LTC3838-2_15 Datasheet, PDF(27/56 Page) Linear Technology – Dual, Fast, Accurate Step-Down DC/DC Controller with xternal Reference Voltage and Dual Differential Output Sensing

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LTC3838-2_15 Datasheet, PDF (27/56 Pages) Linear Technology – Dual, Fast, Accurate Step-Down DC/DC Controller with xternal Reference Voltage and Dual Differential Output Sensing

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LTC3838-2

APPLICATIONS INFORMATION

CIN Selection

In continuous mode, the source current of the top N-

channel MOSFET is a square wave of duty cycle VOUT/

VIN. To prevent large voltage transients, a low ESR input

capacitor sized for the maximum RMS current must be

used. The worst-case RMS current occurs by assuming

a singleâphase application. The maximum RMS capacitor

current is given by:

IRMS

â IOUT(MAX)

â¢

VOUT

VIN

â¢

VIN â 1

VOUT

This formula has a maximum at VIN = 2VOUTâ, where

IRMS = IOUT(MAX)/2. This simple worst-case condition

is commonly used for design because even significant

deviations do not offer much relief. Note that capacitor

manufacturersâ ripple current ratings are often based on

only 2000 hours of life. This makes it advisable to further

derate the capacitor or to choose a capacitor rated at a

higher temperature than required. Several capacitors may

also be paralleled to meet size or height requirements in

the design. Due to the high operating frequency of the

LTC3838-2, additional ceramic capacitors should also be

used in parallel for CIN close to the IC and power switches

to bypass the high frequency switching noises. Typically

multiple X5R or X7R ceramic capacitors are put in parallel

with either conductive-polymer or aluminum-electrolytic

types of bulk capacitors. Because of its low ESR, the

ceramic capacitors will take most of the RMS ripple cur-

rent. Vendors do not consistently specify the ripple current

rating for ceramics, but ceramics could also fail due to

excessive ripple current. Always consult the manufacturer

if there is any question.

Figure 6 represents a simplified circuit model for calculat-

ing the ripple currents in each of these capacitors. The

input inductance (LIN) between the input source and the

input of the converter will affect the ripple current through

the capacitors. A lower input inductance will result in less

ripple current through the input capacitors since more

ripple current will now be flowing out of the input source.

For simulations with this model, look at the ripple current

during steady-state for the case where one phase is fully

loaded and the other was not loaded. This will in general

LIN

1ÂµH

ESR(BULK)

+â VIN

ESL(BULK)

CIN(BULK)

ESR(CERAMIC)

ESL(CERAMIC)

CIN(CERAMIC)

IPULSE(PHASE1)

IPULSE(PHASE2)

38382 F06

Figure 6. Circuit Model for Input Capacitor

Ripple Current Simulation

be the worst case for ripple current since the ripple cur-

rent from one phase will not be cancelled by ripple current

from the other phase.

Note that the bulk capacitor also has to be chosen for

RMS rating with ample margin beyond its RMS current

per simulation with the circuit model provided. For a lower

VIN range, a conductive-polymer type (such as Sanyo

OSâCON) can be used for its higher ripple current rating

and lower ESR. For a wide VIN range that also require

higher voltage rating, aluminum-electrolytic capacitors are

more attractive since it can provide a larger capacitance

for more damping. An aluminum-electrolytic capacitor

with a ripple current rating that is high enough to handle

all of the ripple current by itself will be very large. But

when in parallel with ceramics, an aluminum-electrolytic

capacitor will take a much smaller portion of the RMS

ripple current due to its high ESR. However, it is crucial

that the ripple current through the aluminum-electrolytic

capacitor should not exceed its rating since this will

produce significant heat, which will cause the electrolyte

inside the capacitor to dry over time and its capacitance

to go down and ESR to go up.

The benefit of PolyPhase operation is reduced RMS cur-

rents and therefore less power loss on the input capaci-

tors. Also, the input protection fuse resistance, battery

resistance, and PC board trace resistance losses are also

reduced due to the reduced peak currents in a PolyPhase

system. The details of a close form equation can be found

in Application Note 77 High Efficiency, High Density, Poly-

Phase Converters for High Current Applications. Figure 7

shows the input capacitor RMS ripple currents normalized

against the DC output currents with respect to the duty

For more information www.linear.com/3838-2

38382f

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