LTC3727_15 Datasheet, PDF(16/32 Page) Linear Technology – High Efficiency, 2-Phase Synchronous Step-Down Switching Regulators

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LTC3727_15 Datasheet, PDF (16/32 Pages) Linear Technology – High Efficiency, 2-Phase Synchronous Step-Down Switching Regulators

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LTC3727/LTC3727-1

APPLICATIO S I FOR ATIO

The benefit of the LTC3727 multiphase can be calculated

by using the equation above for the higher power control-

ler and then calculating the loss that would have resulted

if both controller channels switch on at the same time. The

total RMS power lost is lower when both controllers are

operating due to the interleaving of current pulses through

the input capacitorâs ESR. This is why the input capacitorâs

requirement calculated above for the worst-case control-

ler is adequate for the dual controller design. Remember

that input protection fuse resistance, battery resistance

and PC board trace resistance losses are also reduced due

to the reduced peak currents in a multiphase system. The

overall benefit of a multiphase design will only be fully

realized when the source impedance of the power supply/

battery is included in the efficiency testing. The drains of

the two top MOSFETS should be placed within 1cm of each

other and share a common CIN(s). Separating the drains

and CIN may produce undesirable voltage and current

resonances at VIN.

The selection of COUT is driven by the required effective

series resistance (ESR). Typically once the ESR require-

ment is satisfied the capacitance is adequate for filtering.

The output ripple (ÎVOUT) is determined by:

ÎVOUT

ÎIL

âââESR

8fCOUT

â â

Where f = operating frequency, COUT = output capaci-

tance, and ÎIL= ripple current in the inductor. The output

ripple is highest at maximum input voltage since ÎIL

increases with input voltage. With ÎIL = 0.3IOUT(MAX) the

output ripple will typically be less than 50mV at max VIN

assuming:

COUT Recommended ESR < 2 RSENSE

and COUT > 1/(8fRSENSE)

The first condition relates to the ripple current into the

ESR of the output capacitance while the second term

guarantees that the output capacitance does not signifi-

cantly discharge during the operating frequency period

due to ripple current. The choice of using smaller output

capacitance increases the ripple voltage due to the

discharging term but can be compensated for by using

capacitors of very low ESR to maintain the ripple voltage

at or below 50mV. The ITH pin OPTI-LOOP compensation

components can be optimized to provide stable, high

performance transient response regardless of the output

capacitors selected.

Manufacturers such as Nichicon, Nippon Chemi-Con and

Sanyo can be considered for high performance through-

hole capacitors. The OS-CON semiconductor dielectric

capacitor available from Sanyo has the lowest (ESR)(size)

product of any aluminum electrolytic at a somewhat

higher price. An additional ceramic capacitor in parallel

with OS-CON capacitors is recommended to reduce the

inductance effects.

In surface mount applications multiple capacitors may

need to be used in parallel to meet the ESR, RMS current

handling and load step requirements of the application.

Aluminum electrolytic, dry tantalum and special polymer

capacitors are available in surface mount packages. Spe-

cial polymer surface mount capacitors offer very low ESR

but have lower storage capacity per unit volume than other

capacitor types. These capacitors offer a very cost-effec-

tive output capacitor solution and are an ideal choice when

combined with a controller having high loop bandwidth.

Tantalum capacitors offer the highest capacitance density

and are often used as output capacitors for switching

regulators having controlled soft-start. Several excellent

surge-tested choices are the AVX TPS, AVX TPS Series III

or the KEMET T510 series of surface mount tantalums,

available in case heights ranging from 1.2mm to 4.1mm.

Aluminum electrolytic capacitors can be used in cost-

driven applications providing that consideration is given

to ripple current ratings, temperature and long term reli-

ability. A typical application will require several to many

aluminum electrolytic capacitors in parallel. A combina-

tion of the above mentioned capacitors will often result in

maximizing performance and minimizing overall cost. Other

capacitor types include Nichicon PL series, NEC Neocap,

Cornell Dubilier ESRE and Sprague 595D series. Consult

manufacturers for other specific recommendations.

3727fc

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