LTC3413 datasheet(6/16 Pages) LINER

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LTC3413

sn3413 3413fs

OPERATIO

Main Control Loop

The LTC3413 is a monolithic, constant frequency, current

mode step-down DC/DC converter that is capable of sourc-

ing and sinking current at the output. During normal op-

eration, the internal top power switch (P-channel MOSFET)

is turned on at the beginning of each clock cycle. Current

in the inductor increases until the current comparator trips

and turns off the top power MOSFET. The peak inductor

current at which the current comparator shuts off the top

power switch is controlled by the voltage on the ITH pin.

The error amplifier adjusts the voltage on the ITH pin by

comparing the feedback signal on the VFB pin with a refer-

ence voltage that is equal to one-half of the voltage on the

VREF pin. When the load current increases, it causes a

reduction in the feedback voltage relative to the reference.

The error amplifier raises the ITH voltage until the average

inductor current matches the new load current. When the

top power MOSFET shuts off, the synchronous power

switch (N-channel MOSFET) turns on until either the bot-

tom current limit is reached or the beginning of the next

clock cycle. The bottom current limit is set at – 7A.

The operating frequency is set by an external resistor

connected between the RT pin and ground. The switching

frequency can range from 300kHz to 2MHz.

Overvoltage and undervoltage comparators will pull the

PGOOD output low if the output voltage comes out of

regulation by

±10%. In an overvoltage condition, the top

power MOSFET is turned off and the bottom power MOSFET

is switched on until either the overvoltage condition clears

or the bottom MOSFET’s current limit is reached.

Dropout Operation

When the input supply voltage decreases toward the

output voltage, the duty cycle increases toward the maxi-

mum on-time. Further reduction of the supply voltage

forces the main switch to remain on for more than one

cycle until it reaches 100% duty cycle. The output voltage

will then be determined by the input voltage minus the

voltage drop across the internal P-channel MOSFET and

the inductor.

Low Supply Operation

The LTC3413 is designed to operate down to an SVIN input

supply voltage of 2.25V. One important consideration at

low input supply voltages is that the RDS(ON) of the P-

channel and N-channel power switches increases. The

user should calculate the power dissipation when the

LTC3413 is used at 100% duty cycle with low input

voltages to ensure that thermal limits are not exceeded.

Slope Compensation and Inductor Peak Current

Slope compensation provides stability in constant fre-

quency architectures by preventing subharmonic oscilla-

tions at duty cycles greater than 50%. It is accomplished

internally by adding a compensating ramp to the inductor

current signal at duty cycles in excess of 40%. Normally,

the maximum inductor peak current is reduced when slope

compensation is added. In the LTC3413, however, slope

compensation recovery is implemented to keep the maxi-

mum inductor peak current constant throughout the range

of duty cycles.

Short-Circuit Protection

When the output is shorted to ground, the inductor current

decays very slowly during a single switching cycle. To

prevent current runaway from occurring, a secondary

current limit is imposed on the inductor current. If the

inductor valley current increases greater than 5A, the top

power MOSFET will be held off and switching cycles will be

skipped until the inductor current is reduced.

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