LTC1702AIGN datasheet(19/36 Pages) LINER

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LTC1702A

1702afa

APPLICATIONS INFORMATION

from CIN (time point A). 50% of the way through, TG2

turns on and the total current is 13A (time point B).

Shortly thereafter, TG1 turns off and the current drops to

10A (time point C). Finally, TG2 turns off and the current

spends a short time at 0 before TG1 turns on again (time

point D).

AVG =

(

)+(

(

)+(

30 5

130 16

016

018

518

•.

Now we can calculate the RMS current. Using the same

waveform we used to calculate the average DC current,

subtract the average current from each of the DC values.

Square each current term and multiply the squares by the

same period percentages we used to calculate the aver-

age DC current. Sum the results and take the square root.

The result is the approximate RMS current as seen by the

input capacitor with both sides of the LTC1702A at full

load. Actual RMS current will differ due to inductor ripple

current and resistive losses, but this approximate value is

adequate for input capacitor calculation purposes.

TIME

50%

16% 16% 18%

– 5.2

–2.2

4.8

7.8

1702A SB2

Figure SB2. AC Current Calculation

RMS

= (

)+(

(

)+(

)

–.

• .

.• .

– .

• .

218

782

016

482

016

518

018

455

If the circuit is likely to spend time with one side operating

and the other side shut down, the RMS current will need

to be calculated for each possible case (side 1 on, side 2

off; side 1 off, side 2 on; both sides on). The capacitor

must be sized to withstand the largest RMS current of the

three—sometimes this occurs with one side shut down!

Side only

AVE

RMS

AVE

RMS

0 67

0 33

2 01

0 67

0 33

1 42

032

068

032

068

•.

–

•.

.• .

– .

• .

(

)+(

(

)+(

(

)+(

(

)+(

)

= 4466

455

RMS

Consider the case where both sides are operating at the

same load, with a 50% duty cycle at each side. The RMS

current with both sides running is near zero, while the

RMS current with one side active is 1/2 the total load

current of that side. The 2-phase, 5V to 2.5V circuit in the

applications section takes advantage of this phenom-

enon, allowing it to supply 40A of output current with only

120

µF of input capacitance (and only 40µF of output

capacitance!).

the capacitor we chose for the single side application can

support the slightly higher 4.8ARMS current, we can add

the second channel without changing the input capacitor

at all. As a general rule, an input bypass capacitor capable

of supporting the larger output current channel can sup-

port both channels running simultaneously (see the

2-Phase Operation section for more details).

Tantalum capacitors are a popular choice as input capaci-

tors for LTC1702A applications, but they deserve a special

caution here. Generic tantalum capacitors have a destruc-

tive failure mechanism when they are subjected to large

RMS currents (like those seen at the input of a LTC1702A).

At some random time after they are turned on, they can

blow up for no apparent reason. The capacitor manufac-

turers are aware of this and sell special “surge tested”

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

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