LT3669EUFD 查看數據表（PDF） - Linear Technology

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LT3669EUFD

IO-Link Transceiver with Integrated Step-Down Regulator and LDO

Linear Technology 

LT3669/LT3669-2

APPLICATIONS INFORMATION

to check that the LT3669 will be able to deliver the required

output current. Note again that these equations assume

that the inductor current is continuous. Discontinuous

operation occurs when IOUT is less than ΔIL/2.

Input Capacitor

Bypass the input of the LT3669 circuit with a ceramic ca-

pacitor of X7R or X5R type. Do not use Y5V types, which

have poor performance over temperature and applied

voltage. A 4.7μF ceramic capacitor is adequate to bypass

the LT3669 and will easily handle the ripple current. Note

that larger input capacitance is required when a lower

switching frequency is used. 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 lower performance

electrolytic capacitor.

Step-down regulators draw current from the input sup-

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

capacitor is required to reduce the resulting voltage

ripple at the LT3669 and to force this very high frequency

switching current into a tight local loop, minimizing EMI.

A 4.7μF capacitor is capable of this task, but only if it is

placed close to the LT3669 (see the PCB Layout section

for more information). A second precaution regarding

the ceramic input capacitor concerns the maximum input

voltage rating of the LT3669. A ceramic input capacitor

combined with trace or cable inductance forms a high-Q

(underdamped) tank circuit. If the LT3669 circuit is plugged

into a live supply, the input voltage can ring to twice its

nominal value, possibly exceeding the LT3669’s voltage

rating. For guidance see Application Note 88.

Output Capacitor and Output Ripple

The output capacitor has two essential functions. Along

with the inductor, it filters the square wave generated by

the LT3669 to produce the DC output. In this role, it de-

termines the output ripple. Additionally, low impedance at

the switching frequency is important. The second function

is to store energy needed to satisfy transient loads and

stabilize the LT3669’s control loop. Ceramic capacitors

have very low equivalent series resistance (ESR) and

provide the best ripple performance. A good starting

value is:

COUT

=

k

VOUT •

fSW

(k = 17 in LT3669; k = 43 in LT3669-2)

where fSW is in MHz, and COUT is the recommended output

capacitance in μF. Use X5R or X7R types. This choice will

provide low output ripple and good transient response.

Transient performance can be improved with a higher

value capacitor if combined with a phase lead capacitor

(typically 22pF) between the output and the feedback pin

(FBOUT). A lower value of output capacitor can be used to

save space and cost but transient performance will suffer.

When choosing a capacitor, look carefully through the

data sheet to find out what the actual capacitance is under

operating conditions (applied voltage and temperature).

A physically larger capacitor, or one with a higher voltage

rating, may be required. High performance tantalum or

electrolytic capacitors can be used for the output capaci-

tor. Low ESR is important, so choose one that is intended

for use in switching regulators. The ESR should be 0.05Ω

or less. Such a capacitor will be larger than a ceramic

capacitor and will have a larger capacitance, because the

capacitor must be large to achieve low ESR.

LT3669-2 Diode Selection

The catch diode (D1 from the LT3669-2 Block Diagram)

conducts current only during the switch-off time. Average

forward current in normal operation is

ID(AVG) = IOUT • (1−DC)

where DC is the duty cycle. However, a diode with 1A cur-

rent rating is required for overload conditions. For inputs

up to the maximum operating voltage of 40V, use a diode

with a reverse-voltage rating greater than the input voltage.

If transients at the input of up to 60V are expected, use a

diode with a reverse-voltage rating only higher than the

maximum OVLO of 45V. If operating at high ambient tem-

peratures, consider using a Schottky with low reverse leak-

age. For example, Diodes, Inc. SBR1U40LP or DFLS160,

ON Semiconductor MBRM140, and Central Semiconductor

CMMSH1-60 are good choices for the catch diode.

3669fa

30

For more information www.linear.com/LT3669

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