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LT3980IMSE-TRPBF

58V, 2A, 2.4MHz Step-Down Switching Regulator with 85µA Quiescent Current

Linear Technology 

LT3980

Applications Information

Of course, such a simple design guide will not always re-

sult in the optimum inductor for your application. A larger

value inductor provides a slightly higher maximum load

current and will reduce the output voltage ripple. If your

load is lower than 2A, 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 maximum load current. For details of

maximum output current and discontinuous mode oper-

ation, see Linear Technology Application Note 44. Finally,

for duty cycles greater than 50% (VOUT/VIN > 0.5), there

is a minimum inductance required to avoid subharmonic

oscillations. See AN19.

Input Capacitor

Bypass the input of the LT3980 circuit with a ceramic

capacitor of X7R or X5R type. Y5V types have poor

performance over temperature and applied voltage, and

should not be used. A 10µF to 22µF ceramic capacitor is

adequate to bypass the LT3980 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 rip-

ple at the LT3980 and to force this very high frequency

switching current into a tight local loop, minimizing EMI.

A 10µF capacitor is capable of this task, but only if it is

placed close to the LT3980 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 LT3980. A ceramic input capacitor

combined with trace or cable inductance forms a high

quality (under damped) tank circuit. If the LT3980 circuit

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

twice its nominal value, possibly exceeding the LT3980’s

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

Plugging Safety section).

For space sensitive applications, a 4.7µF ceramic ca-

pacitor can be used for local bypassing of the LT3980

input. However, the lower input capacitance will result in

increased input current ripple and input voltage ripple, and

may couple noise into other circuitry. Also, the increased

voltage ripple will raise the minimum operating voltage

of the LT3980 to ~3.7V.

Output Capacitor and Output Ripple

The output capacitor has two essential functions. Along

with the inductor, it filters the square wave generated by the

LT3980 to produce the DC output. In this role it determines

the output ripple, and low impedance at the switching

frequency is important. The second function is to store

energy in order to satisfy transient loads and stabilize the

LT3980’s control loop. Ceramic capacitors have very low

equivalent series resistance (ESR) and provide the best

ripple performance. A good starting value is:

COUT

=

100

VOUT fSW

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 the compensation network is also adjusted

to maintain the loop bandwidth. A lower value of output

capacitor can be used to save space and cost but transient

performance will suffer. See the Frequency Compensation

section to choose an appropriate compensation network.

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 capacitor.

Low ESR is important, so choose one that is intended for

For more information www.linear.com/LT3980

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