LTC3826-1 Datasheet, PDF (20/32 Pages) Linear Technology – 30μA IQ, Dual, 2-Phase Synchronous Step-Down Controller

LTC3826-1
APPLICATIONS INFORMATION
Phase-Locked Loop and Frequency Synchronization
The LTC3826-1 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
of controller 1 to be locked to the rising edge of an external
clock signal applied to the PLLIN/MODE pin. The turn-on
of controller 2’s top MOSFET is thus 180 degrees out of
phase with the external clock. 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 complementary
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 9 and specified in the Electrical
Characteristics table. Note that the LTC3826-1 can only
be synchronized to an external clock whose frequency is
within range of the LTC3826-1’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 10.
If the external clock frequency is greater than the internal
oscillator’s frequency, f_OSC, then current is sourced
continuously 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
components 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
0V
Floating
INTV_CC
RC Loop Filter
PLLIN/MODE PIN
DC Voltage
DC Voltage
DC Voltage
Clock Signal
FREQUENCY
250kHz
390kHz
530kHz
Phase-Locked to External Clock
900
800
700
600
500
400
300
200
100
0
0
0.5
1
1.5
2 2.5
PLLLPF VOLTAGE (V)
38261 F09
Figure 9. Relationship Between Oscillator Frequency and Voltage
at the PLLLPF Pin When Synchronizing to an External Clock
EXTERNAL
OSCILLATOR
PLLIN/
MODE
DIGITAL
PHASE/
FREQUENCY
DETECTOR
2.4V
R_LP
C_LP
PLLLPF
OSCILLATOR
38261 F10
Figure 10. Phase-Locked Loop Block Diagram
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