LTC1702IGN datasheet(20/36 Pages) LINER

20 / 36 page

LTC1702

tantalum capacitors specifically designed for use with

switching regulators. When choosing a tantalum input

capacitor, make sure that it is rated to carry the RMS

current that the LTC1702 will draw. If the data sheet

doesn’t give an RMS current rating, chances are the

capacitor isn’t surge tested. Don’t use it!

OUTPUT BYPASS CAPACITOR

The output bypass capacitor has quite different require-

ments from the input capacitor. The ripple current at the

output of a buck regulator like the LTC1702 is much lower

than at the input, due to the fact that the inductor current

is constantly flowing at the output whenever the LTC1702

is operating in continuous mode. The primary concern at

the output is capacitor ESR. Fast load current transitions

at the output will appear as voltage across the ESR of the

output bypass capacitor until the feedback loop in the

LTC1702 can change the inductor current to match the

new load current value. This ESR step at the output is often

the single largest budget item in the load regulation

calculation. As an example, our hypothetical 1.6V, 10A

switcher with a 0.01

Ω ESR output capacitor would expe-

rience a 100mV step at the output with a 0 to 10A load

step—a 6.3% output change!

Usually the solution is to parallel several capacitors at the

output. For example, to keep the transient response inside

of 3% with the previous design, we’d need an output ESR

better than 0.0048

Ω. This can be met with three 0.014Ω,

470

µF low ESR tantalum capacitors in parallel.

INDUCTOR

The inductor in a typical LTC1702 circuit is chosen prima-

rily for value and saturation current. The inductor value

sets the ripple current, which is commonly chosen at

around 40% of the anticipated full load current. Ripple

current is set by:

RIPPLE

ON Q

OUT

()

In our hypothetical 1.6V, 10A example, we'd set the ripple

current to 40% of 10A or 4A, and the inductor value would

be:

APPLICATIONS INFORMATION

with t

kHz

ON Q

OUT

RIPPLE

ON Q

()

()( )

=µ

=−









=µ

()

550

1 2

The inductor must not saturate at the expected peak

current. In this case, if the current limit was set to 15A, the

inductor should be rated to withstand 15A + 1/2 IRIPPLE,

or 17A without saturating.

FEEDBACK LOOP/COMPENSATION1

Feedback Loop Types

In a typical LTC1702 circuit, the feedback loop consists of

the modulator, the external inductor and output capacitor,

and the feedback amplifier and its compensation network.

All of these components affect loop behavior and need to

be accounted for in the loop compensation. The modulator

consists of the internal PWM generator, the output MOSFET

drivers and the external MOSFETs themselves. From a

feedback loop point of view, it looks like a linear voltage

transfer function from COMP to SW and has a gain roughly

equal to the input voltage. It has fairly benign AC behavior

at typical loop compensation frequencies with significant

phase shift appearing at half the switching frequency.

The external inductor/output capacitor combination makes

a more significant contribution to loop behavior. These

components cause a second order LC roll-off at the

output, with the attendant 180

° phase shift. This roll-off is

what filters the PWM waveform, resulting in the desired

DC output voltage, but the phase shift complicates the

loop compensation if the gain is still higher than unity at

the pole frequency. Eventually (usually well above the LC

pole frequency), the reactance of the output capacitor will

approach its ESR, and the roll-off due to the capacitor will

stop, leaving 6dB/octave and 90

° of phase shift (Figure 8).

So far, the AC response of the loop is pretty well out of the

user’s control. The modulator is a fundamental piece of the

LTC1702 design, and the external L and C are usually

chosen based on the regulation and load current require-

ments without considering the AC loop response. The

1The information in this section is based on the paper “The K Factor: A New Mathematical Tool for

Stability Analysis and Synthesis” by H. Dean Venable, Venable Industries, Inc. For complete paper,

see “Reference Reading #4” at www.linear-tech.com.

LTC1702IGN Arkusz danych(PDF) 20 Page - Linear Technology

LTC1702IGN Arkusz danych(HTML) 20 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

Arkusz danych Pobierz

Link URL