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LTC1983-5

100mA Regulated Charge-Pump Inverters in ThinSOT

Linear Technology 

LTC1983-3/LTC1983-5

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OPERATIO (Refer to Block Diagram)

The LTC1983-3/LTC1983-5 use a switched capacitor

charge pump to invert a positive input voltage to a regu-

lated –3V ±4% (LTC1983-3) or –5 ±4% (LTC1983-5)

output voltage. Regulation is achieved by sensing the

output voltage through an internal resistor divider and

enabling the charge pump when the output voltage droops

above the upper trip point of COMP1. When the charge

pump is enabled, a 2-phase, nonoverlapping clock con-

trols the charge pump switches. Clock 1 closes the S1

switches which enables the flying capacitor to charge up

to the VIN voltage. Clock 2 closes the S2 switches that

invert the VIN voltage and connect the bottom plate of CFLY

to the output capacitor at VOUT. This sequence of charging

and discharging continues at a free-running frequency of

900kHz (typ) until the output voltage has been pumped

down to the lower trip point of COMP1 and the charge

pump is disabled. When the charge pump is disabled, the

LTC1983 draws only 25µA (typ) from VIN which provides

high efficiency at low load conditions.

In shutdown mode, all circuitry is turned off and the part

draws less than 1µA from the VIN supply. VOUT is also

disconnected from VIN and CFLY. The SHDN pin has a

threshold of approximately 0.7V. The part enters shut-

down when a low is applied to the SHDN pin. The SHDN pin

should not be floated; it must be driven with a logic high

or low.

Open-Loop Operation

The LTC1983-3/LTC1983-5 inverting charge pumps regu-

late at –3V/–5V respectively, unless the input voltage is too

low or the output current is too high. The equations for

output voltage regulation are as follows:

VIN –5.06V > IOUT • ROUT (LTC1983-5)

VIN –3.06V > IOUT • ROUT (LTC1983-3)

If this condition is not met, then the part will run in open

loop mode and act as a low output impedance inverter for

which the output voltage will be:

VOUT = –[VIN –(IOUT • ROUT)]

For all ROUT values, check the corresponding curves in

the Typical Performance Characteristics section (Note:

CFLY = 1µF for all ROUT curves). The ROUT value will be

different for different flying caps, as shown in the follow-

ing equation:

Short-Circuit/Thermal Protection

During short-circuit conditions, the LTC1983 will draw

several hundred milliamps from VIN causing a rise in the

junction temperature. On-chip thermal shutdown cir-

cuitry disables the charge pump once the junction tem-

perature exceeds ≈155°C, and reenables the charge pump

once the junction temperature falls back to ≈145°C. The

LTC1983 will cycle in and out of thermal shutdown

indefinitely without latchup or damage until the VOUT

short is removed.

Capacitor Selection

For best performance, it is recommended that low ESR

capacitors be used for both CIN and COUT to reduce noise

and ripple. The CIN and COUT capacitors should be either

ceramic or tantalum and should be 4.7µF or greater.

Aluminum electrolytic are not recommended because of

their high equivalent series resistance (ESR). If the source

impedance is very low, CIN may not be needed. Increasing

the size of COUT to 10µF or greater will reduce output

voltage ripple. The flying capacitor and COUT should also

have low equivalent series inductance (ESL). The board

layout is critical as well for inductance for the same reason

(the suggested board layout should be used).

A ceramic capacitor is recommended for the flying capaci-

tor with a value in the range of 0.1µF to 4.7µF. Note that

a large value flying cap (>1µF) will increase output ripple

unless COUT is also increased. For very low load applica-

tions, C1 may be reduced to 0.01µF to 0.047µF. This will

reduce output ripple at the expense of efficiency and

maximum output current.

1983fa

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