LTC3829_15 Datasheet, PDF (31/40 Pages) Linear Technology – 3-Phase, Single Output Synchronous Step-Down DC/DC Controller with Diffamp

APPLICATIONS INFORMATION
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0
0
0.5
1
1.5
2 2.5
FREQ PIN VOLTAGE (V)
38501 F15
Figure 15. Relationship Between Oscillator
Frequency and Voltage at the FREQ Pin
PLLIN
EXTERNAL
OSCILLATOR
2.4V 5V
10µA
R_SET
DIGITAL _SYNC
PHASE/
FREQUENCY
DETECTOR
FREQ
VCO
LTC3829
3829 F16
Figure 16. Phase-Locked Loop Block Diagram
Minimum On-Time Considerations
Minimum on-time, t_ON(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:
t_ON(MIN)
<
V_OUT
V_IN ( 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 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.
For more information www.linear.com/LTC3829
3829fc
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