5
LTC1517-5
APPLICATIONS INFORMATION
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Operation
The LTC1517-5 uses a switched-capacitor charge pump
to boost V
IN
to a 5V ±4% regulated output. The part
achieves regulation by sensing the output voltage through
an internal resistor divider and enabling the charge pump
when the divided output droops below the comparator’s
lower trip point (set by V
REF
). When the charge pump is
enabled, a 2-phase nonoverlapping clock controls the
internal charge pump switches. Flying capacitor C1 is
charged to V
IN
on phase one of the clock. On phase two of
the clock, C1 is stacked in series with V
IN
and connected
to V
OUT
through an internal switch. This sequence of
charging and discharging the flying capacitor occurs at a
free running frequency of 800kHz (typ) and continues until
the divided output voltage reaches the upper trip point of
the comparator. Once the output is back in regulation, the
charge pump is disabled. This method of bursting the
charge pump on and off enables the LTC1517-5 to achieve
high efficiency at extremely low output loads.
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 3.3µF or greater.
Ceramic capacitors will provide the smallest size and
lowest ESR for a given capacitance. If the input source
impedance is very low (< 0.5Ω), C
IN
may not be needed.
Ceramic capacitors are recommended for the flying ca-
pacitor C1 with values of 0.1µF or 0.22µF. Smaller value
flying capacitors may be used in low I
OUT
applications.
Output Ripple
Normal LTC1517-5 operation produces voltage ripple on
the V
OUT
pin. Output voltage ripple is required for the part
to regulate. Low frequency ripple exists due to the hyster-
esis 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
with V
IN
= 3V under maximum load is 100mV peak-to-peak
with a low ESR (<0.5Ω) 3.3µF output capacitor (minimum
recommended C
OUT
). For applications requiring V
IN
to
exceed 3.3V or for applications requiring < 100mV of
peak-to-peak ripple, a 6.8µF to 10µF C
OUT
capacitor is
recommended. Slight further decreases in output ripple
can be achieved by using C
OUT
capacitors larger than
10µF.
Short-Circuit/Thermal Protection
During short-circuit conditions, the LTC1517-5 will draw
between 50mA and 200mA from V
IN
, causing a rise in
junction temperature. On-chip thermal shutdown circuitry
disables the charge pump once the junction temperature
exceeds approximately 160°C. The charge pump is
reenabled once the junction temperature drops to approxi-
mately 145°C. The LTC1517-5 will cycle in and out of
thermal shutdown indefinitely without latchup or damage
until the V
OUT
short is removed.
