LT1054L Datasheet, PDF (5/16 Pages) Linear Technology – Switched-Capacitor Voltage Converter with Regulator

LT1054/LT1054L
TYPICAL PERFOR A CE CHARACTERISTICS
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 dis-
charged 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
applications 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 dis-
charged, 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 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.
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 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)
1054lfe
5