LTC1983-3/LTC1983-5
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
to the V
IN
voltage. Clock 2 closes the S2 switches that
invert the V
IN
voltage and connect the bottom plate of C
FLY
to the output capacitor at V
OUT
. 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 V
IN
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 V
IN
supply. V
OUT
is also
disconnected from V
IN
and C
FLY
. 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:
V
IN
–5.06V > I
OUT
• R
OUT
(LTC1983-5)
V
IN
–3.06V > I
OUT
• R
OUT
(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:
V
OUT
= –[V
IN
–(I
OUT
• R
OUT
)]
6
U
(Refer to Block Diagram)
For all R
OUT
values, check the corresponding curves in
the Typical Performance Characteristics section (Note:
C
FLY
= 1µF for all R
OUT
curves). The R
OUT
value will be
different for different flying caps, as shown in the follow-
ing equation:
1
R
OUT
=
R
OUT
(curve ) – 1.11
Ω +
f
OSC
• C
FLY
Short-Circuit/Thermal Protection
During short-circuit conditions, the LTC1983 will draw
several hundred milliamps from V
IN
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 V
OUT
short is removed.
Capacitor Selection
For best performance, it is recommended that low ESR
capacitors be used for both C
IN
and C
OUT
to reduce noise
and ripple. The C
IN
and C
OUT
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, C
IN
may not be needed. Increasing
the size of C
OUT
to 10µF or greater will reduce output
voltage ripple. The flying capacitor and C
OUT
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 C
OUT
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.
sn1983 1983fs
LINEAR [ LINEAR INTEGRATED SYSTEMS ]
