LTC7813 datasheet(30/36 Pages) LINER

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LTC7821

Rev A

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APPLICATIONS INFORMATION

Minimum On-Time Considerations

Minimum on-time tON(MIN) is the smallest time duration

that the LTC7821 is capable of turning on the top MOSFET.

It is determined by internal timing delays and the gate

charge required to turn on the top MOSFET. Low duty

cycle applications may approach this minimum on-time

limit and care should be taken to ensure that:

tON(MIN)<

2 • VOUT

VIN(f)

If the duty cycle falls below what can be accommodated

by the minimum on-time, the controller will begin to skip

cycles. The output voltage will continue to be regulated,

but the ripple voltage and current will increase.

The minimum on-time for the LTC7821 is approximately

210ns, with reasonably good PCB layout, minimum 30%

inductor current ripple and at least 10mV – 15mV ripple

on the current sense signal. The minimum on-time can be

affected by PCB switching noise in the voltage and current

loop. As the peak sense voltage decreases the minimum

on-time increases. This is of particular concern in forced

continuous applications with low ripple current at light

loads. If the duty cycle drops below the minimum on-time

limit in this situation, a significant amount of cycle skip-

ping can occur with correspondingly larger current and

voltage ripple.

Multiphase Operation

For high output power applications, two LTC7821's can be

paralleled to create a dual phase single output configura-

tion. Figure 22 shows the key signal connections between

the two LTC7821s.

LTC7813 Datasheet(PDF) 30 Page - Linear Technology