Datasheet

LTC3736-2
20
37362fb
APPLICATIONS INFORMATION
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 fi lter capacitor C
LP
holds the voltage.
The loop fi lter 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 fi lter
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 SYNC/FCB pin) input high
level is 1.6V, while the input low level is 1.2V.
Table 2 summarizes the different states in which the
PLLLPF pin can be used.
Table 2
PLLLPF PIN SYNC/FCB PIN FREQUENCY
0V DC Voltage 300kHz
Floating DC Voltage 550kHz
V
IN
DC Voltage 750kHz
RC Loop Filter Clock Signal Phase-Locked to External Clock
Auxiliary Winding Control Using SYNC/FCB Pin
The SYNC/FCB can be used as an auxiliary feedback to
provide a means of regulating a fl yback winding output.
When this pin drops below its ground-referenced 0.6V
threshold, continuous mode operation is forced.
During continuous mode, current fl ows continuously in the
transformer primary. The auxiliary winding draws current
only when the bottom, synchronous N-channel MOSFET is
on. When primary load currents are low and/or the V
IN
/V
OUT
ratio is close to unity, the synchronous MOSFET may not be
on for a suffi cient amount of time to transfer power from
the output capacitor to the auxiliary load. Forced continu-
ous operation will support an auxiliary winding as long
as there is a suffi cient synchronous MOSFET duty factor.
The FCB input pin removes the requirement that power
must be drawn from the transformer primary in order to
extract power from the auxiliary winding. With the loop in
continuous mode, the auxiliary output may nominally be
loaded without regard to the primary output load.
The auxiliary output voltage V
AUX
is normally set as shown
in Figure 10 by the turns ratio N of the transformer:
V
AUX
≅ (N + 1) V
OUT
However, if the controller goes into pulse-skipping operation
and halts switching due to a light primary load current, then
V
AUX
will droop. An external resistor divider from V
AUX
to
the FCB sets a minimum voltage V
AUX(MIN)
:
VV
R
R
AUX MIN()
.=+
⎛
⎝
⎜
⎞
⎠
⎟
06 1
6
5
Figures 9. Phase-Locked Loop Block Diagram
DIGITAL
PHASE/
FREQUENCY
DETECTOR
OSCILLATOR
2.4V
R
LP
C
LP
37362 F09
PLLLPF
EXTERNAL
OSCILLATOR
SYNC/
FCB