8
LTC1550L/LTC1551L
The LTC1550L/LTC1551L consist of two major blocks
(see Block Diagram): an inverting charge pump and a
negative linear regulator. The charge pump uses two
external capacitors, C1 and C
CP
to generate a negative
voltage at CP
OUT
. It operates by charging and discharging
C1 on alternate phases of the internal 900kHz clock. C1 is
initially charged to V
CC
through switches S1 and S3. When
the internal clock changes phase, S1 and S3 open and S2
and S4 close, shorting the positive side of C1 to ground.
This forces the negative side of C1 below ground, and
charge is transferred to C
CP
through S4. As this cycle
repeats, the magnitude of the negative voltage approaches
V
CC
. The 900kHz internal clock frequency helps keep noise
out of the 400kHz to 600kHz IF bands commonly used by
portable radio frequency systems and reduces the size of
the external capacitors required. Most applications can
use standard 0.1µF ceramic capacitors for C1 and C
CP
.
Increasing C1 and C
CP
beyond 0.1µF has little effect on the
output ripple or the output current capacity of the
LTC1550L/LTC1551L.
The negative voltage at CP
OUT
supplies the input to the
negative regulator block. This block consists of an
N-channel MOSFET pass device and a feedback amplifier
that monitors the output voltage and compares it to the
internal reference. The regulated output appears at the
V
OUT
pin. The regulation loop is optimized for fast tran-
sient response, enabling it to remove most of the switch-
ing artifacts present at the CP
OUT
pin. Output ripple is
typically below 1mV
P-P
with output loads between 0mA
and 10mA. The output voltage is set by a pair of internal
divider resistors for the fixed voltage versions. The N-
channel pass device minimizes dropout, allowing the
output to remain in regulation with supply voltages as low
as 2.7V for an output voltage of –2V. An output capacitor
of at least 4.7µF from V
OUT
to ground is required to keep
the regulator loop stable; for optimum stability and mini-
mum output ripple, at least 10µF is recommended.
Adjustable Hook-Up
For the adjustable LTC1550L/LTC1551L, the output volt-
age is set with a resistor divider from GND to V
OUT
(Figure␣ 2). Note that the internal reference and the internal
feedback amplifier are set up as a positive-output regula-
tor referenced to the V
OUT
pin, not as a negative regulator
APPLICATIONS INFORMATION
WUU
U
Figure 2. External Resistor Connections
referenced to ground. The output resistor divider must be
set to provide 1.225V at the ADJ pin with respect to V
OUT
.
For example, a –3V output would require a 17.4k resistor
from GND to ADJ, and a 12.1k resistor to V
OUT
.
CAPACITOR SELECTION
The LTC1550L/LTC1551L requires four external capaci-
tors: an input bypass capacitor, two 0.1µF charge pump
capacitors and an output filter capacitor. The overall
behavior of the LTC1550L/LTC1551L is strongly affected
by how the capacitors are used, and by how the capacitors
are laid out on the printed circuit board (PCB). In particu-
lar, the output capacitor’s value and ESR have a significant
effect on the output ripple and noise performance. In
addition, the ground connections for the V
CC
bypass
capacitor, the CP
OUT
capacitor and the V
OUT
bypass ca-
pacitor must employ star-ground techniques at the GND
pin of the LTC1550L/LTC1551L. Proper capacitor selec-
tion is critical for optimum performance of the LTC1550L/
LTC1551L.
Output Ripple vs Output Capacitor
Figure 4 shows the effect of using different output capaci-
tor values on the LTC1550L/LTC1551L output ripple.
These curves are taken using the LTC1551L circuit in
Figure 3, with C
IN
= 2.2µF and I
LOAD
= 5mA. The upper
curve shows the performance with a standard tantalum
capacitor alone and the lower curve shows that of the
tantalum capacitor in parallel with a 0.1µF ceramic capaci-
tor. As a general rule, larger output capacitors provide
lower output ripple. To keep output voltage ripple below
1mV
P–P
, 10µF, or greater, in parallel with a 0.1µF ceramic
capacitor is required. To guarantee loop stability under all
conditions, a minimum of 4.7µF is required at the output.
PGND, AGND
LTC1550L
V
OUT
ADJ
R1
R2
V
OUT
= –1.225V
R1 + R2
R2
()
1550L/51L • F02
