Datasheet

LTC3835
19
3835fd
APPLICATIONS INFORMATION
Phase-Locked Loop and Frequency Synchronization
The LTC3835 has a phase-locked loop (PLL) comprised of
an internal voltage-controlled oscillator (VCO) and a phase
detector. This allows the turn-on of the top MOSFET (TG)
to be locked to the rising edge of an external clock signal
applied to the PLLIN/MODE pin. The phase detector is
an edge sensitive digital type that provides zero degrees
phase shift between the external and internal oscillators.
This type of phase detector does not exhibit false lock to
harmonics of the external clock.
The output of the phase detector is a pair of comple-
mentary current sources that charge or discharge the
external filter network connected to the PLLLPF pin. The
relationship between the voltage on the PLLLPF pin and
operating frequency, when there is a clock signal applied
to PLLIN/MODE, is shown in Figure 7 and specified in the
Electrical Characteristics table. Note that the LTC3835 can
only be synchronized to an external clock whose frequency
is within range of the LTC3835’s internal VCO, which is
nominally 115kHz to 800kHz. This is guaranteed to be
between 140kHz and 650kHz. A simplified block diagram
is shown in Figure 8.
If the external clock frequency is greater than the internal
oscillator’s frequency, f
OSC
, then current is sourced con-
tinuously from the phase detector output, pulling up the
PLLLPF pin. When the external clock frequency is less
than f
OSC
, current is sunk continuously, pulling down
the PLLLPF pin. If the external and internal frequencies
are the same but exhibit a phase difference, the current
sources turn on for an amount of time corresponding to
the phase difference. The voltage on the PLLLPF pin is
adjusted until the phase and frequency of the internal and
external oscillators are identical. At the stable operating
point, the phase detector output is high impedance and
the filter capacitor C
LP
holds the voltage.
The loop filter components, C
LP
and R
LP
, smooth out the
current pulses from the phase detector and provide a stable
input to the voltage-controlled oscillator. The filter compo-
nents C
LP
and R
LP
determine how fast the loop acquires
lock. Typically R
LP
= 10k and C
LP
is 2200pF 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.
Table 2 summarizes the different states in which the
PLLLPF pin can be used.
Table 2
PLLLPF PIN PLLIN/MODE
PIN
FREQUENCY
0V DC Voltage 250kHz
Floating DC Voltage 400kHz
INTV
CC
DC Voltage 530kHz
RC Loop Filter Clock Signal Phase-Locked to External Clock
Figure 7. Relationship Between Oscillator Frequency and Voltage
at the PLLLPF Pin When Synchronizing to an External Clock
PLLLPF PIN VOLTAGE (V)
0
FREQUENCY (kHz)
0.5 1 1.5 2
3835 F07
2.5
0
100
300
400
500
900
800
700
200
600
Figure 8. Phase-Locked Loop Block Diagram
DIGITAL
PHASE/
FREQUENCY
DETECTOR
OSCILLATOR
2.4V
R
LP
C
LP
3835 F08
PLLLPF
EXTERNAL
OSCILLATOR
PLLIN/
MODE