LTC4091 datasheet(17/26 Pages) LINER

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LTC4091

4091fa

For more information www.linear.com/LTC4091

APPLICATIONS INFORMATION

Table 1. Inductor Vendors

VENDOR URL

PART SERIES

TYPE

Murata

www.murata.com

LQH55D

Open

TDK

www.componenttdk.com

SLF7045

SLF10145

Shielded

Toko

www.toko.com

D62CB

D63CB

D75C

D75F

Shielded

Open

Sumida

www.sumida.com

CR54

CDRH74

CDRH6D38

CR75

Open

Shielded

Open

Catch Diode

The catch diode conducts current only during switch-off

time. Average forward current in normal operation can

be calculated from:

ID(AVG) =IHVOUT •

VIN – VHVOUT

(

)

VIN

where IHVOUTistheoutputloadcurrent.Theonlyreasonto

consideradiodewithalargercurrentratingthannecessary

for nominal operation is for the worst-case condition of

shorted output. The diode current will then increase to the

typical peak switch current. Peak reverse voltage is equal

to the regulator input voltage. Use a Schottky diode with

a reverse voltage rating greater than the input voltage.

The overvoltage protection feature in the high voltage

regulator will keep the switch off when VIN > 40V which

allows the use of a 40V rated Schottky even when VVIN

ranges up to 60V. Table 2 lists several Schottky diodes

and their manufacturers.

Table 2. Diode Vendors

PART NUMBER

(V)

IAVE

(A)

VF AT 1A

(MV)

VF AT 2A

(MV)

On Semiconductor

MBRM120E

MBRM140

530

550

595

Diodes Inc.

B120

B130

B220

B230

DFLS240L

500

International Rectifier

10BQ030

20BQ030

420

470

Switching Regulator Capacitor Selection

Ceramic capacitors are small, robust and have very low

ESR providing the best ripple performance. However, ce-

ramic capacitors can cause problems when used with the

high voltage switching regulator due to their piezoelectric

nature. When in Burst Mode operation, the LTC4091’s

switching frequency depends on the load current, and

at very light loads the LTC4091 can excite the ceramic

capacitor at audio frequencies, generating audible noise.

Since the LTC4091 operates at a lower current limit during

Burst Mode operation, the noise is typically very quiet to a

casual ear. If this is unacceptable, use a high performance

tantalum or electrolytic capacitor at the output.

Only a small amount of capacitance is required on HVOUT

– about 3.3µF. This choice will provide low output ripple

andgoodtransientresponse.Transientperformancecanbe

improvedwithahighervaluecapacitorifthecompensation

network is also adjusted to maintain the loop bandwidth. A

lower value of output capacitor can be used to save space

and cost but transient performance will suffer. See the

Switching Regulator Frequency Compensation section to

choose an appropriate compensation network.

When choosing a capacitor, look carefully through the

data sheet to find out what the actual capacitance is under

operating conditions (applied voltage and temperature).

A physically larger capacitor, or one with a higher voltage

rating, may be required. Low ESR is important, so choose

one that is intended for use in switching regulators. The

ESR should be specified by the supplier, and should be

0.05Ω or less.

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