Datasheet

5
LT1054
PIN FUNCTIONS
UUU
V
+
(Pin 8): Input Supply. The LT1054 alternately charges
C
IN
to the input voltage when C
IN
is switched in parallel with
the input supply and then transfers charge to C
OUT
when
C
IN
is switched in parallel with C
OUT
. Switching occurs at
the oscillator frequency. During the time that C
IN
is charg-
ing, the peak supply current will be approximately equal to
2.2 times the output current. During the time that C
IN
is
delivering charge to C
OUT
the supply current drops to
approximately 0.2 times the output current. An input
supply bypass capacitor will supply part of the peak input
current drawn by the LT1054 and average out the current
drawn from the supply. A minimum input supply bypass
capacitor of 2µF, preferably tantalum or some other low
ESR type is recommended. A larger capacitor may be
desirable in some cases, for example, when the actual input
supply is connected to the LT1054 through long leads, or
when the pulse current drawn by the LT1054 might affect
other circuitry through supply coupling.
V
OUT
(Pin 5): In addition to being the output pin the pin is
also tied to the substrate of the device. Special care must
be taken in LT1054 circuits to avoid pulling this pin
positive with respect to any of the other pins. Pulling pin
5 positive with respect to pin 3 (GND) will forward bias the
substrate diode which will prevent the device from starting.
This condition can occur when the output load driven by the
LT1054 is referred to its positive supply (or to some other
positive voltage). Note that most op amps present just such
a load since their supply currents flow from their V
+
terminals to their V
–
terminals. To prevent start-up prob-
lems with this type of load an external transistor must be
added as shown in Figure 1. This will prevent V
OUT
(pin 5)
from being pulled above the ground pin (pin 3) during start-
up. Any small, general purpose transistor such as 2N2222
or 2N2219 can be used. R
X
should be chosen to provide
enough base drive to the external transistor so that it is
saturated under nominal output voltage and maximum
output current conditions. In some cases an N-channel
enhancement mode MOSFET can be used in place of the
transistor.
 R
X
≤
(
| V
OUT
| )β
I
OUT
–
+
LOAD
C
IN
C
OUT
LT1054 • F01
I
L
V
+
R
X
LT1054
FB/SHDN
CAP
+
GND
CAP
–
V
+
OSC
V
REF
V
OUT
+
+
I
Q
I
OUT
Figure 1
V
REF
(Pin 6): 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 synchro-
nization.
CAP
+
/CAP
–
(Pin 2/Pin 4): Pin 2, the positive side of the
input capacitor (C
IN
), is alternately driven between V
+
and
ground. When driven to V
+
, pin 2 sources current from V
+
.
When driven to ground pin 2 sinks current to ground. Pin
4, the negative side of the input capacitor, is driven alter-
nately between ground the V
OUT
. When driven to ground,
pin 4 sinks current to ground. When driven to V
OUT
pin 4
sources current from C
OUT
. In all cases current flow in the
switches is unidirectional as should be expected using
bipolar switches.