LTC3306CACBZ-R7 datasheet(8/16 Pages) AD

8 / 16 page

LTC3306

Rev. 0

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OPERATION

Voltage Regulation

The LTC3306 is a monolithic, constant frequency, peak

current mode, step-down DC/DC converter. The synchro-

nous buck switching regulator is factory programmed and

requires only input and output capacitors and an inductor

for a complete solution. A factory-programmed oscillator

turns on the internal top power switch at the beginning of

each clock cycle. Current in the inductor then increases

until the top switch current comparator trips and turns off

the top power switch. The peak inductor current at which

the top switch turns off is controlled by an internal control

voltage (VC). The error amplifier regulates VC by compar-

ing the voltage on the FB or VOUT node with an internal

reference. When the load current increases, it causes a

reduction in the output voltage relative to the reference,

leading the error amplifier to raise the VC voltage until the

average inductor current matches the new load current.

When the top power switch turns off, the synchronous

power switch turns on and ramps down the inductor cur-

rent for the remainder of the clock cycle or, if configured

for Burst Mode operation, until the inductor current falls

to zero. If overload conditions result in excessive current

flowing through the bottom switch, the next clock cycle

will be skipped until switch current returns to a safe level.

The enable pin has a precision 400mV threshold that may

be used to provide event-based power-up sequencing by

connecting the EN pin to the output of another regulator

through a resistor divider. If the EN pin is low, the device

is shut down and in a low quiescent current state. When

the EN pin is above its threshold, the switching regulator

will be enabled.

The LTC3306 has forward and reverse-current limiting,

short-circuit protection, output overvoltage protection,

and soft-start to limit inrush current during start-up or

recovery from a short-circuit.

Mode Selection

The LTC3306 is available in two operational modes: forced

continuous mode, and Burst Mode operation.

In forced continuous mode, the switches turn on every

cycle, maintaining regulation by allowing the inductor cur-

rent to reverse at light load. This mode allows the buck

to run at a fixed frequency with a consistent amount of

current ripple, generating minimal output voltage ripple.

In Burst Mode operation, reverse current is prevented on

a cycle-by-cycle basis, allowing discontinuous switching

operation at lower average load currents. While operat-

ing at light loads, the output voltage charges to a slightly

higher voltage than its normal regulation voltage. The

regulator then suspends switching and disables most of

the internal circuitry, minimizing input power. The output

capacitor provides required load current until the output

voltage drops below its programmed value, when the cir-

cuitry is powered on and switching cycles commence.

Output Power Good

When the LTC3306 output voltage is above 98% of the

regulation voltage, the output is considered good and the

open-drain PGOOD pin becomes high impedance and can

be pulled high using an external resistor. When the output

voltage falls below 96.8%, the internal pull-down device

will pull the PGOOD pin low. The PGOOD pin is also pulled

low during the following fault conditions: EN pin is low,

VIN is too low, VOUT is above 110% of the nominal volt-

age, or during thermal shutdown. To filter noise and short

duration output voltage transients, the lower threshold

has a hysteresis of 1%, the upper threshold has a hys-

teresis of 2%, and both have a built-in 100μs time delay

to report a PGOOD change of state.

Output Overvoltage Protection

During an output overvoltage event, when VOUT is greater

than 110% of the nominal regulated voltage, the LTC3306

top power switch will be turned off. If the output remains

out of regulation for more than 100µs, the PGOOD pin

will be pulled low. An output overvoltage event should not

happen under normal operating conditions.

Overtemperature Protection

To prevent thermal damage to the LTC3306 and its sur-

rounding components, the device incorporates an over-

temperature (OT) function. When the die temperature

reaches 160°C (typical), the switcher is disabled until the

die temperature falls to 155°C (typical).

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