LT1054_15 Datasheet, PDF (9/33 Pages) Linear Technology – Switched-Capacitor Voltage Converter with Regulator

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7 Detailed Description
LT1054
SLVS033G – FEBRUARY 1990 – REVISED JULY 2015
7.1 Overview
LT1054 is a "negative voltage generator" or "negative charge pump" that will output a negative voltage that is
proportional to the input voltage (or V_CC). With proper supply voltage, V_OUT will regulate to an unregulated V_OUT
that is approximately –V_CC (reduced by a small voltage loss). If a lower absolute voltage is desired, V_OUT can be
regulated to that value when proper feedback resistors are applied.
LT1054 regulates up to 100mA with minimal loss and has a shutdown mode that makes this part optimal across
a wide range of applications.
7.2 Functional Block Diagram
V_REF
6
V_CC
8
Ref
FB/SD ¹
7
OSC
2.5 V
R
+
−
R
Q
OSC
Q
Drive
Drive
CAP +
2
C_IN†
4
CAP −
Drive
† _{External capacitors}
Pin numbers shown are for the P package.
Drive
3
GND
C_OUT†
5
V_OUT
7.3 Feature Description
7.3.1 Reference and Error Amplifier for Regulation
The feedback/shutdown (FB/SD) terminal has two functions. Pulling FB/SD below the shutdown threshold (≈ 0.45
V) 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 100 µA. Any open-collector gate can be used to put the LT1054 into shutdown.
For normal (unregulated) operation, the device will restart when the external gate is shut off. In LT1054 circuits
that use the regulation feature, the external resistor divider can provide enough pulldown to keep the device in
shutdown until the output capacitor (C_OUT) has fully discharged. For most applications, where the LT1054 is run
intermittently, this does not present a problem because the discharge time of the output capacitor is 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 FB/SD of the LT1054. Using the circuit
shown in Figure 11, the restart signal can be either a pulse (t_p > 100 µs) or a logic high. Diode coupling the
restart signal into FB/SD allows the output voltage to rise and regulate without overshoot. The resistor divider
R3/R4 shown in Figure 11 should be chosen to provide a signal level at FB/SD of 0.7−1.1 V.
FB/SD also is the inverting input of the LT1054 error amplifier and, as such, can be used to obtain a regulated
output voltage.
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