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LTC3416

3416f

OPERATIO

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.

Voltage Tracking

Some microprocessors, ASIC and DSP chips need two

power supplies with different voltage levels. These sys-

tems often require voltage sequencing between the core

power supply and the I/O power supply. Without proper

sequencing, latch-up failure or excessive current draw

may occur that could result in damage to the processor’s

I/O ports or the I/O ports of supporting system devices

such as memory, FPGAs or data converters. To ensure

that the I/O loads are not driven until the core voltage is

properly biased, tracking of the core supply voltage and

the I/O supply voltage is necessary.

Voltage tracking is enabled by applying a voltage to the

TRACK pin. When the voltage on the TRACK pin is below

0.8V, the feedback voltage will regulate to this tracking

voltage. When the tracking voltage exceeds 0.8V, tracking

is disabled and the feedback voltage will regulate to the

internal reference voltage.

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, eventually reaching 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 LTC3416 is designed to operate down to an 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

LTC3416 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 LTC3416, however,

slope compensation recovery is implemented to keep the

maximum inductor peak current constant throughout the

range of duty cycles. This keeps the maximum output

current relatively constant regardless of duty cycle.

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 exceeds 7.8A, the top power

MOSFET will be held off and switching cycles will be

skipped until the inductor current is reduced.

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