ltc3858-2 Linear Technology Corporation, ltc3858-2 Datasheet - Page 25

• Manufacturer Part Number: ltc3858-2
• Description: Ltc3858-2 - Low Iq, Dual 2-phase Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

Note that the LTC3858-2 can only be synchronized to an
external clock whose frequency is within range of the
LTC3858-2’s internal VCO, which is nominally 55kHz to
1MHz. This is guaranteed to be between 75kHz and 850kHz.
Typically, the external clock (on the PLLIN/MODE pin) input
high threshold is 1.6V, while the input low threshold is 1.1V.
Rapid phase-locking can be achieved by using the FREQ
pin to set a free-running frequency near the desired
synchronization 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-locking
and synchronization. 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.
APPLICATIONS INFORMATION
Figure 10. Relationship Between Oscillator Frequency
and Resistor Value at the FREQ Pin
1000
200
100
900
800
700
600
500
400
300
0
15
25
35 45 55
FREQ PIN RESISTOR (kΩ)
65 75 85 95 105 115 125
38582 F10
Table 3 summarizes the different states in which the FREQ
pin can be used.
Table 3
Minimum On-Time Considerations
Minimum on-time, t
tion that the LTC3858-2 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:
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 LTC3858-2 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 applica-
tions 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 cor-
respondingly larger current and voltage ripple.
Any of the Above
t
ON(MIN)
FREQ PIN
Resistor
INTV
0V
CC
<
V
V
IN
OUT
( )
f
ON(MIN)
PLLIN/MODE PIN
External Clock
DC Voltage
DC Voltage
DC Voltage
, is the smallest time dura-
LTC3858-2
50kHz to 900kHz
Phase-Locked to
External Clock
FREQUENCY
350kHz
535kHz
25
38582f