LTC3418 查看數據表（PDF） - Linear Technology

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LTC3418

8A, 4MHz, Monolithic Synchronous Step-Down Regulator

Linear Technology 

LTC3418

OPERATION

Dropout Operation

When the input supply voltage decreases toward the output

voltage, the duty cycle increases toward the maximum

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 sup-

ply 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 cur-

rent 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 cur-

rent 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,

control over the feedback voltage is gradually released.

Full release of tracking control over the feedback voltage

is achieved when the tracking voltage exceeds 1.05V.

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.

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