LTC1983-3/LTC1983-5

6

sn1983 1983fs

OPERATIO

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

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 25

high efficiency at low load conditions.

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

draws less than 1

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:

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:

the Typical Performance Characteristics section (Note:

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

ing equation:

R

R

curve

fC

OUT

OUT

OSC

FLY

=Ω +









() – .

•

111

1

Short-Circuit/Thermal Protection

During short-circuit conditions, the LTC1983 will draw

junction temperature. On-chip thermal shutdown cir-

cuitry disables the charge pump once the junction tem-

perature exceeds

≈155°C,andreenablesthechargepump

once the junction temperature falls back to

≈145°C. The

LTC1983 will cycle in and out of thermal shutdown

short is removed.

Capacitor Selection

For best performance, it is recommended that low ESR

ceramic or tantalum and should be 4.7

µF or greater.

Aluminum electrolytic are not recommended because of

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

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 0.1

µF to 4.7µF. Note that

a large value flying cap (>1

µF) will increase output ripple

tions, C1 may be reduced to 0.01

µF to 0.047µF. This will

reduce output ripple at the expense of efficiency and

maximum output current.

(Refer to Block Diagram)