LTC3402 datasheet(18/36 Pages) LINER

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LTC1871

1871fe

APPLICATIONS INFORMATION

Boost Converter: Input Capacitor Selection

The input capacitor of a boost converter is less critical

than the output capacitor, due to the fact that the inductor

is in series with the input and the input current waveform

is continuous (see Figure 12b). The input voltage source

impedance determines the size of the input capacitor,

which is typically in the range of 10μF to 100μF. A low ESR

capacitor is recommended, although it is not as critical as

for the output capacitor.

The RMS input capacitor ripple current for a boost con-

verter is:

IRMS(CIN) = 0.3 •

VIN(MIN)

L• f

•DMAX

Please note that the input capacitor can see a very high

surge current when a battery is suddenly connected to

the input of the converter and solid tantalum capacitors

can fail catastrophically under these conditions. Be sure

to specify surge-tested capacitors!

Burst Mode Operation and Considerations

The choice of MOSFET RDS(ON) and inductor value also

determines the load current at which the LTC1871 enters

Burst Mode operation. When bursting, the controller

clamps the peak inductor current to approximately:

IBURST(PEAK) =

30mV

RDS(ON)

which represents about 20% of the maximum 150mV

SENSE pin voltage. The corresponding average current

depends upon the amount of ripple current. Lower inductor

values (higher ΔIL) will reduce the load current at which

Burst Mode operations begins, since it is the peak current

that is being clamped.

The output voltage ripple can increase during Burst Mode

operation if ΔIL is substantially less than IBURST. This can

occur if the input voltage is very low or if a very large

inductor is chosen. At high duty cycles, a skipped cycle

causes the inductor current to quickly decay to zero.

However, because ΔIL is small, it takes multiple cycles

for the current to ramp back up to IBURST(PEAK). Dur-

ing this inductor charging interval, the output capacitor

must supply the load current and a signiﬁcant droop in

the output voltage can occur. Generally, it is a good idea

to choose a value of inductor ΔIL between 25% and 40%

of IIN(MAX). The alternative is to either increase the value

of the output capacitor or disable Burst Mode operation

using the MODE/SYNC pin.

Burst Mode operation can be defeated by connecting the

MODE/SYNC pin to a high logic-level voltage (either with

a control input or by connecting this pin to INTVCC). In

this mode, the burst clamp is removed, and the chip can

operate at constant frequency from continuous conduction

mode (CCM) at full load, down into deep discontinuous

conduction mode (DCM) at light load. Prior to skipping

pulses at very light load (i.e., < 5% of full load), the control-

ler will operate with a minimum switch on-time in DCM.

VIN

12a. Circuit Diagram

12b. Inductor and Input Currents

COUT

VOUT

IIN

12c. Switch Current

ISW

tON

12d. Diode and Output Currents

12e. Output Voltage Ripple Waveform

VOUT

(AC)

tOFF

ΔVESR

RINGING DUE TO

TOTAL INDUCTANCE

(BOARD + CAP)

ΔVCOUT

Figure 12. Switching Waveforms for a Boost Converter

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