LT1933HS6(RevD) データシートの表示（PDF） - Linear Technology

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LT1933HS6
(Rev.:RevD)

600mA, 500kHz Step-Down Switching Regulator in SOT-23 and DFN Packages

Linear Technology 

LT1933

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

output. If the VIN pin is allowed to float and the SHDN pin

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

to VIN), then the LT1933’s internal circuitry will pull its

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

zero. However, if the VIN pin is grounded while the output

is held high, then parasitic diodes inside the LT1933 can

pull large currents from the output through the SW pin

and the VIN pin. Figure 5 shows a circuit that will run only

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

at the VIN pin of the LT1933 can ring to twice the nominal

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.

D4

VIN

VIN

BOOST

LT1933

SHDN

GND

SW

FB

VOUT

BACKUP

D4: MBR0540

1933 F05

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

1933fd

13

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