20

LTC1702A

1702afa

current that the LTC1702A 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 LTC1702A is much

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

current is constantly flowing at the output whenever the

LTC1702A 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 LTC1702A 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 LTC1702A circuit is chosen

primarily 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:

I

tV

L

RIPPLE

ON Q

OUT

=

()

()

2

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:

L

tV

I

sV

A

H

with t

V

V

kHz

s

ON Q

OUT

RIPPLE

ON Q

=

()

=

µ

()( )

=µ

= −

⎛

⎝⎜

⎞

⎠⎟

=µ

()

()

..

.

.

/.

2

2

12

16

4

05

1

16

5

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

or 17A without saturating.

Feedback Loop Types

In a typical LTC1702A circuit, the feedback loop consists

of the modulator, the external inductor and output capaci-

tor, 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 them-

selves. 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 frequen-

cies 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

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

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

0

10A

32%

68%

0

10A

32% 18%

18%

18%

32%

–3.2A

0

6.8A

32%

68%

Q1 CURRENT, SIDE 1 ONLY

(FOR 1-PHASE, 2 SIDES:

MULTIPLY CURRENT BY 2)

BOTH SIDES EQUAL LOAD

Q11 CURRENT

Q21 CURRENT

BOTH SIDES EQUAL LOAD

2-PHASE OPERATION

–6.4A

0

3.6A

32%

18%

1702A F07

32%

Figure 7. RMS Input Current