LTC3890
applicaTions inForMaTion
Note that the LTC3890 can only be synchronized to an
external clock whose frequency is within range of the
LTC3890’s internal VCO, which is nominally 55kHz to
1MHz.Thisisguaranteedtobebetween75kHzand850kHz.
Minimum On-Time Considerations
Minimum on-time, t , is the smallest time duration
that the LTC3890 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:
ON(MIN)
Typically,theexternalclock(onthePLLIN/MODEpin)input
highthresholdis1.6V,whiletheinputlowthresholdis1.1V.
RapidphaselockingcanbeachievedbyusingtheFREQpin
to set a free-running frequency near the desired synchro-
nization frequency. The VCO’s input voltage is prebiased
at a frequency corresponding to the frequency set by the
FREQ pin. Once prebiased, the PLL only needs to adjust
the frequency slightly to achieve phase lock and synchro-
nization. Although it is not required that the free-running
frequency be near external clock frequency, doing so will
prevent the operating frequency from passing through a
large range of frequencies as the PLL locks.
VOUT
V f
IN ( )
tON(MIN)
<
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 LTC3890 is approximately
95ns. However, as the peak sense voltage decreases
the minimum on-time gradually increases up to about
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.
Table 2 summarizes the different states in which the FREQ
pin can be used.
Table 2
FREQ PIN
PLLIN/MODE PIN
DC Voltage
FREQUENCY
350kHz
0V
INTV
DC Voltage
535kHz
CC
Resistor
DC Voltage
50kHz to 900kHz
Any of the Above
External Clock
Phase Locked to
External Clock
3890fb
24
Linear Systems [ Linear Systems ]
