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LTC3418

3418f

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 LTC3418 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

LTC3418 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. Normally, the maximum inductor peak

current is reduced when slope compensation is added. In

the LTC3418, 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 regard-

less 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 increases larger than 15A, the top

power MOSFET will be held off and switching cycles will be

skipped until the inductor current is reduced.

Voltage Tracking

Some microprocessors and DSP chips need two power

supplies with different voltage levels. These systems often

require voltage sequencing between the core power sup-

ply 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 a supporting system device such as memory,

an FPGA or a data converter. To ensure that the I/O loads

are not driven until the core voltage is properly biased,

tracking of the core supply and the I/O supply voltage is

necessary.

Voltage tracking is enabled by applying a ramp 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.

Voltage Reference Output

The LTC3418 provides a 1.25V reference voltage that is

capable of sourcing up to 5mA of output current. This

reference voltage is generated from a linear regulator and

is intended for applications requiring a low noise reference

voltage. To ensure that the output is stable, the reference

voltage pin should be decoupled with a minimum of 2.2µF.

OPERATIO

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