LT3503 Просмотр технического описания (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

Table 1. Inductor Vendors

Vendor

URL

Sumida

www.sumida.com

Toko

www.toko.com

Würth Elektronik www.we-online.com

Part Series

CDRH4D28

CDRH5D28

CDRH8D28

A916CY

D585LC

WE-TPC(M)

WE-PD2(M)

WE-PD(S)

Inductance Range (µH)

1.2 to 4.7

2.5 to 10

2.5 to 33

2 to 12

1.1 to 39

1 to 10

2.2 to 22

1 to 27

Size (mm)

4.5 × 4.5

5.5 × 5.5

8.3 × 8.3

6.3 × 6.2

8.1 × 8.0

4.8 × 4.8

5.2 × 5.8

7.3 × 7.3

transient response. If your load is lower than 1A, then you

can decrease the value of the inductor and operate with

higher ripple current. This allows you to use a physically

smaller inductor, or one with a lower DCR resulting in

higher efficiency. There are several graphs in the Typical

Performance Characteristics section of this data sheet that

show the maximum load current as a function of input

voltage and inductor value for several popular output

voltages. Low inductance may result in discontinuous

mode operation, which is okay, but further reduces maxi-

mum load current. For details of the maximum output

current and discontinuous mode operation, see Linear

Technology Application Note 44.

Catch Diode

Depending on load current, a 1A to 2A Schottky diode is

recommended for the catch diode, D1. The diode must

have a reverse voltage rating equal to or greater than the

maximum input voltage. The ON Semiconductor MBRM140

is a good choice; it is rated for 1A continuous forward

current and a maximum reverse voltage of 40V.

Input Capacitor

Bypass the input of the LT3503 circuit with a 1µF or higher

value ceramic capacitor of X7R or X5R type. Y5V types

have poor performance over temperature and applied

voltage and should not be used. A 1µF ceramic is adequate

to bypass the LT3503 and will easily handle the ripple

current. However, if the input power source has high

impedance, or there is significant inductance due to long

wires or cables, additional bulk capacitance may be nec-

essary. This can be provided with a low performance

electrolytic capacitor.

Step-down regulators draw current from the input supply

in pulses with very fast rise and fall times. The input

capacitor is required to reduce the resulting voltage ripple

at the LT3503 and to force this very high frequency

switching current into a tight local loop, minimizing EMI.

A 1µF capacitor is capable of this task, but only if it is

placed close to the LT3503 and the catch diode; see the

PCB Layout section. A second precaution regarding the

ceramic input capacitor concerns the maximum input

voltage rating of the LT3503. A ceramic input capacitor

combined with trace or cable inductance forms a high

quality (underdamped) tank circuit. If the LT3503 circuit is

plugged into a live supply, the input voltage can ring to

twice its nominal value, possibly exceeding the LT3503’s

voltage rating. This situation is easily avoided; see the Hot

Plugging Safely section.

Output Capacitor

The output capacitor has two essential functions. Along

with the inductor, it filters the square wave generated by

the LT3503 to produce the DC output. In this role it

determines the output ripple so low impedance at the

switching frequency is important. The second function is

to store energy in order to satisfy transient loads and

stabilize the LT3503’s control loop.

3503f

10

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