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LTC3829

3829f

applicaTions inForMaTion

Minimum On-Time Considerations

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

that the LTC3829 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:

V f

ON MIN

OUT

(

) <

( )

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 LTC3829 is approximately 90ns,

with reasonably good PCB layout, minimum 30% induc-

tor current ripple and at least 10mV 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

gradually increases to 130ns. 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

skipping can occur with correspondingly larger current

and voltage ripple.

Figure 15. Relationship Between Oscillator

Frequency and Voltage at the FREQ Pin

Figure 16. Phase-Locked Loop Block Diagram

FREQ PIN VOLTAGE (V)

0.5

1.5

38501 F15

2.5

100

300

400

500

900

800

700

200

600

DIGITAL

PHASE/

FREQUENCY

DETECTOR

VCO

2.4V 5V

10µA

RSET

3829 F16

FREQ

SYNC

EXTERNAL

OSCILLATOR

PLLIN

LTC3829 Datasheet(PDF) 32 Page - Linear Technology

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