Datasheet

LTC3809
17
3809fc
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 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/MODE pin) input
high level is 1.6V, while the input low level is 1.2V.
Table 1 summarizes the different states in which the
PLLLPF pin can be used.
Table 1. The States of the PLLLPF Pin
PLLLPF PIN SYNC/MODE PIN FREQUENCY
0V DC Voltage (<1.2V or V
IN
) 300kHz
Floating DC Voltage (<1.2V or V
IN
) 550kHz
V
IN
DC Voltage (<1.2V or V
IN
) 750kHz
RC Loop Filter Clock Signal Phase-Locked to
External Clock
Filter Caps DC Voltage (>1.35V and <V
IN
– 0.5V) Spread Spectrum
460kHz to 635kHz
Auxiliary Winding Control Using SYNC/MODE Pin
The SYNC/MODE pin 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 0.4V
threshold, continuous mode operation is forced.
During continuous mode, current fl ows continuously in
the transformer primary side. 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 synchro-
nous MOSFET may not be on for a suffi cient amount of
time to transfer power from the output capacitor to the
auxiliary load. Forced continuous operation will sup-
port an auxiliary winding as long as there is a suffi cient
synchronous MOSFET duty factor. The SYNC/MODE
input pin removes the requirement that power must be
drawn from the transformer primary side 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
i s n o r m a l l y s e t , a s s h o w n
in Figure 7, 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 SYNC/MODE sets a minimum voltage V
AUX(MIN)
:
VV
R
R
AUX MIN()
.•=+
⎛
⎝
⎜
⎞
⎠
⎟
04 1
6
5
If V
AUX
drops below this value, the SYNC/MODE voltage
forces temporary continuous switching operation until
V
AUX
is again above its minimum.
APPLICATIONS INFORMATION
LTC3809
+
+
R6
R5
1μF
V
OUT
V
AUX
C
OUT
L1
1:N
SYNC/MODE
BG
SW
TG
3809 F07
V
IN
Figure 7. Auxilliary Output Loop Connection
Spread Spectrum Modulation with SYNC/MODE and
PLLLPF Pins
Switching regulators, which operate at fi xed frequency,
conduct electromagnetic interference (EMI) to their down-
stream load(s) with high spectral power density at this
fundamental and harmonic frequencies. The peak energy