LTC3245_15 Datasheet, PDF (9/18 Pages) Linear Technology – Wide VIN Range, Low Noise, 250mA Buck-Boost Charge Pump

LTC3245
Applications Information
General Operation
The LTC3245 uses switched capacitor based DC/DC
conversion to provide the efficiency advantages associ-
ated with inductor based circuits as well as the cost and
simplicity advantages of a linear regulator. The LTC3245’s
unique constant frequency architecture provides a low
noise regulated output as well as lower input noise than
conventional switch capacitor charge pump regulators. The
LTC3245 uses an internal switch network and fractional
conversion ratios to achieve high efficiency and regula-
tion over widely varying V_IN and output load conditions.
Internal control circuitry selects the appropriate conver-
sion ratio based on V_IN and load conditions. The device
has three possible conversion modes: 2:1 step-down
mode, 1:1 step-down mode and 1:2 step-up mode. Only
one external flying capacitor is needed to operate in all
three modes. 2:1 mode is chosen when V_IN is greater than
two times the desired V_OUT. 1:1 mode is chosen when
V_IN falls between two times V_OUT and V_OUT. 1:2 mode is
chosen when V_IN falls below the desired V_OUT. An internal
load current sense circuit controls the switch point of the
conversion ratio as needed to maintain output regulation
over all load conditions.
Regulation is achieved by sensing the output voltage and
regulating the amount of charge transferred per cycle. This
method of regulation provides much lower input and output
ripple than that of conventional switched capacitor charge
pumps. The constant frequency charge transfer also makes
additional output or input filtering much less demanding
than conventional switched capacitor charge pumps.
The LTC3245 has a Burst Mode operation pin that allows
the user to trade output ripple for better efficiency/lower
quiescent current. The device has two SEL pins that select
the output regulation (fixed 5V, fixed 3.3V or adjustable)
as well as shutdown. The device includes soft-start func-
tion to limit in-rush current at startup. The device is also
short-circuit and overtemperature protected.
V_OUT Regulation and Mode Selection
As shown in the Simplified Block Diagram, the device uses
a control loop to adjust the strength of the charge pump to
match the current required at the output. The error signal
of this loop is stored directly on the output charge storage
capacitor. As the load on V_OUT increases, V_OUT will drop
slightly increasing the amount of charge transferred until
the output current matches the output load. This method
of regulation applies regardless of the conversion ratio.
The optimal conversion ratio is chosen based on V_IN,
V_OUT and output load conditions. Two internal compara-
tors are used to select the default conversion ratio. Each
comparator has an adjustable offset built in that increases
(decreases) in proportion to the increasing (decreasing)
output load current. In this manner, the conversion ratio
switch point is optimized to provide peak efficiency over
all supply and load conditions while maintaining regulation.
Each comparator also has built-in hysteresis to reduce the
tendency of oscillating between modes when a transition
point is reached.
Low Noise vs Burst Mode Operation
Burst Mode operation is selected by driving the BURST
pin low. In Burst Mode operation the LTC3245 delivers a
minimum amount of charge each cycle forcing V_OUT above
regulation at light output loads. When the LTC3245 detects
that V_OUT is above regulation the device stops charge
transfer and goes into a low current sleep state. During
this sleep state, the output load is supplied by the output
capacitor. The device will remain in the sleep state until the
output drops enough to require another burst of charge.
Burst Mode operation allows the LTC3245 to achieve high
efficiency even at light loads. If the output load exceeds
the minimum charge transferred per cycle, then the device
will operate continuously to maintain regulation.
Unlike traditional charge pumps who’s burst current is
dependant on many factors (i.e., supply, switch strength,
capacitor selection, etc.), the LTC3245 burst current is
regulated which helps to keep burst output ripple voltage
relatively constant and is typically 50mV for C_OUT = 10μF.
Driving the BURST pin high puts the LTC3245 in low noise
operation. In low noise operation the minimum amount
of charge delivered each cycle and sleep hysteresis
are reduced compared to Burst Mode operation. This
results in lower burst output ripple (typically 20mV for
C_OUT = 10µF) and will transition to constant frequency
operation at lighter loads.
For more information www.linear.com/LTC3245
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