaks of the sawtooth waveform. This is due to the fast switching of the output switch and

the parasitic inductance of COUT. These voltage spikes can be minimized through the use of low-inductance

capacitors.

There are several ways to reduce the output ripple voltage: a larger inductor, a larger COUT, or both. Another

method is to use a small LC filter (20

μH and 100 μF) at the output. This filter can reduce the output ripple

voltage by a factor of 10 (see Figure 16).

8.2.2.6 Feedback Connection

FEEDBACK must be connected between the two programming resistors. Again, both of these resistors should be

in close proximity to the regulator, and each should be less than 100 k

Ω to minimize noise pickup.

8.2.2.7

ON/OFF Input

ON/OFF should be grounded or be a low-level TTL voltage (typically <1.6 V) for normal operation. To shut down

the LM2575 and put it in standby mode, a high-level TTL or CMOS voltage must be supplied to this pin. ON/OFF

must not be left open and safely can be pulled up to VIN with or without a pullup resistor.

8.2.2.8 Grounding

The power and ground connections of the LM2575 must be low-impedance to help maintain output stability. With

the 16-pin package, all the ground pins (including signal and power grounds) must be soldered directly to wide

PCB copper traces to ensure low-inductance connections and good thermal dissipation.

8.2.2.9 Reverse Current Considerations

There is an internal diode from the output to VIN. Therefore, the device does not protect against reverse current

and take care to limit current in this scenario.

8.2.2.10 Buck Regulator Design Procedure

PROCEDURE

EXAMPLE

Known:

Known:

VOUT(Nom)

VOUT = 10 V

VIN(Max) = Maximum input voltage

VIN(Max) = 25 V

ILOAD(Max) = Maximum load current

ILOAD(Max) = 1 A

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