LT1054 Datasheet, PDF (12/26 Pages) Linear Technology – Switched-Capacitor Voltage Converter with Regulator

LT1054
SWITCHEDĆCAPACITOR VOLTAGE CONVERTERS
WITH REGULATORS
SLVS033F − FEBRUARY 1990 − REVISED NOVEMBER 2004
APPLICATION INFORMATION
regulation (continued)
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. Figure 16 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. 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.
The functional block diagram shows that the maximum regulated output voltage is limited by the supply voltage.
For the basic configuration, V_OUT  referenced to the ground terminal of the LT1054 must be less than the total
of the supply voltage minus the voltage loss due to the switches. The voltage loss versus output current due
to the switches can be found in the typical performance curves. Other configurations, such as the negative
doubler, can provide higher voltages at reduced output currents.
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.
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
+
I_OUT
2fC_OUT
(5)
Where:
∆V = peak-to-peak ripple
f_OSC = oscillator frequency
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:
ǒ_2IOUTǓǒ_{ESR of COUT}Ǔ
(6)
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