LT1054_15 Datasheet, PDF (11/33 Pages) Linear Technology – Switched-Capacitor Voltage Converter with Regulator

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LT1054
SLVS033G – FEBRUARY 1990 – REVISED JULY 2015
Feature Description (continued)
the reference pin (Pin 6). A 20k pull-up is recommended. An open collector gate or an NPN transistor can then
be used to drive the oscillator pin at the external clock frequency as shown in Figure 12. Pulling up Pin 7 to an
external voltage is not recommended. For circuits that require both frequency synchronization and regulation, an
external reference can be used as the reference point for the top of the R1/R2 divider allowing Pin 6 to be used
as a pullup point for Pin 7.
7.3.3 Output Current and Voltage Loss
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.
7.3.4 Reference Voltage
Reference Output. This pin provides a 2.5V reference point for use in LT1054-based regulator circuits. The
temperature coefficient of the reference voltage has been adjusted so that the temperature coefficient of the
regulated output voltage is close to zero. This requires the reference output to have a positive temperature
coefficient as can be seen in the typical performance curves. This nonzero drift is necessary to offset a drift term
inherent in the internal reference divider and comparator network tied to the feedback pin. The overall result of
these drift terms is a regulated output which has a slight positive temperature coefficient at output voltages below
5V and a slight negative TC at output voltages above 5V. Reference output current should be limited, for
regulator feedback networks, to approximately 60µA. The reference pin will draw ≈100µA when shorted to
ground and will not affect the internal reference/regulator, so that this pin can also be used as a pull-up for
LT1054 circuits that require synchronization.
7.4 Device Functional Modes
7.4.1 Main Operation
A review of a basic switched-capacitor building block is helpful in understanding the operation of the LT1054.
When the switch shown in Figure 13 is in the left position, capacitor C1 charges to the voltage at V1. The total
charge on C1 is q1 = C1*V1. When the switch is moved to the right, C1 is discharged to the voltage at V2. After
this discharge time, the charge on C1 is q2 = C1*V2. The charge has been transferred from the source V1 to the
output V2. The amount of charge transferred is shown in Equation 1.
Δq = q1 – q2 = C1(V1 – V2)
(1)
If the switch is cycled f times per second, the charge transfer per unit time (that is, current) is as shown in
Equation 2.
I = f × L\q = f × C1(1 – V2)
(2)
To obtain an equivalent resistance for a switched-capacitor network, this equation can be rewritten in terms of
voltage and impedance equivalence as shown in Equation 3.
I
=
V1- V2
(1/ fC1)
=
V1- V2
R_EQUIV
(3)
V1
V2
f
R_L
C1
C2
Figure 13. Switched-Capacitor Building Block
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