LTC3736-2 Datasheet, PDF (20/28 Pages) Linear Technology – Dual 2-Phase, No RSENSE Synchronous Controller with Output Tracking

LTC3736-2
APPLICATIO S I FOR ATIO
EXTERNAL
OSCILLATOR
SYNC/
FCB
DIGITAL
PHASE/
FREQUENCY
DETECTOR
2.4V
R_LP
C_LP
PLLLPF
OSCILLATOR
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Figure 9. Phase-Locked Loop Block Diagram
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 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_IN
RC Loop Filter
SYNC/FCB PIN
DC Voltage
DC Voltage
DC Voltage
Clock Signal
FREQUENCY
300kHz
550kHz
750kHz
Phase-Locked to External Clock
20
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 pulse-skipping op-
eration 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_AUX(MIN)
=
⎛
0.6V_⎝⎜1+
R6^⎞
R5⎠⎟
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