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LTC3787

3787fc

APPLICATIONS INFORMATION

CIN and COUT Selection

The input ripple current in a boost converter is relatively

low (compared with the output ripple current), because this

current is continuous. The input capacitor CINvoltagerating

should comfortably exceed the maximum input voltage.

Although ceramic capacitors can be relatively tolerant of

overvoltage conditions, aluminum electrolytic capacitors

are not. Be sure to characterize the input voltage for any

possible overvoltage transients that could apply excess

stress to the input capacitors.

The value of CIN is a function of the source impedance, and

in general, the higher the source impedance, the higher the

required input capacitance. The required amount of input

capacitance is also greatly affected by the duty cycle. High

output current applications that also experience high duty

cycles can place great demands on the input supply, both

in terms of DC current and ripple current.

In a boost converter, the output has a discontinuous current,

so COUT must be capable of reducing the output voltage

ripple. The effects of ESR (equivalent series resistance) and

the bulk capacitance must be considered when choosing

the right capacitor for a given output ripple voltage. The

steady ripple voltage due to charging and discharging

the bulk capacitance in a single phase boost converter

is given by:

RIPPLE =

OUT(MAX) •(VOUT − VIN(MIN))

OUT •VOUT •f

where COUT is the output filter capacitor.

The steady ripple due to the voltage drop across the ESR

is given by:

ΔVESR = IL(MAX) • ESR

The LTC3787 is configured as a 2-phase single output

converter where the outputs of the two channels are

connected together and both channels have the same

duty cycle. With 2-phase operation, the two channels

are operated 180 degrees out-of-phase. This effectively

interleaves the output capacitor current pulses, greatly

reducing the output capacitor ripple current. As a result,

the ESR requirement of the capacitor can be relaxed.

Because the ripple current in the output capacitor is a

square wave, the ripple current requirements for the output

capacitor depend on the duty cycle, the number of phases

and the maximum output current. Figure 3 illustrates the

normalized output capacitor ripple current as a function of

duty cycle in a 2-phase configuration. To choose a ripple

current rating for the output capacitor, first establish the

duty cycle range based on the output voltage and range

of input voltage. Referring to Figure 3, choose the worst-

case high normalized ripple current as a percentage of the

maximum load current.

Multiple capacitors placed in parallel may be needed to

meet the ESR and RMS current handling requirements.

Dry tantalum, special polymer, aluminum electrolytic and

ceramic capacitors are all available in surface mount

packages. Ceramic capacitors have excellent low ESR

characteristics but can have a high voltage coefficient.

Capacitors are now available with low ESR and high ripple

current ratings (e.g., OS-CON and POSCAP).

Figure 3. Normalized Output Capacitor Ripple

Current (RMS) for a Boost Converter

0.1

0.9

3787 F03

0.3

0.5

0.7

0.8

0.2

0.4

0.6

3.25

3.00

2.75

2.50

2.25

2.00

1.75

1.50

1.25

1.00

0.75

0.50

0.25

DUTY CYCLE OR (1-VIN/VOUT)

1-PHASE

2-PHASE

PolyPhase Operation

For output loads that demand high current, multiple

LTC3787s can be cascaded to run out-of-phase to provide

more output current and at the same time to reduce input

and output voltage ripple. The PLLIN/MODE pin allows the

LTC3787 to synchronize to the CLKOUT signal of another

LTC3787. The CLKOUT signal can be connected to the

PLLIN/MODE pin of the following LTC3787 stage to line

up both the frequency and the phase of the entire system.

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