LTC3370 datasheet(13/22 Pages) AD

13 / 22 page

LTC3315B

Rev. 0

For more information www.analog.com

Operating Frequency Selection and Trade-Offs

Selection of the operating frequency is a trade-off between

efficiency, component size, transient response, and input

voltage range. The LTC3315B can operate at frequencies

between 3MHz and 10MHz.

The advantage of high frequency operation is that smaller

inductor and capacitor values may be used. Higher

switching frequencies allow for higher control loop

bandwidth and, therefore, faster transient response. The

disadvantages of higher switching frequencies are lower

efficiency, because of increased switching losses, and a

smaller input voltage range, because of minimum switch

on-time limitations.

The minimum on-time of the buck regulator imposes a

minimum operating duty cycle. The highest switching

frequency (fSW(MAX)) for a given application can be cal-

culated as follows:

fSW(MAX) =

VOUT

tON(MIN) • VIN(MAX)

(2)

where VIN(MAX) is the maximum input voltage, VOUT is the

output voltage and tON(MIN) is the minimum top switch

on-time. This equation shows that a slower switching

frequency might be necessary to accommodate a high

VIN/VOUT ratio.

The LTC3315B is capable of a maximum duty cycle of

100%, therefore, the VIN-to-VOUT dropout is limited by

the RDS(ON) of the PMOS, the inductor DCR, and the load

current.

Inductor Selection and Maximum Output Current

Considerations in choosing an inductor are inductance,

RMS current rating, saturation current rating, DCR, and

core loss.

Selecttheinductorvaluebasedonthefollowingequations:

L ≥

VOUT

0.6A • fSW

• 1−

VOUT

VIN(MAX)

⎛

⎝

⎜⎜

⎞

⎠

⎟⎟ for

VOUT

VIN(MAX)

≤ 0.5 (3)

L ≥

VIN(MAX)

2.4A • fSW

for

VOUT

VIN(MAX)

> 0.5

(4)

where fSW is the switching frequency and VIN(MAX) is the

maximum applied input voltage.

To avoid overheating of the inductor, choose an inductor

with an RMS current rating that is greater than the maxi-

mumexpectedoutputloadoftheapplication.Overloadand

short-circuitconditionsneedtobetakenintoconsideration.

In addition ensure that the saturation current rating (typi-

cally labeled ISAT) is either higher than 3.5A, the maximum

currentlimitoftheLTC3315B,orhigherthanthemaximum

expected load plus half the inductor ripple:

ISAT >ILOAD(MAX)+

ΔIL

(5)

where ILOAD(MAX) is the maximum output load current

for the application and ΔIL is the inductor ripple current

calculated as:

ΔIL =

VOUT

L • fSW

• 1−

VOUT

VIN

⎛

⎝

⎜

⎞

⎠

⎟

(6)

To keep the efficiency high, choose an inductor with

the lowest series resistance (DCR) and a core material

intended for high frequency applications. Table 1 shows

recommended inductors from several manufacturers.

Input Capacitors

BypasstheinputoftheLTC3315Bwithatleasttwoceramic

capacitors, one near each VIN pin for best performance.

Connect the ground of each capacitor to a wide PCB trace

on the top layer of the PCB that connects pins 8 and 9

with the exposed pad. These capacitors should be 0603

or 0805 in size. Smaller 0201 capacitors can also be

placed as close as possible to the LTC3315B directly on

the traces leading from VIN (Pin 7) and GND (Pin 8) and

on the traces leading from VIN (Pin 10) and GND (Pin 9)

to reduce input noise with minimal (if at all) increase in

application footprint. See the layout section for more

detail. X7R or X5R capacitors are recommended for best

performance across temperature and input voltage varia-

tions (see Table 2). Note that larger input capacitance is

required when a lower switching frequency is used. If

the input power source has high impedance, or if there

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