LT1933

13

1933fe

APPLICATIONS INFORMATION

Shorted and Reversed Input Protection

If the inductor is chosen so that it won’t saturate exces-

sively, an LT1933 buck regulator will tolerate a shorted

output. There is another situation to consider in systems

where the output will be held high when the input to the

LT1933 is absent. This may occur in battery charging ap-

plications or in battery backup systems where a battery

or some other supply is diode OR-ed with the LT1933’s

is held high (either by a logic signal or because it is tied

quiescent current through its SW pin. This is fine if your

system can tolerate a few mA in this state. If you ground

the SHDN pin, the SW pin current will drop to essentially

is held high, then parasitic diodes inside the LT1933 can

pull large currents from the output through the SW pin

when the input voltage is present and that protects against

a shorted or reversed input.

Hot Plugging Safely

The small size, robustness and low impedance of ceramic

capacitors make them an attractive option for the input

bypass capacitor of LT1933 circuits. However, these capaci-

tors can cause problems if the LT1933 is plugged into a

live supply (see Linear Technology Application Note 88 for

a complete discussion). The low loss ceramic capacitor

combined with stray inductance in series with the power

source forms an under damped tank circuit, and the voltage

input voltage, possibly exceeding the LT1933’s rating and

damaging the part. If the input supply is poorly controlled

or the user will be plugging the LT1933 into an energized

supply, the input network should be designed to prevent

this overshoot.

Figure 5. Diode D4 Prevents a Shorted Input from Discharging a Backup

Battery Tied to the Output; It Also Protects the Circuit from a Reversed

Input. The LT1933 Rns Only When the Input is Present

BOOST

GND

FB

SHDN

SW

D4

LT1933

1933 F05

BACKUP

D4: MBR0540