LT1054/LT1054L
6
1054lfh
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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
OSC (Pin 7): Oscillator Pin. This pin can be used to raise
or lower the oscillator frequency or to synchronize the
device to an external clock. Internally Pin 7 is connected
to the oscillator timing capacitor (C
t
≈ 150pF) which is
alternately charged and discharged by current sources of
±7µA so that the duty cycle is ≈50%. The LT1054 oscillator
is designed to run in the frequency band where switch
-
ing losses are minimized. However the frequency can be
raised, lowered, or synchronized to an external system
clock if necessary
.
The frequency can be lowered by adding an external
capacitor (C1, Figure 2) from Pin 7 to ground. This will
increase the charge and discharge times which lowers the
oscillator frequency. The frequency can be increased by
adding an external capacitor (C2, Figure 2, in the range
of 5pF to 20pF) from Pin 2 to Pin 7. This capacitor will
couple charge into C
T
at the switch transitions, which will
shorten the charge and discharge time, raising the oscil-
lator frequency. Synchronization can be accomplished
by adding an external resistive pull-up from Pin 7 to 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 2. Pulling up Pin 7 to an external volt-
age 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 pull-
up point for Pin 7.
–
+
LOAD
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
+
+
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 af
-
fect the internal reference/regulator, so that this pin can
also be used as a pull-up for LT1054 cir
cuits that require
synchronization.
Figure 1
Figure 2
V
IN
C
OUT
C
IN
C2
C1
LT1054
FB/SHDN
CAP
+
GND
CAP
–
V
+
OSC
V
REF
V
OUT
+
+
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 oc-
curs at the oscillator frequency. During the time that C
IN
pin Functions
