LTC7813 datasheet(23/36 Pages) LINER

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LTC7821

Rev A

For more information www.analog.com

APPLICATIONS INFORMATION

Ceramic capacitors also have a large voltage coefficient,

losing close to half their capacitance when the DC bias

across a given capacitor is half its rated voltage. The DC

bias effect on a capacitor is greater when the case size

is smaller. Factor in these effects when deciding on the

capacitance.

The ripple voltage of CFLY and CMID is given by:

VCFLY _RIPPLE =

IOUT • tON

2 •CFLY

VCMID _RIPPLE =

IOUT • tON

2 •CMID

where IOUT is the output current and tON is the on-time

of M1 and M3.

The ripple voltage on CFLYandCMID,(VCFLY_RIPPLE,VCMID_

RIPPLE ), contributes significantly to the power dissipated

in M2 and M4 (see Power MOSFET Selection section).

As a good starting point, select enough capacitance such

that the ripple on each capacitor is less than 1% of the DC

bias voltage of the capacitor. For example, if the DC bias

voltage of the capacitor is 24V, keep the ripple to be less

than 240mV. For the lowest conduction loss of MOSFETs

M1 and M3 (see Power MOSFET Selection section), select

the capacitance of CMID to be the same as that of CFLY.

Schottky Diode and Bootstrap Capacitors Selection

Three diodes are used to form a charge pump circuit

to provide the drive voltages for MOSFETs M1 to M3.

Figure 14 shows the diodes configuration. The voltages

acrossthefollowingbootstrapcapacitorsareapproximately:

VBST1_SW1 = VINTVCC – VF_D1 – VF_D2 – VF_D3

VBST2_MID = VINTVCC – VF_D2 – VF_D3

VBST2_SW3 = VINTVCC – VF_D3

where VF_DX is the forward voltage of diode X.

Figure 14. External Charge Pump

CBST1

CBST2

CBST3

7821 F14

LTC7821

VIN

TG1

TG2

BG1

BG2

SW1

MID

SW3

INTVCC

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