LT1054_10 Datasheet, PDF (8/16 Pages) Linear Technology – Switched-Capacitor Voltage Converter with Regulator

LT1054/LT1054L
APPLICATIONS INFORMATION
R3
V_IN 2.2µF
FB/SHDN V⁺
+
R4
C_IN
10µF
TANTALUM
CAP⁺ OSC
LT1054
GND V_REF
CAP ^– V_OUT
RESTART SHUTDOWN
) ) ^R2
R1
=
|V_OUT|
V_REF
2
–
40mV
+
1
≈
|V_OUT|
1.21V
+
1
V_OUT
WHERE V_REF = 2.5V NOMINAL
FOR EXAMPLE: TO GET V_OUT = –5V REFERRED TO THE GROUND
PIN OF THE LT1054, CHOOSE R1 = 20k, THEN
) ^{R2 = 20k}
|–5V|
2.5V
2
– 40mV
+
1
= 102.6k*
*CHOOSE THE CLOSEST 1% VALUE
R1
R2
C1
C_OUT
+ ^100µF
TANTALUM
LT1054 • F05
Figure 5
be 20k or greater because the reference output current
is limited to ≈100µA. R2 should be chosen to be in the
range of 100k to 300k. For optimum results the ratio of
C_IN/C_OUT is recommended to be 1/10. C1, required for
good load regulation at light load currents, should be
0.002µF for all output voltages.
A new die layout was required to fit into the physical
dimensions of the S8 package. Although the new die
of the LT1054CS8 will meet all the specifications of the
existing LT1054 data sheet, subtle differences in the
layout of the new die require consideration in some ap-
plication circuits. In regulating mode circuits using the
1054CS8 the nominal values of the capacitors, C_IN and
C_OUT, must be approximately equal for proper operation
at elevated junction temperatures. This is different from
the earlier part. Mismatches within normal production
tolerances for the capacitors are acceptable. Making the
nominal capacitor values equal will ensure proper opera-
tion at elevated junction temperatures at the cost of a
small degradation in the transient response of regulator
circuits. For unregulated circuits the values of C_IN and
C_OUT are normally equal for all packages. For S8 applica-
tions assistance in unusual applications circuits, please
consult the factory.
It can be seen from the circuit block diagram that the
maximum regulated output voltage is limited by the supply
voltage. For the basic configuration, |V_OUT| referred to the
8
ground pin 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 Typical Performance Characteristics. Other
configurations such as the negative doubler can provide
higher output voltages at reduced output currents (see
Typical Applications).
Capacitor Selection
For unregulated circuits the nominal values of C_IN and C_OUT
should be equal. For regulated circuits see the section on
Regulation. 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 will
be multiplied by four due to the fact that switch currents
are approximately two times higher than output current and
losses will occur on both the charge and discharge cycle.
This means that using a capacitor with 1Ω of ESR for C_IN
will have the same effect as increasing the output imped-
ance of the LT1054 by 4Ω. This represents a significant
increase in the voltage losses. For C_OUT the affect of ESR is
less dramatic. C_OUT is alternately charged and discharged
at a current approximately equal to the output current and
the ESR of the capacitor will cause a step function to oc-
cur in the output ripple at the switch transitions. This step
function will degrade the output regulation for changes
in output load current and should be avoided. Realizing
that large value tantalum capacitors can be expensive, a
technique that can be used is to parallel a smaller tantalum
capacitor with a large aluminum electrolytic capacitor to
gain both low ESR and reasonable cost. Where physical
size is a concern some of the newer chip type surface
mount tantalum capacitors can be used. These capacitors
are normally rated at working voltages in the 10V to 20V
range and exhibit very low ESR (in the range of 0.1Ω).
Output Ripple
The peak-to-peak output ripple is determined by the value
of the output capacitor and the output current. Peak-to-
peak output ripple may be approximated by the formula:
dV
=
I_OUT
2fC_OUT
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