ltc3730 Linear Technology Corporation, ltc3730 Datasheet - Page 19

• Manufacturer Part Number: ltc3730
• Description: 3-phase, 5-bit Intel Mobile Vid, 600khz, Synchronous Buck Controller
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
Undervoltage Reset
In the event that the input power source to the IC (V
drops below 3.8V, the RUN/SS capacitor will be dis-
charged to ground. When V
SS capacitor will be allowed to recharge and initiate
another soft-start turn-on attempt. This may be useful in
applications that switch between two supplies that are not
diode connected, but note that this cannot make up for the
resultant interruption of the regulated output.
Phase-Locked Loop and Frequency Synchronization
The IC has a phase-locked loop comprised of an internal
voltage controlled oscillator and phase detector. This
allows the top MOSFET of output stage 1’s turn-on to be
locked to the rising edge of an external source. The
frequency range of the voltage controlled oscillator is
±50% around the center frequency f
the PLLFLTR pin of 1.2V corresponds to a frequency of
approximately 400kHz. The nominal operating frequency
range of the IC is 225kHz to 680kHz.
The phase detector used is an edge sensitive digital type
that provides zero degrees phase shift between the exter-
nal and internal oscillators. This type of phase detector will
not lock the internal oscillator to harmonics of the input
frequency. The PLL hold-in range, ∆f
capture range, ∆f
The output of the phase detector is a complementary pair
of current sources charging or discharging the external
filter components on the PLLFLTR pin. A simplified block
diagram is shown in Figure 9.
∆f
H
= ∆f
PLLIN
EXTERNAL
Figure 9. Phase-Locked Loop Block Diagram
OSC
C
= ±0.5 f
PHASE
DETECTOR/
OSCILLATOR
50k
C
:
U
O
FREQUENCY
DETECTOR
DIGITAL
PHASE/
OSC
U
CC
rises above 3.8V, the RUN/
2.4V
W
O
. A voltage applied to
3730 F09
H
, is equal to the
R
PLLFLTR
10k
LP
U
C
LP
CC
)
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
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
current pulses from the phase detector and provide a
stable input to the voltage controlled oscillator. The filter
components C
acquires lock. Typically R
0.01µF to 0.1µF.
Minimum On-Time Considerations
Minimum on-time, t
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:
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.
t
ON MIN
(
)
<
OSC
V f
V
LP
IN
OUT
, current is sunk continuously, pulling
( )
OSC
and R
ON(MIN)
, current is sourced continuously,
LP
PLLIN
LP
determine how fast the loop
, is the smallest time duration
=10k and C
LP
) is greater than the oscil-
LP
holds the voltage. The IC
, R
LP
) smooth out the
LTC3730
LP
ranges from
19
3730fa