LTC3707-SYNC Ver la hoja de datos (PDF) - Linear Technology

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LTC3707-SYNC

High Efficiency, 2-Phase Synchronous Step-Down Switching Regulator

Linear Technology 

LTC3707-SYNC

APPLICATIO S I FOR ATIO

paralleled to meet size or height requirements in the

design. Always consult the manufacturer if there is any

question.

The benefit of the LTC3707-SYNC multiphase can be

calculated by using the equation above for the higher

power controller 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 capacitor’s ESR. This is why the

input capacitor’s requirement calculated above for the

worst-case controller 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 imped-

ance of the power supply/battery is included in the effi-

ciency 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 unde-

sirable 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 (∆VOUT) is determined by:

∆VOUT

≈



∆ILESR +

1

8fCOUT 

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 = 0.3IOUT(MAX) the output

ripple will typically be less than 50mV at max VIN assum-

ing:

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

16

due to ripple current. The choice of using smaller output

capacitance increases the ripple voltage due to the dis-

charging 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, United Chemicon 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 TPSV or the

KEMET T510 series of surface mount tantalums, available

in case heights ranging from 2mm to 4mm. Aluminum

electrolytic capacitors can be used in cost-driven applica-

tions providing that consideration is given to ripple current

ratings, temperature and long term reliability. A typical

application will require several to many aluminum electro-

lytic capacitors in parallel. A combination of the above

mentioned capacitors will often result in maximizing per-

formance and minimizing overall cost. Other capacitor

types include Nichicon PL series, NEC Neocap, Cornell

Dubilier ESRE and Sprague 595D series. Consult manu-

facturers for other specific recommendations.

3707sf

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