LT1432 Ver la hoja de datos (PDF) - Linear Technology

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LT1432

5V High Efficiency Step-Down Switching Regulator Controller

Linear Technology 

LT1432

APPLICATI S I FOR ATIO

0.2A RMS. Theoretically the output capacitor size would

be minimized by using one which just met this ripple

current, but in practice, this would yield such high output

ripple voltage that an additional output filter would have to

be added. A better solution in the case of buck converters

is usually just to increase the size of the output capacitor

to meet output ripple voltage requirements.

50mV/DIV

1A/DIV

0

5µs/DIV

Figure 8. Output Ripple Current

1A/DIV

0

tures, so be sure to check ESR ratings at the lowest

expected operating temperature. Ripple voltage can be

reduced by increasing the inductor value, but this has

rapidly diminishing returns because of typical size re-

straints.

Figure 9 shows diode current under normal load condi-

tions of 2A, and with the output shorted. Current limit has

been set at 3A. Average diode current at IOUT = 2A is only

about 1A because of duty cycle considerations. Under

short circuit conditions, duty cycle is nearly 100% for the

diode (switch duty cycle is near zero), and diode average

current is nearly 3A. Designs which must tolerate continu-

ous short circuit conditions should be checked carefully

for diode heating. Foldback current limiting can be used if

necessary.

Figure 10 shows inductor current (0.5A/DIV) with a 2A and

100mA load. Average inductor current is always equal to

output current, but it is obvious that with 100mA load,

inductor current drops to zero for part of the switching

cycle, indicating dicontinuous mode. When selecting an

inductor, keep in mind that RMS current determines

copper losses, peak-to-peak current determines core loss,

and peak current must be calculated to avoid core satura-

tion. Also, remember that during short circuit conditions,

inductor current will increase to the full current limit value.

Inductor failure is normally caused by overheating of the

winding insulation with resultant turn-to-turn shorts.

Foldback current limiting will be helpful.

1A/DIV

0

5µs/DIV

Figure 9. Diode Current

Figure 8 shows output ripple voltage at the top and switch

current below. Peak-to-peak ripple voltage is 80mV. This

implies an output capacitor effective series resistance

(ESR) of 80mV/0.7A = 0.11Ω. Capacitor ESR varies sig-

nificantly with temperature, increasing at low tempera-

0.5A/DIV

0

5µs/DIV

Figure 10. Inductor Current

11

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