LTC1983-3 LINER [Linear Technology], LTC1983-3 Datasheet - Page 6

• Manufacturer Part Number: LTC1983-3
• Description: 100mA REGULATED CHARGE-PUMP INVERTERS IN THINSOT
• Manufacturer: LINER [Linear Technology]

OPERATIO
LTC1983-3/LTC1983-5
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
invert the V
to the output capacitor at V
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 A (typ) from V
high efficiency at low load conditions.
In shutdown mode, all circuitry is turned off and the part
draws less than 1 A from the V
disconnected from V
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:
6
V
V
V
IN
IN
OUT
–5.06V > I
–3.06V > I
IN
= –[V
voltage. Clock 2 closes the S2 switches that
IN
voltage and connect the bottom plate of C
IN
–(I
OUT
OUT
U
OUT
• R
• R
IN
(Refer to Block Diagram)
• R
and C
OUT
OUT
OUT
OUT
(LTC1983-5)
(LTC1983-3)
. This sequence of charging
)]
FLY
IN
. The SHDN pin has a
supply. V
IN
which provides
OUT
is also
FLY
For all R
the Typical Performance Characteristics section (Note:
C
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 V
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
short is removed.
Capacitor Selection
For best performance, it is recommended that low ESR
capacitors be used for both C
and ripple. The C
ceramic or tantalum and should be 4.7 F or greater.
Aluminum electrolytic are not recommended because of
their high equivalent series resistance (ESR). If the source
impedance is very low, C
the size of C
voltage ripple. The flying capacitor and C
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 F to 4.7 F. Note that
a large value flying cap (>1 F) will increase output ripple
unless C
tions, C1 may be reduced to 0.01 F to 0.047 F. This will
reduce output ripple at the expense of efficiency and
maximum output current.
FLY
R
OUT
= 1 F for all R
OUT
OUT
R
is also increased. For very low load applica-
OUT
values, check the corresponding curves in
OUT
(
IN
curve
to 10 F or greater will reduce output
and C
OUT
) – .
IN
curves). The R
OUT
may not be needed. Increasing
1 11
IN
capacitors should be either
and C
IN
causing a rise in the
OUT
f
OSC
OUT
OUT
to reduce noise
•
1
C
value will be
should also
FLY
sn1983 1983fs
OUT