LTC3499B datasheet(11/16 Pages) LINER

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LTC3499/LTC3499B

3499f

Low ESR capacitors should be used to minimize output

voltage ripple. A 4.7

µF to 10µF output capacitor is suffi-

cient for most applications and should be placed as close

to VOUT as possible. Larger values may be used to obtain

even lower output ripple and improve transient response.

X5R and X7R dielectric materials are preferred for their

ability to maintain capacitance over wide voltage and

temperature ranges.

Input Capacitor Selection

The input filter capacitor reduces peak currents drawn

from the input source and reduces input switching noise.

Ceramic capacitors are a good choice for input decoupling

due to their low ESR and ability to withstand reverse

voltage (i.e. non-polar nature). The capacitor should be

located as close as possible to the device. In most appli-

cations a 2.2

µF input capacitor is sufficient. Larger values

may be used without limitations. Table 2 shows a list of

several ceramic capacitor manufacturers.

Table 2. Capacitor Vendor Information

SUPPLIER

WEB SITE

AVX

www.avxcorp.com

Murata

www.murata.com

TDK

www.component.tdk.com

Taiyo Yuden

www.t-yuden.com

Thermal Considerations

For the LTC3499/LTC3499B to deliver full output power, it

is imperative that a good thermal path be provided to

dissipate the heat generated within the package. For the

DFN package, this can be accomplished by taking advan-

tage of the large thermal pad on the underside of the

device. It is recommended that multiple vias in the printed

circuit board be used to conduct heat away from the part

and into a copper plane with as much area as possible. If

the junction temperature continues to rise, the part will go

into thermal shutdown where switching will stop until the

temperature drops.

Closing the Feedback Loop

The LTC3499/LTC3499B utilize current mode control,

with internal slope compensation. Current mode control

eliminates the 2nd order filter due to the inductor and

output capacitor exhibited in voltage mode controllers,

thus simplifying it to a single pole filter response. The

product of the modulator control to output DC gain and the

error amp open loop gain gives the DC gain of the system:

CONTROL

REF

OUT

CURRENT SENSE

CONTROL

OUT

CURRENT SENSE

DS ON

()

••

•

•,

1000

The output filter pole is given by:

FILTER POLE

OUT

••

()

where COUT is the output filter capacitor.

The output filter zero is given by:

FILTER ZERO

ESR

OUT

••

•

()

where RESR is the capacitor equivalent series resistance.

A troublesome feature of the boost regulator topology is

the right half plane (RHP) zero, given by:

RPHZ

OUT

()

2•

•

APPLICATIO S I FOR ATIO

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