LT1054L Datasheet, PDF (6/16 Pages) Linear Technology – Switched-Capacitor Voltage Converter with Regulator

LT1054/LT1054L
PIN FUNCTIONS
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
V⁺
+
C_IN
FB/SHDN V⁺
CAP⁺ OSC
LT1054
GND V_REF
CAP ^– V_OUT
I_L
+
LOAD
–
R_X
+ ^COUT
I_Q
I_OUT
LT1054 • F01
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 tempera-
ture 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.
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 switching 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 oscillator fre-
quency. 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 voltage is
not recommended. For circuits that require both fre-
quency synchronization and regulation, an external refer-
ence 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.
+
C_IN
FB/SHDN V⁺
CAP⁺ OSC
LT1054
GND V_REF
CAP ^– V_OUT
V_IN
C2
C1
+ ^COUT
LT1054 • F02
Figure 2
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
1054lfe
6