LM1575, LM2575-N, LM2575HV

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SNVS106E – MAY 1999 – REVISED APRIL 2013

Table 2. Inductor Selection by Manufacturer's Part Number (continued)

Inductor Code

Inductor Value

H1500

1500

μH

67127120

PE-53121

RL1958

H2200

2200

μH

67127130

PE-53122

RL2448

APPLICATION HINTS

To maintain stability, the regulator input pin must be bypassed with at least a 47

μF electrolytic capacitor. The

capacitor's leads must be kept short, and located near the regulator.

If the operating temperature range includes temperatures below

−25°C, the input capacitor value may need to be

larger. With most electrolytic capacitors, the capacitance value decreases and the ESR increases with lower

temperatures and age. Paralleling a ceramic or solid tantalum capacitor will increase the regulator stability at cold

temperatures. For maximum capacitor operating lifetime, the capacitor's RMS ripple current rating should be

greater than

(8)

INDUCTOR SELECTION

All switching regulators have two basic modes of operation: continuous and discontinuous. The difference

between the two types relates to the inductor current, whether it is flowing continuously, or if it drops to zero for a

period of time in the normal switching cycle. Each mode has distinctively different operating characteristics,

which can affect the regulator performance and requirements.

The LM2575 (or any of the Simple Switcher family) can be used for both continuous and discontinuous modes of

operation.

The inductor value selection guides in Figure 27 through Figure 31 were designed for buck regulator designs of

the continuous inductor current type. When using inductor values shown in the inductor selection guide, the

peak-to-peak inductor ripple current will be approximately 20% to 30% of the maximum DC current. With

relatively heavy load currents, the circuit operates in the continuous mode (inductor current always flowing), but

under light load conditions, the circuit will be forced to the discontinuous mode (inductor current falls to zero for a

period of time). This discontinuous mode of operation is perfectly acceptable. For light loads (less than

approximately 200 mA) it may be desirable to operate the regulator in the discontinuous mode, primarily because

of the lower inductor values required for the discontinuous mode.

The selection guide chooses inductor values suitable for continuous mode operation, but if the inductor value

chosen is prohibitively high, the designer should investigate the possibility of discontinuous operation. The

computer design software Switchers Made Simple will provide all component values for discontinuous (as well

as continuous) mode of operation.

Inductors are available in different styles such as pot core, toriod, E-frame, bobbin core, etc., as well as different

core materials, such as ferrites and powdered iron. The least expensive, the bobbin core type, consists of wire

wrapped on a ferrite rod core. This type of construction makes for an inexpensive inductor, but since the

magnetic flux is not completely contained within the core, it generates more electromagnetic interference (EMI).

This EMI can cause problems in sensitive circuits, or can give incorrect scope readings because of induced

voltages in the scope probe.

The inductors listed in the selection chart include ferrite pot core construction for AIE, powdered iron toroid for

Pulse Engineering, and ferrite bobbin core for Renco.

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