ltc1629-6 Linear Technology Corporation, ltc1629-6 Datasheet - Page 19

• Manufacturer Part Number: ltc1629-6
• Description: Polyphase, Synchronous Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
center frequency f
of 1.2V corresponds to a frequency of approximately
220kHz. The nominal operating frequency range of the
LTC1629-6 is 140kHz to 310kHz.
The phase detector used is an edge sensitive digital type
which provides zero degrees phase shift between the
external and internal oscillators. This type of phase detec-
tor will not lock up on input frequencies close to the
harmonics of the VCO center frequency. The PLL hold-in
range, f
The output of the phase detector is a complementary pair
of current sources charging or discharging the external
filter network on the PLLFLTR pin. A simplified block
diagram is shown in Figure 7.
If the external frequency (f
lator frequency f
pulling up the PLLFLTR pin. When the external frequency
is less than f
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
LTC1629-6 PLLIN pin must be driven from a low imped-
ance source such as a logic gate located close to the pin.
When using multiple LTC1629-6’s 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
DC voltage of 1.6V to 1.7V applied to the master oscillator’s
PLLFLTR pin is recommended in order to meet this
requirement. The resultant operating frequency will be
approximately 300kHz.
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
0.1 F.
f
H
= f
H
, is equal to the capture range, f
C
= 0.5 f
0SC
LP
, current is sunk continuously, pulling
0SC
O
and R
. A voltage applied to the PLLFLTR pin
U
, current is sourced continuously,
O
(150kHz-300kHz)
LP
LP
U
PLLIN
determine how fast the loop
=10k and C
LP
) is greater than the oscil-
LP
, R
holds the voltage. The
W
LP
) smooth out the
LP
C:
is 0.01 F to
U
Minimum On-Time Considerations
Minimum on-time t
that the LTC1629-6 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 LTC1629-6 will begin to skip
cycles resulting in nonconstant 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 LTC1629-6 is generally less
than 200ns. However, as the peak sense voltage decreases
the minimum 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 corre-
spondingly larger current and voltage ripple.
If an application can operate close to the minimum on-
time limit, an inductor must be chosen that has a low
enough inductance to provide sufficient ripple amplitude
to meet the minimum on-time requirement. As a general
rule, keep the inductor ripple current of each phase equal
to or greater than 15% of I
PLLIN
t
ON MIN
EXTERNAL
OSC
Figure 7. Phase-Locked Loop Block Diagram
50k
V f
V
IN
OUT
FREQUENCY
DETECTOR
DETECTOR
ON(MIN)
DIGITAL
PHASE/
PHASE
OUT(MAX)
is the smallest time duration
2.4V
LTC1629-6
/N at V
IN(MAX)
PLLFLTR
R
10k
LP
1629 F07
19
.
OSC
C
LP
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