LTC3736 LINER [Linear Technology], LTC3736 Datasheet - Page 20

• Manufacturer Part Number: LTC3736
• Description: Dual 2-Phase, No RSENSE Synchronous Controller with Output Tracking
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
If the external clock frequency is greater than the internal
oscillator’s frequency, f
tinuously from the phase detector output, pulling up the
PLLLPF pin. When the external clock frequency is less than
f
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 external oscillators are
identical. At the stable operating point, the phase detector
output is high impedance and the filter capacitor C
the voltage.
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
0.01 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
0V
Floating
V
RC Loop Filter
LTC3736
20
OSC
IN
, current is sunk continuously, pulling down the
LP
SYNC/FCB PIN
Clock Signal
DC Voltage
DC Voltage
DC Voltage
and R
U
OSCILLATOR
OSC
EXTERNAL
LP
U
LP
, then current is sourced con-
determine how fast the loop
= 10k and C
LP
Phase-Locked to External Clock
and R
SYNC/
FCB
W
FREQUENCY
LP
300kHz
550kHz
750kHz
Figure 9. Phase-Locked Loop Block Diagram
, smooth out the
FREQUENCY
LP
DETECTOR
DIGITAL
PHASE/
is 2200pF to
U
LP
holds
2.4V
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
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
in Figure 10 by the turns ratio N of the transformer:
However, if the controller goes into Burst Mode operation
and halts switching due to a light primary load current,
then V
V
AUX
V
V
PLLLPF
AUX
AUX MIN
R
to the FCB sets a minimum voltage V
IN
LP
OSCILLATOR
AUX
/V
(
OUT
(N + 1) V
3736 F09
will droop. An external resistor divider from
)
C
LP
ratio is close to unity, the synchronous
. 0 6
V
OUT
1
R
R
6
5
AUX
is normally set as shown
AUX(MIN)
:
3736f