LT1054 Datasheet, PDF (5/16 Pages) Linear Technology – Switched-Capacitor Voltage Converter with Regulator

TYPICAL PERFORMANCE CHARACTERISTICS
LT1054/LT1054L
Regulated Output Voltage
–4.7
–4.8
–4.9
–5.0
–5.1
–11.6
–11.8
–12.0
–12.2
–12.4
–12.6
–50 –25
0 25 50 75 100 125
TEMPERATURE (°C)
LT1054 • TPC09
Reference Voltage Temperature
Coefficient
100
80 ^{VREF AT 0 = 2.500V}
60
40
20
0
–20
–40
–60
–80
–100
–50 –25
0 25 50 75 100 125
TEMPERATURE (°C)
LT1054 • TPC10
PIN FUNCTIONS
FB/SHDN (Pin 1): Feedback/Shutdown Pin. This pin has
two functions. Pulling Pin 1 below the shutdown threshold
(≈ 0.45V) puts the device into shutdown. In shutdown the
reference/regulator is turned off and switching stops. The
switches are set such that both C_IN and C_OUT are discharged
through the output load. Quiescent current in shutdown
drops to approximately 100µA (see Typical Performance
Characteristics). Any open-collector gate can be used to
put the LT1054 into shutdown. For normal (unregulated)
operation the device will start back up when the external
gate is shut off. In LT1054 circuits that use the regulation
feature, the external resistor divider can provide enough
pull-down to keep the device in shutdown until the output
capacitor (C_OUT) has fully discharged. For most applica-
tions where the LT1054 would be run intermittently, this
does not present a problem because the discharge time
of the output capacitor will be short compared to the off-
time of the device. In applications where the device has
to start up before the output capacitor (C_OUT) has fully
discharged, a restart pulse must be applied to Pin 1 of
the LT1054. Using the circuit of Figure 5, the restart signal
can be either a pulse (t_p > 100µs) or a logic high. Diode
coupling the restart signal into Pin 1 will allow the output
voltage to come up and regulate without overshoot. The
resistor divider R3/R4 in Figure 5 should be chosen to
provide a signal level at pin 1 of 0.7V to 1.1V.
Pin 1 is also the inverting input of the LT1054’s error
amplifier and as such can be used to obtain a regulated
output voltage.
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 alternately between ground and 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.
V_OUT (Pin 5): In addition to being the output pin this 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 problems with this type of load an external
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