LTC3355 datasheet(13/18 Pages) AD

13 / 18 page

LTC3355-1

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

The current in the inductor is a triangle wave with an

average value equal to the load current. The peak inductor

and switch current is:

ISW(PEAK) =IL(PEAK) =IOUT(MAX) +

ΔIL

where IL(PEAK) is the peak inductor current, IOUT(MAX) is

the maximum output load current and ∆IL is the inductor

ripple current. The LTC3355-1 limits the switch current

in order to protect the part. Therefore, the maximum

output current that the buck will deliver depends on the

switch current limit, the inductor value, and the input and

output voltages.

When the switch is off, the potential across the inductor

is the output voltage plus the catch diode drop. This gives

the peak-to-peak ripple current in the inductor:

ΔIL = 1–DC

(

)• VOUT +VD

L • fSW

where fSW is the switching frequency of the buck, DC is

the duty cycle and L is the value of the inductor.

To maintain output regulation, the inductor peak current

must be less than the buck switch current limit. The max-

imum output current is:

IOUT(MAX) =ILIM –

ΔIL

Choosing an inductor value so that the ripple current is

small will allow a maximum output current near the switch

current limit.

Table 1. Inductor Vendors

VENDOR

URL

PART SERIES

TYPE

Murata

www.murata.com

LQH5BPB

Shielded

TDK

www.tdk.com

LTF5022T

Shielded

Toko

www.toko.com

FDS50xx

Shielded

Coilcraft

www.coilcraft.com

XAL40xx, LPS40xx Shielded

Sumida

www.sumida.com

DCRH5D, CDRH6D Shielded

Viashay

www.vishay.com

IHLP2020

Shielded

One approach to choosing the inductor is to start with

the simple rule above, look at the available inductors, and

choose one to meet cost or space goals. Then use the

equations to check that the buck will be able to deliver the

required output current. These equations assume that the

inductor current is continuous. Discontinuous operation

occurs when IOUT is less than ∆IL/2.

Buck Input Capacitor

Bypass VIN and VINS with a ceramic capacitor of X7R or

X5R type. A 10µF to 22µF ceramic capacitor is adequate

for bypassing. Note that a larger VINS bypass capacitor

may be required if the input power supply source imped-

ance is high or there is significant inductance due to long

wires or cables. This can be provided with a lower perfor-

mance electrolytic capacitor in parallel with the ceramic

capacitor.

Buck regulators draw current from the input supply in

pulses with very fast rise and fall times. The input capac-

itors are required to reduce the resulting voltage ripple at

VINS and VIN and to force this very high frequency switch-

ing into a tight local loop, minimizing EMI. The capacitors

must be placed close to the LTC3355-1 pins.

Output Capacitor and Output Ripple

The output capacitor has two essential functions. Along

with the inductor, it filters the square wave generated by

the buck regulator 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 sta-

bilize the buck regulator control loop. Ceramic capacitors

have very low equivalent series resistance (ESR) and pro-

vide the best ripple performance. A good starting value is:

COUT = fSW

100

VOUT

⎛

⎝⎜

⎞

⎠⎟

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.

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