LT3503 데이터 시트보기 (PDF) - Linear Technology

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LT3503

1A, 2.2MHz Step-Down Switching Regulator in 2mm × 3mm DFN

Linear Technology 

LT3503

APPLICATIO S I FOR ATIO

D4

VIN

VIN

BOOST

LT3503

SHDN

GND

SW

FB

VOUT

BACKUP

3503 F09

Figure 9. 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 LT3503 Runs Only When the Input

is Present

Hot Plugging Safely

The small size, robustness and low impedance of ceramic

capacitors make them an attractive option for the input

bypass capacitor of LT3503 circuits. However, these ca-

pacitors can cause problems if the LT3503 is plugged into

a live supply (see Linear Technology Application Note 88

for a complete discussion). The low loss ceramic capaci-

tor combined with stray inductance in series with the

power source forms an underdamped tank circuit, and the

voltage at the VIN pin of the LT3503 can ring to twice the

nominal input voltage, possibly exceeding the LT3503’s

rating and damaging the part. If the input supply is poorly

controlled or the user will be plugging the LT3503 into an

energized supply, the input network should be designed to

prevent this overshoot.

Figure 10 shows the waveforms that result when an

LT3503 circuit is connected to a 20V supply through six

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

with a 2.2µF ceramic capacitor at the input. The input

voltage rings as high as 35V and the input current peaks

at 20A. One method of damping the tank circuit is to add

another capacitor with a series resistor to the circuit. In

Figure 10b an aluminum electrolytic capacitor has been

added. This capacitor’s high equivalent series resistance

CLOSING SWITCH

SIMULATES HOT PLUG

IIN VIN

LT3503

+

2.2µF

VIN

20V/DIV

LOW

IMPEDANCE

ENERGIZED

20V SUPPLY

STRAY

INDUCTANCE

DUE TO 6 FEET

(2 METERS) OF

TWISTED PAIR

IIN

5A/DIV

(10a)

DANGER!

RINGING VIN MAY EXCEED

ABSOLUTE MAXIMUM

RATING OF THE LT3503

20µs/DIV

+

10µF +

35V

AI.EI.

LT3503

2.2µF

VIN

20V/DIV

IIN

5A/DIV

(10b)

1Ω

LT3503

VIN

+

20V/DIV

0.1µF

2.2µF

20µs/DIV

IIN

5A/DIV

(10c)

20µs/DIV

3503 F10

Figure 10. A Well Chosen Input Network Prevents Input Voltage Overshoot and

Ensures Reliable Operation When the LT3503 is Connected to a Live Supply

3503f

15

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