ltc3827ig-1 Linear Technology Corporation, ltc3827ig-1 Datasheet - Page 20

• Manufacturer Part Number: ltc3827ig-1
• Description: Low Iq, Dual, 2-phase Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC3827-1
Figure 9. Relationship Between Oscillator Frequency and Voltage
at the PLLLPF Pin When Synchronizing to an External Clock
The loop filter components, C
current pulses from the phase detector and provide a
stable input to the voltage-controlled oscillator. The filter
components C
acquires lock. Typically R
to 0.01μF.
Typically, the external clock (on PLLIN/MODE pin) input
high threshold is 1.6V, while the input low threshold
is 1.2V.
20
OSCILLATOR
EXTERNAL
Figure 10. Phase-Locked Loop Block Diagram
PLLIN/
MODE
900
800
700
600
500
400
300
200
100
0
0
LP
and R
FREQUENCY
DETECTOR
U
DIGITAL
PHASE/
0.5
PLLLPF PIN VOLTAGE (V)
LP
U
1
LP
determine how fast the loop
LP
= 10k and C
1.5
and R
2.4V
W
LP
2
, smooth out the
38271 F09
PLLLPF
R
LP
2.5
LP
OSCILLATOR
U
is 2200pF
38271 F10
C
LP
Table 2 summarizes the different states in which the
PLLLPF pin can be used.
Table 2
PLLLPF PIN
0V
Floating
INTV
RC Loop Filter
Minimum On-Time Considerations
Minimum on-time t
that the LTC3827-1 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 LTC3827-1 is approximately
180ns. However, as the peak sense voltage decreases the
minimum on-time gradually increases up to about 200ns.
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
correspondingly larger current and voltage ripple.
t
CC
ON MIN
(
)
<
V
PLLIN/MODE PIN
V
IN
OUT
Clock Signal
DC Voltage
DC Voltage
DC Voltage
( )
f
ON(MIN)
is the smallest time duration
Phase-Locked to External Clock
FREQUENCY
250kHz
400kHz
530kHz
38271fd