LT1054_06 Datasheet, PDF (13/27 Pages) Texas Instruments – SWITCHED-CAPACITOR VOLTAGE CONVERTERS WITH REGULATORS

LT1054
SWITCHEDĆCAPACITOR VOLTAGE CONVERTERS
WITH REGULATORS
SLVS033F − FEBRUARY 1990 − REVISED NOVEMBER 2004
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 [ ^ǒV_CC * ^ŤV_OUT^ŤǓ I_OUT ) ^ǒV_CC^ǓǒI_OUT^Ǔ(0.2)
(7)
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
(8)
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 * |*5 V|)(100 mA) ) (12 V)(100 mA)(0.2)
P + 700 mW ) 240 mW + 940 mW
(9)
1
FB/SD
VIN
8
VCC
Rx ₂
CAP+
7
OSC
LT1054
+
CIN
3
GND
6
R1
VREF
4
CAP−
5
R2
VOUT
VOUT
C1
+ ^COUT
Pin numbers shown are for the P package.
Figure 17. Power-Dissipation-Limiting Resistor in Series With C_IN
• POST OFFICE BOX 655303 DALLAS, TEXAS 75265
13