LTC3255_15 Datasheet, PDF (8/16 Pages) Linear Technology – Wide VIN Range Fault Protected 50mA Step-Down Charge Pump

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.
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For more information www.linear.com/LTC3255