LT1936H データシートの表示（PDF） - Linear Technology

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LT1936H

1.4A, 500kHz Step-Down Switching Regulator

Linear Technology 

LT1936

APPLICATIONS INFORMATION

Hot Plugging Safely

The small size, robustness and low impedance of ceramic

capacitors make them an attractive option for the input

bypass capacitor of LT1936 circuits. However, these capaci-

tors can cause problems if the LT1936 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 LT1936 can ring to twice the nominal

input voltage, possibly exceeding the LT1936’s rating and

damaging the part. If the input supply is poorly controlled

or the user will be plugging the LT1936 into an energized

supply, the input network should be designed to prevent

this overshoot.

Figure 8 shows the waveforms that result when an LT1936

circuit is connected to a 24V supply through six feet of

24-gauge twisted pair. The first plot is the response with

a 4.7μF ceramic capacitor at the input. The input voltage

rings as high as 50V and the input current peaks at 26A.

CLOSING SWITCH

SIMULATES HOT PLUG

IIN VIN

LT1936

+

4.7μF

VIN

20V/DIV

DANGER

RINGING VIN MAY EXCEED

ABSOLUTE MAXIMUM

RATING OF THE LT1936

LOW

IMPEDANCE

ENERGIZED

24V SUPPLY

STRAY

INDUCTANCE

DUE TO 6 FEET

(2 METERS) OF

TWISTED PAIR

IIN

10A/DIV

(8a)

20μs/DIV

+

22μF +

35V

AI.EI.

LT1936

4.7μF

VIN

20V/DIV

IIN

10A/DIV

(8b)

20μs/DIV

0.7Ω

LT1936

VIN

20V/DIV

+

0.1μF

4.7μF

IIN

10A/DIV

(8c)

20μs/DIV

1936 F08

Figure 8. A Well Chosen Input Network Prevents Input Voltage Overshoot and

Ensures Reliable Operation When the LT1936 is Connected to a Live Supply

1936fd

15

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