LTC3255
8
3255f
For more information www.linear.com/LTC3255
APPLICATIONS INFORMATION
General Operation
The LTC3255 uses switched-capacitor-based DC/DC
conversion to provide efficiency advantages associated
with inductor-based circuits together with the cost and
simplicity advantages of linear regulators. No inductors
are required. The LTC3255 uses an internal switch network
and fractional conversion ratios to achieve high efficiency
and regulation over widely varying V
IN
and output load
conditions. A defeatable V
IN
shunt regulator allows the
LTC3255 to operate with current-fed V
IN
supplies, such
as 4mA to 20mA current loops.
Automatic 2:1/1:1 Mode Switching with V
IN
Shunt
Disabled (SHUNT Pin Connected to GND)
Connecting the SHUNT pin to GND defeats the V
IN
shunt
regulator. With the shunt regulator defeated, the LTC3255
functions as a general purpose step-down charge pump
offering two conversion modes: 2:1 and 1:1. Internal cir-
cuitry automatically
chooses the optimal conversion ratio
based on V
IN
, V
OUT
, and output load conditions, generally
preferring 2:1 mode when V
IN
exceeds twice V
OUT
, but
falling back to 1:1 mode as needed to maintain regulation.
Forced 2:1 Mode Operation When V
IN
Shunt Regulator
is Enabled (SHUNT Pin Connected to BIAS)
With the SHUNT pin connected to BIAS, the V
IN
shunt
regulator is enabled, and the LTC3255 expects a
high
impedance power source at V
IN
, such as a 4mA to 20mA
current loop, or a resistor to a DC supply. With the shunt
regulator enabled, the charge pump runs in 2:1 conversion
mode only, extending its output current capability beyond
that of the V
IN
source. For example, the LTC3255 can
typically boost the current capability of a 4mA source to
power a 7.4mA load continuously. See
V
IN
Shunt Regula-
tor in the Operation section for V
IN
compliance and other
operating information.
Regulation Loop
Regulation is achieved via a Burst Mode control loop that
allows the LTC3255 to achieve high efficiency even at
light loads. As shown in the Block Diagram, a comparator
monitors the output voltage via a feedback pin, FB, which
receives a fraction of V
OUT
via an external resistor divider.
While V
FB
is below regulation, the LTC3255 transfers fixed
packets of charge from V
IN
to V
OUT
, paced by an internal
oscillator. This causes V
OUT
and hence FB to rise. When
V
FB
enters regulation, the LTC3255 stops charge transfer
and enters a low quiescent current sleep state. During this
sleep state, the output load is supplied entirely by the output
capacitor. The LTC3255
remains in sleep until the output
drops enough to require another burst of charge. As load
current decreases, the output capacitor takes longer to
discharge, so sleep time increases.
Shutdown and Undervoltage Lockout (UVLO)
Driving the EN pin low puts the LTC3255 in shutdown,
which disables all circuitry except the internal bias. V
IN
supply current is minimized. When the EN pin is high, the
charge pump
will enable if V
IN
satisfies the V
IN
undervolt-
age lockout (UVLO) threshold. If the shutdown feature is
not needed, the EN pin can be connected to V
IN
, as both
pins share the same Absolute Maximum rating.
Reverse Polarity Input Protection
The V
IN
and EN pins are designed to withstand connection
to voltages below ground without damage. When V
IN
is
below ground, the LTC3255 prevents V
OUT
from going more
than a diode drop below GND to protect the load circuit.
Short-Circuit/Thermal Protection
The LTC3255 has built-in short-circuit current limiting as
well as overtemperature protection. During short-circuit
conditions output current is automatically limited by the
output current limit circuitry.
