20
LTC3736
3736fa
APPLICATIO S I FOR ATIO
W
U
U
U
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 differ-
ence. The voltage on the PLLLPF pin is adjusted until the
phase and frequency of the internal and external oscilla-
tors 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
0.01
µ
F.
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 flyback winding output.
When this pin drops below its ground-referenced 0.6V
threshold, continuous mode operation is forced.
During continuous mode, current flows 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 sufficient amount of time to
transfer power from the output capacitor to the auxiliary
load. Forced continuous operation will support an auxil-
iary winding as long as there is a sufficient synchronous
MOSFET duty factor. The FCB input pin removes the
requirement that power must be drawn from the trans-
former 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 Burst Mode 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)
:
V
V
R
R
AUX MIN
(
)
.
=
+
⎛
⎝
⎜
⎞
⎠
⎟
0 6
1
6
5
DIGITAL
PHASE/
FREQUENCY
DETECTOR
OSCILLATOR
2.4V
R
LP
C
LP
3736 F09
PLLLPF
EXTERNAL
OSCILLATOR
SYNC/
FCB
Figure 9. Phase-Locked Loop Block Diagram
