LTC3548EDD-2(RevA) Просмотр технического описания (PDF) - Linear Technology

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LTC3548EDD-2
(Rev.:RevA)

Dual Synchronous, Fixed/Adjustable Output, 2.25MHz Step-Down DC/DC Regulator

Linear Technology 

LTC3548-2

APPLICATIO S I FOR ATIO

series resistance (ESR) input capacitor sized for the maxi-

mum RMS current must be used. The maximum RMS

capacitor current is given by:

IRMS ≈IMAX

VOUT(VIN – VOUT

VIN

where the maximum average output current IMAX equals

the peak current minus half the peak-to-peak ripple cur-

rent, IMAX = ILIM – ∆IL/2.

This formula has a maximum at VIN = 2VOUT, where IRMS

= IOUT/2. This simple worst-case is commonly used to

design because even significant deviations do not offer

much relief. Note that capacitor manufacturer’s ripple

current ratings are often based on only 2000 hours life-

time. This makes it advisable to further derate the capaci-

tor, or choose a capacitor rated at a higher temperature

than required. Several capacitors may also be paralleled to

meet the size or height requirements of the design. An

additional 0.1µF to 1µF ceramic capacitor is also recom-

mended on VIN for high frequency decoupling, when not

using an all ceramic capacitor solution.

Output Capacitor (COUT) Selection

The selection of COUT is driven by the required ESR to

minimize voltage ripple and load step transients. Typically,

once the ESR requirement is satisfied, the capacitance is

adequate for filtering. The output ripple (∆VOUT) is deter-

mined by:

∆VOUT

≈

∆IL

⎛

⎝⎜

ESR

+

8

•

fO

1

• COUT

⎞

⎠⎟

where f = operating frequency, COUT = output capacitance

and ∆IL = ripple current in the inductor. The output ripple

is highest at maximum input voltage since ∆IL increases

with input voltage. With ∆IL = 0.3 • IOUT(MAX) the output

ripple will be less than 100mV at maximum VIN and

fO = 2.25MHz with:

ESRCOUT < 150mΩ

Once the ESR requirements for COUT have been met, the

RMS current rating generally far exceeds the IRIPPLE(P-P)

requirement, except for an all ceramic solution.

In surface mount applications, multiple capacitors may

have to be paralleled to meet the capacitance, ESR or RMS

current handling requirement of the application. Alumi-

num electrolytic, special polymer, ceramic and dry tantulum

capacitors are all available in surface mount packages. The

OS-CON semiconductor dielectric capacitor available from

Sanyo has the lowest ESR(size) product of any aluminum

electrolytic at a somewhat higher price. Special polymer

capacitors, such as Sanyo POSCAP, Panasonic Special

Polymer (SP), and Kemet A700, offer very low ESR, but

have a lower capacitance density than other types. Tanta-

lum capacitors have the highest capacitance density, but

they have a larger ESR and it is critical that the capacitors

are surge tested for use in switching power supplies. An

excellent choice is the AVX TPS series of surface mount

tantalums, available in case heights ranging from 2mm to

4mm. Aluminum electrolytic capacitors have a signifi-

cantly larger ESR, and are often used in extremely cost-

sensitive applications provided that consideration is given

to ripple current ratings and long term reliability. Ceramic

capacitors have the lowest ESR and cost, but also have the

lowest capacitance density, a high voltage and tempera-

ture coefficient, and exhibit audible piezoelectric effects.

In addition, the high Q of ceramic capacitors along with

trace inductance can lead to significant ringing.

In most cases, 0.1µF to 1µF of ceramic capacitors should

also be placed close to the LTC3548-2 in parallel with the

main capacitors for high frequency decoupling.

Ceramic Input and Output Capacitors

Higher value, lower cost ceramic capacitors are now

becoming available in smaller case sizes. These are tempt-

ing for switching regulator use because of their very low

ESR. Unfortunately, the ESR is so low that it can cause

loop stability problems. Solid tantalum capacitor ESR

generates a loop “zero” at 5kHz to 50kHz that is instrumen-

tal in giving acceptable loop phase margin. Ceramic ca-

pacitors remain capacitive to beyond 300kHz and usually

resonate with their ESL before ESR becomes effective.

Also, ceramic caps are prone to temperature effects which

requires the designer to check loop stability over the

operating temperature range. To minimize their large

temperature and voltage coefficients, only X5R or X7R

35482fa

9

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