LT3645 datasheet(11/24 Pages) LINER

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LT3645

3645f

APPLICATIONS INFORMATION

Now the required on time has decreased below the mini-

mum on time of 100ns. Instead of the switch pulse width

becoming narrower to accommodate the lower duty cycle

requirement, the part skips a few pulses so that the aver-

age inductor current meets and does not exceed the load

current requirement.

The LT3645 is robust enough to survive prolonged opera-

tion under these conditions as long as the peak inductor

current does not exceed 1.2A. Inductor saturation due

to high current may further limit performance in this

operating region.

Inductor Selection and Maximum Output Current

Choose the inductor value according to:

L = 2.2 •(VOUT + VD)/ƒ

where VD is the forward voltage drop of the catch diode

(~0.4V), f is the switching frequency in MHz and L is in

μH. With this value, there will be no subharmonic oscilla-

tion for applications with 50% or greater duty cycle. For

robust operation in fault conditions, the saturation current

should be above 1.5A. To keep efﬁciency high, the series

resistance (DCR) should be less than 0.1Ω. Table 1 lists

several inductor vendors. If the buck load current is less

than 500mA, then a lower valued inductor can be used.

Catch Diode

Depending on load current, a 500mA to 1A Schottky diode

is recommended for the catch diode, D1. The diode must

have a reverse voltage rating equal to or greater than the

overvoltage lockout voltage (38.5V). The ON Semiconduc-

tor MBRA140T3 and Central Semiconductor CMMSH1-40

are good choices, as they are rated for 1A continuous

forward current and a maximum reverse voltage of 40V.

Input Filter Network

Bypass VIN with a 1μF or higher ceramic capacitor of X7R

or X5R type. Y5V types have poor performance over tem-

perature and applied voltage and should not be used. A 1μF

ceramic capacitor is adequate to bypass the LT3645 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 might be necessary. This can be provided

with a low performance (high ESR) electrolytic capacitor

in parallel with the ceramic device. 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 LT3645 input

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

LT3645 and catch diode (see the PCB layout section). A

second precaution regarding the ceramic input capacitor

concerns the maximum input voltage rating of the LT3645.

A ceramic input capacitor combined with trace or cable

inductance forms a high quality (underdamped) tank cir-

cuit. If the LT3645 circuit is plugged into a live supply, the

input voltage can ring to twice its nominal value, possibly

exceeding the LT3645’s voltage rating. This situation can

easily be avoided. For more details, see Linear Technology

Application Note 88.

Table 1. Inductor Vendors

Vendor

URL

Part Series

Inductance Range (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

× 8.0

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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