Datasheet
LT1941
20
1941fb
APPLICATIONS INFORMATION
current capability of the third channel can be calculated
from the output currents and voltages of the other chan-
nels, the switching regulator effi ciency (η), the ambient
temperature (T
A
), the maximum junction temperature
(T
JMAX
) and the thermal resistance from junction to ambi-
ent (θ
JA
) as follows:
P
TT
P
P
VI V I
I
DISS
JMAX A
JA
DISS
=
=
=
–
–
–•– •
θ
η
3
1
12
12
3
PP
V
3
3
Example: LT1941 at V1 = 2.5V, I
1
= 2A, V2 = 3.3V, I
2
=
1A, V3 = 12V, η = 80%, T
A
= 75°C, T
JMAX
= 125°C, θ
JA
=
25°C/W:
P
CC
CW
W
P
W
VA
DISS
=
°°
°
=
=
125 75
25
2
3
2
108
25 2
–
/
–.
–. •
––. • .
.
.
33 1 17
17
12
0 141
3
VA W
I
W
V
A
=
==
Note that decreasing θ
JA
increases the power output
capability. The power output capability of the individual
channels can be calculated from the following:
Channel 1 Output Power (P1) = V1 • I
1
Channel 2 Output Power (P2) = V2 • I
2
Channel 3 Output Power (P3) = V3 • I
3
Total Output Power (P123) = P
DISS
/η = P1 + P2 + P3
Figure 9 shows power output capability if overall system
effi ciency (η) is 75% and maximum allowable power
dissipation (P
DISS
) is either 1W or 2W. For example, if al-
lowable power dissipation is 2W, Channel 3 output power
is 2W and Channel 2 output power is 1W, then Channel 1
output power can be up to 5W.
RELATED LINEAR TECHNOLOGY PUBLICATIONS
Application notes 19, 35, 44, 76 and 88 contain more
detailed descriptions and design information for buck
regulators and other switching regulators. The LT1375
data sheet has a more extensive discussion of output
ripple, loop compensation, and stability testing. Design
Notes 100 and 318 show how to generate a dual polarity
output supply using a buck regulator.
Figure 9. Power Output Capability of an Individual Channel Depends on the Output Power of the Other Channels
Power Output Capability for P
DISS
= 2W, η = 0.75 Power Output Capability for P
DISS
= 1W, η = 0.75
CHANNEL 1 OUTPUT POWER (WATTS)
0
0
CHANNEL 2 OUTPUT POWER (WATTS)
1
2
3
4
6
1
234
1941 F09a
56
5
CHANNEL 3
OUTPUT POWER (P3)
P3 = 2W
P3 = 4W
P3 = 6W
CHANNEL 1 OUTPUT POWER (WATTS)
0
0
CHANNEL 2 OUTPUT POWER (WATTS)
0.5
1.0
1.5
2.0
3.0
12
1941 F09b
3
2.5
CHANNEL 3
OUTPUT POWER (P3)
P3 = 1W
P3 = 2W
P3 = 3W