LTC1873EG#TR Linear Technology, LTC1873EG#TR Datasheet - Page 19

• Manufacturer Part Number: LTC1873EG#TR
• Description: IC REG SW 2PH DUAL SYNC 28SSOP
• Manufacturer: Linear Technology

Specifications of LTC1873EG#TR

• Pwm Type: Voltage Mode
• Number Of Outputs: 2
• Frequency - Max: 750kHz
• Duty Cycle: 93%
• Voltage - Supply: 3 V ~ 7 V
• Buck: Yes
• Boost: No
• Flyback: No
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: No
• Operating Temperature: -40°C ~ 85°C
• Package / Case: 28-SSOP
• Frequency-max: 750kHz
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATIO S I FOR ATIO
can support both channels running simultaneously (see
the 2-Phase Operation section for more information).
Details on how to calculate the maximum RMS input
current can be found in Application Note 77.
Tantalum capacitors are a popular choice as input capaci-
tors for LTC1873 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 LTC1873).
from C
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).
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 LTC1873 at full
load. Actual RMS current will differ due to inductor ripple
I
AVG
IN
–2.2
– 5.2
(time point A). 50% of the way through, TG2
7.8
4.8
3
10
0
A
Figure SB2. AC Current Calculation
0
A
• .
0 5
• .
0 16
U
50%
A
13
U
A
0
TIME
A
• .
16% 16% 18%
0 16
B
• .
0 18
C
W
D
5 18
1873 SB2
.
A
U
current and resistive losses, but this approximate value
is adequate for input capacitor calculation purposes.
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!
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.
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
current that the LTC1873 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!
Side only
Side only
I
I
I
I
I
RMS
AVE
RMS
AVE
RMS
1
2
1
2
1
2
4 55
:
:
.
4 4 66
3
10
– .
4 82
.
A
2 18
.
1 0 67
6 8
A
2
A
.
• .
A
RMS
• .
0 67
• .
2
2
RMS
0 32
2
• .
• .
0 16
• .
0 32
0 5
4 55
0
.
–
0
A
2
A
7 82
– .
• .
– .
A
2
.
5 18
0 33
• .
3 2
RMS
• .
0 68
0 33
2
2
2
• .
• .
0 16
• .
0 68
0 18
2 01
.
3 2
LTC1873
1 42
.
.
A
A
A
RMS
19