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Inductor

Output Voltage Ripple and Transients

Feedback Connection

ON/OFF Input

TL2575, TL2575HV

1-A SIMPLE STEP-DOWN SWITCHING VOLTAGE REGULATORS

SLVS638B – MAY 2006 – REVISED JANUARY 2007

APPLICATION INFORMATION (continued)

Proper inductor selection is key to the performance-switching power-supply designs. One important factor to

consider is whether the regulator is used in continuous mode (inductor current flows continuously and never

drops to zero) or in discontinuous mode (inductor current goes to zero during the normal switching cycle). Each

mode has distinctively different operating characteristics and, therefore, can affect the regulator performance

and requirements. In many applications, the continuous mode is the preferred mode of operation, since it offers

greater output power with lower peak currents, and also can result in lower output ripple voltage. The

advantages of continuous mode of operation come at the expense of a larger inductor required to keep inductor

current continuous, especially at low output currents and/or high input voltages.

The TL2575 and TL2575HV can operate in either continuous or discontinuous mode. With heavy load currents,

the inductor current flows continuously and the regulator operates in continuous mode. Under light load, the

inductor fully discharges and the regulator is forced into the discontinuous mode of operation. For light loads

(approximately 200 mA or less), this discontinuous mode of operation is perfectly acceptable and may be

desirable solely to keep the inductor value and size small. Any buck regulator eventually operates in

discontinuous mode when the load current is light enough.

The type of inductor chosen can have advantages and disadvantages. If high performance/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 not 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 TL2575 and TL2575HV) and can result in

overheating of the inductor and the IC itself. Note that different types of inductors have different saturation

characteristics.

As with any switching power supply, the output of the TL2575 and TL2575HV have a sawtooth ripple voltage at

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

may contain small voltage spikes at the peaks of the sawtooth waveform. This is due to the fast switching of the

low-inductance capacitors.

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 2).

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.

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

the TL2575 or TL2575HV and put it in standby mode, a high-level TTL or CMOS voltage should be supplied to

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