LTC3458L

12

3458lf

Input Capacitor Selection

The input filter capacitor reduces peak currents drawn

from the input source and reduces input switching noise.

In most applications >1

µF per amp of peak input current

is recommended. See Table 3 for a list of capacitor

manufacturers for input and output capacitor selection.

Table 3. Capacitor Vendor Information

Supplier

Phone

Website

AVX

(803) 448 - 9411

www.avxcorp.com

Sanyo

(619) 661 - 6322

www.sanyovideo.com

TDK

(847) 803 - 6100

www.component.tdk.com

Murata

USA: (814) 237-1431

(800) 831-9172

www.murata.com

Taiyo Yuden

(408) 573 - 4150

www.t-yuden.com

Operating Frequency Selection

There are several considerations in selecting the operating

frequency of the converter. The first is staying clear of

sensitive frequency bands, which cannot tolerate any

spectral noise. For example in products incorporating RF

communications the 455kHz IF frequency is sensitive to

any noise, therefore switching above 600kHz is desired.

Some communications have sensitivity to 1.1MHz and in

that case a 1.5MHz switching converter frequency may be

employed. The second consideration is the physical size of

the converter. As the operating frequency goes up, the

inductor and filter capacitors go down in value and size.

The trade off is in efficiency, since the switching losses due

to gate charge increase proportional with frequency.

Thermal Considerations

For the LTC3458L to deliver its full output power, it is

imperative that a good thermal path be provided to dissi-

pate the heat generated within the package. This can be

accomplished by taking advantage of the large thermal

pad on the underside of the IC. It is recommended that

multiple vias in the printed circuit board be used to

conduct heat away from the IC and into a copper plane with

as much area as possible. If the junction temperature rises

above ~150

°C, the part will go into thermal shutdown, and

all switching will stop until the temperature drops.

Compensating the Feedback Loop

The LTC3458L uses current mode control, with internal

adaptive slope compensation. Current mode control elimi-

nates the 2nd order filter due to the inductor and output

capacitor exhibited in voltage mode controllers, and sim-

plifies the power loop 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:

GG

G

V

V

G

G

V

I

GG

R

DC

CONTROL

EA

REF

OUT

CURRENT SENSE

CONTROL

IN

OUT

EA

CURRENT SENSE

DS ON

=

=

≈

=

••

•

•

,

,

_

_

()

2

1000

1

The output filter pole is given by:

f

I

VC

FILTER POLE

OUT

OUT

OUT

_

••

,

=

π

The output filter zero is given by:

f

RC

FILTER ZERO

ESR

OUT

_

••

,

=

1

2

π

resistance.

A troublesome feature of the boost regulator topology is

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

f

V

IV

L

RHPZ

IN

OUT

OUT

=

2

2

π ••

•

At heavy loads this gain increase with phase lag can occur

at a relatively low frequency. The loop gain is typically

rolled off before the RHP zero frequency.

APPLICATIO S I FOR ATIO