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LT3502/LT3502A

3502fc

APPLICATIONS INFORMATION

Catch Diode

A low capacitance 500mA 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 Phillips PMEG4005AEA is a good choice; it

is related for 500mA continuous forward current and a

maximum reverse voltage of 40V.

Input Capacitor

Bypass the input of the LT3502/LT3502A 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 LT3502/LT3502A 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

necessary. 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 ca-

pacitor is required to reduce the resulting voltage ripple at

the LT3502/LT3502A 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 LT3502/LT3502A and the catch diode (see the

PCB Layout section). A second precaution regarding the

ceramic input capacitor concerns the maximum input volt-

age rating of the LT3502/LT3502A. A ceramic input capaci-

tor combined with trace or cable inductance forms a high

quality (underdamped) tank circuit. If the LT3502/LT3502A

circuit is plugged into a live supply, the input voltage can

ring to twice its nominal value, possibly exceeding the

LT3502/LT3502A’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 LT3502/LT3502A 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 LT3502/LT3502A’s control loop. Ceramic capacitors

have very low equivalent series resistance (ESR) and

provide the best ripple performance. A good value is:

COUT =

VOUT

for the LT3502A

COUT =

VOUT

for the LT3502

where COUT is in μF. Use an X5R or X7R type and keep

in mind that a ceramic capacitor biased with VOUT will

have less than its nominal capacitance. This choice will

provide low output ripple and good transient response.

Transient performance can be improved with a high value

capacitor, but a phase lead capacitor across the feedback

resistor, R1, may be required to get the full beneﬁt (see

the Compensation section).

Table 1

VENDOR

URL

PART SERIES

INDUCTANCE RATE (μH)

SIZE (mm)

Sumida

www.sumida.com

CDRH4D28

CDRH5D28

CDRH8D28

1.2 to 4.7

2.5 to 10

2.5 to 33

4.5 × 4.5

5.5 × 5.5

8.3 × 8.3

Toko

www.toko.com

A916CY

D585LC

2 to 12

1.1 to 39

6.3 × 6.2

8.1 × 8

Würth Elektronik

www.we-online.com

WE-TPC(M)

WE-PD2(M)

WE-PD(S)

1 to 10

2.2 to 22

1 to 27

4.8 × 4.8

5.2 × 5.8

7.3 × 7.3

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