LTC3255 datasheet(8/16 Pages) LINER

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LTC3255

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

arerequired.TheLTC3255usesaninternalswitchnetwork

and fractional conversion ratios to achieve high efficiency

and regulation over widely varying VIN and output load

conditions. A defeatable VIN shunt regulator allows the

LTC3255 to operate with current-fed VIN supplies, such

as 4mA to 20mA current loops.

Automatic 2:1/1:1 Mode Switching with VIN Shunt

Disabled (SHUNT Pin Connected to GND)

Connecting the SHUNT pin to GND defeats the VIN 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 VIN, VOUT, and output load conditions, generally

preferring 2:1 mode when VIN exceeds twice VOUT, but

falling back to 1:1 mode as needed to maintain regulation.

Forced 2:1 Mode Operation When VIN Shunt Regulator

is Enabled (SHUNT Pin Connected to BIAS)

With the SHUNT pin connected to BIAS, the VIN shunt

regulator is enabled, and the LTC3255 expects a high

impedance power source at VIN, such as a 4mA to 20mA

current loop, or a resistor to a DC supply. With the shunt

regulatorenabled,thechargepumprunsin2:1conversion

mode only, extending its output current capability beyond

that of the VIN source. For example, the LTC3255 can

typically boost the current capability of a 4mA source to

power a 7.4mA load continuously. See VIN Shunt Regula-

tor in the Operation section for VIN 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 VOUT via an external resistor divider.

While VFB is below regulation, the LTC3255 transfers fixed

packets of charge from VIN to VOUT, paced by an internal

oscillator. This causes VOUT and hence FB to rise. When

VFB enters regulation, the LTC3255 stops charge transfer

and enters a low quiescent current sleep state. During this

sleepstate,theoutputloadissuppliedentirelybytheoutput

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. VIN

supply current is minimized. When the EN pin is high, the

charge pump will enable if VIN satisfies the VIN undervolt-

age lockout (UVLO) threshold. If the shutdown feature is

not needed, the EN pin can be connected to VIN, as both

pins share the same Absolute Maximum rating.

Reverse Polarity Input Protection

The VIN and EN pins are designed to withstand connection

to voltages below ground without damage. When VIN is

belowground,theLTC3255preventsVOUTfromgoingmore

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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