LTC1703CG#TRPBF Linear Technology, LTC1703CG#TRPBF Datasheet - Page 20

• Manufacturer Part Number: LTC1703CG#TRPBF
• Description: IC REG SW DUAL SYNC VID 28SSOP
• Manufacturer: Linear Technology

Specifications of LTC1703CG#TRPBF

• Applications: Controller, Mobile Intel Pentium® III
• Voltage - Input: 3 ~ 7 V
• Number Of Outputs: 2
• Voltage - Output: 0.9 ~ 2 V
• Operating Temperature: 0°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-SSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

APPLICATIO S I FOR ATIO
LTC1703
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:
20
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 LTC1703 at full load.
Actual RMS current will differ due to inductor ripple
L
with t
I
AVG
=
t
IN
ON QB
=
ON QB
(
(time point A). 50% of the way through, TG2
I
(
(
RIPPLE
(
3
10
–2.2
– 5.2
A
Figure SB2. AC Current Calculation
7.8
4.8
)
)
0
(
A
• .
V
=
0
0 5
• .
OUT
⎛
⎜
⎝
0 16
1
U
)
−
)
+
=
1 6
)
(
5
.
13
+
50%
(
V
A
1 2
V
U
(
.
A
0
⎞
⎟
⎠
µ
A
• .
/
TIME
s
3
0 16
550
• .
)(
A
16% 16% 18%
0 18
B
1 6
.
kHz
)
W
V
C
+
)
)
=
=
=
D
5 18
1 2
0 64
.
.
.
1703 SB2
µ
A
s
µ
U
H
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.
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 I
or 17A without saturating.
Side only
Side only
I
I
I
I
I
RMS
AVE
RMS
AVE
RMS
1
2
1
2
1
2
=
=
=
=
=
=
= 4 4 66
4 55
:
:
.
(
(
(
(
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
1 42
.
)
)
.
+
)
A
A
A
RMS
RIPPLE
1703fa
,