LT1054CS8 datasheet(5/16 Pages) LINER

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LT1054/LT1054L

HARA TERISTICS

TYPICALPERFOR

TEMPERATURE (°C)

–50

–100

–80

–40

–20

100

LT1054 • TPC10

–60

–25

100

125

VREF AT 0 = 2.500V

TEMPERATURE (°C)

–50

–12.6

–12.4

–12.0

–11.8

–11.6

–4.7

–5.0

LT1054 • TPC09

–12.2

–4.9

–4.8

–5.1

–25

100

125

Regulated Output Voltage

Reference Voltage Temperature

Coefficient

PIN FUNCTIONS

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 (CIN), 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 VOUT. When driven to ground,

Pin 4 sinks current to ground. When driven to VOUT Pin 4

sources current from COUT. In all cases current flow in the

switches is unidirectional as should be expected using

bipolar switches.

VOUT (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 VOUT (Pin 5)

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 CIN and COUT are dis-

charged through the output load. Quiescent current in

shutdown drops to approximately 100

µA (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 (COUT) 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 (COUT) 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 (tp > 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.

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