LTC3731H Datasheet, PDF (20/32 Pages) Linear Technology – 3-Phase, 600kHz, Synchronous Buck Switching Regulator Controller

LTC3731H
APPLICATIO S I FOR ATIO
PHASE
DETECTOR/
2.4V
OSCILLATOR
EXTERNAL
OSC
OSC
PLLIN
DIGITAL
PHASE/
FREQUENCY
50k
DETECTOR
R_LP
10k
C_LP
PLLFLTR
3731H F09
Figure 9. Phase-Locked Loop Block Diagram
down the PLLFLTR pin. If the external and internal fre-
quencies are the same, but exhibit a phase difference, the
current sources turn on for an amount of time correspond-
ing to the phase difference. Thus, the voltage on the
PLLFLTR pin is adjusted until the phase and frequency of
the external and internal oscillators are identical. At this
stable operating point, the phase comparator output is
open and the filter capacitor C_LP holds the voltage. The IC
PLLIN pin must be driven from a low impedance source
such as a logic gate located close to the pin. When using
multiple ICs for a phase-locked system, the PLLFLTR pin
of the master oscillator should be biased at a voltage that
will guarantee the slave oscillator(s) ability to lock onto the
master’s frequency. A voltage of 1.7V or below applied to
the master oscillator’s PLLFLTR pin is recommended in
order to meet this requirement. The resultant operating
frequency will be approximately 550kHz for 1.7V.
The loop filter components (C_LP, R_LP) smooth out the
current pulses from the phase detector and provide a
stable input to the voltage controlled oscillator. The filter
components C_LP and R_LP determine how fast the loop
acquires lock. Typically R_LP =10k and C_LP ranges from
0.01µF to 0.1µF.
Minimum On-Time Considerations
Minimum on-time, t_ON(MIN), is the smallest time duration
that the IC is capable of turning on the top MOSFET. It is
determined by internal timing delays and the gate charge
of 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 IC will begin to skip every other
cycle, resulting in half-frequency operation. The output
voltage will continue to be regulated, but the ripple current
and ripple voltage will increase.
The minimum on-time for the IC is generally about 110ns.
However, as the peak sense voltage decreases the mini-
mum on-time gradually 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 skipping can occur with correspondingly
larger current and voltage ripple.
If an application can operate close to the minimum on-
time limit, an inductor must be chosen that is low enough
in value to provide sufficient ripple amplitude to meet the
minimum on-time requirement. As a general rule, keep
the inductor ripple current for each channel equal to or
greater than 30% of I_OUT(MAX) at V_IN(MAX).
3731hf
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