5
LTC1514-3.3/LTC1514-5
PIN FUNCTIONS
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battery comparator and 1.145V reference are kept alive in
shutdown.
GND (Pin 4): Ground. Should be tied to a ground plane for
best performance.
C1
–
(Pin 5): Charge Pump Flying Capacitor, Negative
Terminal.
C1
+
(Pin 6): Charge Pump Flying Capacitor, Positive
Terminal.
V
IN
(Pin 7): Charge Pump Input Voltage. May be between
2V and 8V (LTC1514-3.3) or between 2.7V and 10V
(LTC1514-5). V
IN
should be bypassed with a ≥10µF low
ESR capacitor as close as possible to the pin for best
performance.
V
OUT
(Pin 8): Regulated Output Voltage. The output volt-
age is internally set to either 3.3V (LTC1514-3.3) or to 5V
(LTC1514-5) using an internal resistor divider. V
OUT
should
be bypassed with a ≥10µF low ESR capacitor as close as
possible to the pin for best performance.
APPLICATIONS INFORMATION
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Regulator Operation
The regulator section of the LTC1514-3.3/LTC1514-5
consists of a charge pump, reference, comparator and
some logic. The divided down output voltage is com-
pared to the internal reference voltage. When the divided
output drops below the reference voltage, the charge
pump is enabled, which boosts the output back into
regulation. Hysteresis in the comparator forces the regu-
lator to burst on and off and causes approximately
100mV of peak-to-peak ripple to appear at the output. By
enabling the charge pump only when needed, the
LTC1514-3.3 and LTC1514-5 are able to achieve high
efficiencies with low output load currents.
Each part’s charge pump has a unique architecture that
allows the input voltage to be either stepped up or
stepped down to produce a regulated output. Internal
circuitry senses the V
IN
to V
OUT
differential voltage and
controls the charge pump operating mode. In addition,
the effective output impedance of the charge pump is
internally adjusted to prevent large inrush currents and
allow for a wide input voltage range. When the input
voltage is lower than the output voltage, the charge pump
operates as a step-up voltage doubler. When the input
voltage is greater than the output, the charge pump
operates as a step-down gated switch.
Capacitor Selection
For best performance, low ESR capacitors are recom-
mended 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 10µF or greater. If the input
source impedance is very low (<0.5Ω), C
IN
may not be
needed. Increasing the size of C
OUT
to 22µF or greater will
reduce output voltage ripple—particularly with high V
IN
voltages (8V or greater). A ceramic capacitor is recom-
mended for the flying capacitor C1 with a value of 0.1µF
or 0.22µF. Smaller value flying capacitors may be used in
low output current applications.
Output Ripple
Normal LTC1514-3.3/LTC1514-5 operation produces
voltage ripple on the V
OUT
pin. Output voltage ripple is
required for the parts to regulate. Low frequency ripple
exists due to the hysteresis in the sense comparator and
propagation delays in the charge pump enable/disable
circuits. High frequency ripple is also present mainly
from the ESR (equivalent series resistance) in the output
capacitor. Typical output ripple (V
IN
< 8V) under maxi-
mum load is 100mV peak-to-peak with a low ESR (< 0.5Ω)
10µF output capacitor. For applications requiring V
IN
to
exceed 8V, a 22µF or larger C
OUT
capacitor is recom-
mended to maintain max ripple in the 100mV range.
The magnitude of the ripple voltage depends on several
factors. High input voltages increase the output ripple
since more charge is delivered to C
OUT
per charging
cycle. A large C1 flying capacitor (>0.22µF) also
increases ripple in step-up mode for the same reason.
Large output current load and/or a small output capacitor
(< 10µF) results in higher ripple due to higher output
voltage dV/dt. High ESR capacitors (ESR > 0.5Ω) on the
