LT1054_15 Datasheet, PDF (14/33 Pages) Linear Technology – Switched-Capacitor Voltage Converter with Regulator

LT1054
SLVS033G – FEBRUARY 1990 – REVISED JULY 2015
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8.2.2 Detailed Design Procedure
When using LT1054 as a basic voltage inverter, determine the following:
• Input Voltage
• Desired output Voltage
• Desired Ripple
• Power Dissipation
8.2.2.1 Output Voltage Programming
The error amplifier of the LT1054 drives the PNP switch to control the voltage across the input capacitor (C_IN),
which determines the output voltage. When the reference and error amplifier of the LT1054 are used, an external
resistive divider is all that is needed to set the regulated output voltage. shows the basic regulator configuration
and the formula for calculating the appropriate resistor values. R1 should be 20 kΩ or greater because the
reference current is limited to ±100 μA. R2 should be in the range of 100 kΩ to 300 kΩ.
8.2.2.2 Capacitor Selection
While the exact values of C_IN and C_OUT are noncritical, good-quality low-ESR capacitors, such as solid tantalum,
are necessary to minimize voltage losses at high currents. For C_IN, the effect of the ESR of the capacitor is
multiplied by four, because switch currents are approximately two times higher than output current. Losses occur
on both the charge and discharge cycle, which means that a capacitor with 1 Ω of ESR for C_IN has the same
effect as increasing the output impedance of the LT1054 by 4 Ω. This represents a significant increase in the
voltage losses. C_OUT alternately is charged and discharged at a current approximately equal to the output
current. The ESR of the capacitor causes a step function to occur in the output ripple at the switch transitions.
This step function degrades the output regulation for changes in output load current and should be avoided. A
technique used to gain both low ESR and reasonable cost is to parallel a smaller tantalum capacitor with a large
aluminum electrolytic capacitor.
Frequency compensation is accomplished by adjusting the ratio of C_IN to C_OUT.
For best results, this ratio should be approximately 1:10. Capacitor C1, required for good load regulation, should
be 0.002 μF for all output voltages.
8.2.2.3 Output Ripple
The peak-to-peak output ripple is determined by the output capacitor and the output current values. Peak-to-peak
output ripple is approximated as:
DV = ^IOUT
2fC_OUT
where
• ΔV = peak-to-peak ripple
• f_OSC = oscillator frequency
(4)
For output capacitors with significant ESR, a second term must be added to account for the voltage step at the
switch transitions. This step is approximately equal to:
(2I_OUT)(ESR of C_OUT)
(5)
8.2.2.4 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 = (V_CC – V_OUT ) I_OUT + (V_CC)(I_OUT)(0.2)
where
• both V_CC and V_OUT are referenced to ground
(6)
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