SLVS033F − FEBRUARY 1990 − REVISED NOVEMBER 2004
13
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
power dissipation
The power dissipation of any LT1054 circuit must be limited so that the junction temperature of the device does
not exceed the maximum junction-temperature ratings. The total power dissipation is calculated from two
components–the power loss due to voltage drops in the switches, and the power loss due to drive-current
losses. The total power dissipated by the LT1054 is calculated as:
P [
ǒ
V
CC
*
Ť
V
OUT
Ť
Ǔ
I
OUT
)
ǒ
V
CC
Ǔǒ
I
OUT
Ǔ
(
0.2
)
where both V
CC
and V
OUT
are referenced to ground. The power dissipation is equivalent to that of a linear
regulator. Limited power-handling capability of the LT1054 packages causes limited output-current
requirements, or steps can be taken to dissipate power external to the LT1054 for large input or output
differentials. This is accomplished by placing a resistor in series with C
IN
as shown in Figure 17. A portion of
the input voltage is dropped across this resistor without affecting the output regulation. Since switch current is
approximately 2.2 times the output current and the resistor causes a voltage drop when C
IN
is both charging
and discharging, the resistor chosen is as shown:
R
X
+
V
X
4.4 I
OUT
Where:
V
X
≈ V
CC
− [(LT1054 voltage loss)(1.3) + |V
OUT
|]
and
I
OUT
= maximum required output current
The factor of 1.3 allows some operating margin for the LT1054.
When using a 12-V to −5-V converter at 100-mA output current, calculate the power dissipation without an
external resistor.
P +
(
12 V *
|
*5V
|)(
100 mA
)
)
(
12 V
)(
100 mA
)(
0.2
)
P + 700 mW ) 240 mW + 940 mW
V
IN
C
OUT
R2
R1
C1
LT1054
V
OUT
V
REF
OSC
V
CC
CAP−
GND
CAP+
FB/SD
V
OUT
Rx
C
IN
Pin numbers shown are for the P package.
+
+
1
2
3
4
8
7
6
5
Figure 17. Power-Dissipation-Limiting Resistor in Series With C
IN
