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LTC1876

1876fa

Medium voltage (20V to 35V) ceramic, tantalum, OS-CON

and switcher-rated electrolytic capacitors can be used as

input capacitors, but each has drawbacks: ceramic voltage

coefficients are very high and may have audible piezoelec-

tric effects; tantalums need to be surge-rated; OS-CONs

suffer from higher inductance, larger case size and limited

surface-mount applicability; electrolytics’ higher ESR and

dryout possibility require several to be used. Multiphase

systems allow the lowest amount of capacitance overall.

As little as one 22

µF or two to three 10µF ceramic capaci-

tors are an ideal choice in a 20W to 35W power supply due

to their extremely low ESR. Even though the capacitance

at 20V is substantially below their rating at zero-bias, very

low ESR loss makes ceramics an ideal candidate for

highest efficiency battery operated systems. Also con-

sider parallel ceramic and high quality electrolytic capaci-

tors as an effective means of achieving ESR and bulk

capacitance goals.

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

nel 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 of one channel must

be used. The maximum RMS capacitor current is given by:

quired I

RMS

MAX

OUT

≈

−

()

[]12

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

IRMS = IOUT/2. This simple worst case condition is com-

monly used for 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

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. Always consult the manufacturer if there is any

question.

The benefit of the LTC1876 multiphase controllers 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 reduced overlap of

current pulses required 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 protection fuse

resistance, battery resistance and PC board trace resis-

tance 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 pro-

duce undesirable voltage and current resonances at VIN.

For the boost regulator, the ripple requirement for the

input capacitor is less stringent. If the supply to the

regulator is obtained from one of the LTC1876 step-down

outputs, a 1

µF to 4.7µF ceramic capacitor is sufficient.

However, if the step-down output is within close proximity

(< 1cm) to the boost supply input, there is no need for the

capacitor.

COUT Selection

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.

For the step-down regulators, the output ripple (

∆VOUT) is

determined by:

∆∆

I ESR

OUT

≈+









Where f = operating frequency, COUT = output capacitance,

and

∆L= ripple current in the inductor. The output ripple is

highest at maximum input voltage since

∆IL increases

with input voltage. With

∆IL = 0.4IOUT(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 guaran-

tees that the output capacitance does not significantly

APPLICATIO S I FOR ATIO

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