LT1054_15 Datasheet, PDF (15/33 Pages) Linear Technology – Switched-Capacitor Voltage Converter with Regulator

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LT1054
SLVS033G – FEBRUARY 1990 – REVISED JULY 2015
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 16. 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|]
• I_OUT = maximum required output current
(7)
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
(8)
+
C_IN
1
FB/SD
Rx ₂
CAP+
3
GND
LT1054
4
CAP−
8
V_CC
7
OSC
6
V_REF
5
V_OUT
V_IN
R1
R2
V_OUT
C1
C_OUT
+
Pin numbers shown are for the P package.
Figure 16. Power-Dissipation-Limiting Resistor in Series With CIN
At R_θJA of 130°C/W for a commercial plastic device, a junction temperature rise of 122°C occurs. The device
exceeds the maximum junction temperature at an ambient temperature of 25°C. To calculate the power
dissipation with an external resistor (R_X), determine how much voltage can be dropped across RX. The maximum
voltage loss of the LT1054 in the standard regulator configuration at 100 mA output current is 1.6 V.
V_X = 12 V – [(1.6 V)(1.3) + |–5 V|] = 4.9 V
(9)
and
R_x
=
4.9 V
(4.4)(100 mA)
= 11 W
(10)
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