Datasheet

LTC3766
20
3766fa
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applicaTions inForMaTion
control. If excessive pulse skipping occurs in applications
that use a peak charge circuit to generate bias voltage, this
can cause the bias supply to fall, preventing proper start-
up. To preclude this possibility, use the RUN pin to inhibit
the LTC3766 start-up until the output voltage is at least:
V
OUT(ON)
> 300ns
N
S
f
SW
V
IN(MAX)
N
P
Note that in self-starting applications, direct RUN/STOP
control should be handled only on the primary side using
the LTC3765. If the LTC3765 gets disabled, the LTC3766
will sense that the primary side is no longer switching
and automatically shut down. To avoid a possible output
overvoltage, do not manually disable the LTC3766 unless
the LTC3765 is also manually disabled.
If the RUN pin function is not needed, it can be tied directly
to the V
IN
pin.
Setting the Switching Frequency and Synchronization
The switching frequency of the LTC3766 is set using the
FS/SYNC pin. This pin sources a 20μA current, and a re
-
sistor to ground on this pin sets the switching frequency
to a value equal to:
f
SW
= 4R
FS
Alternatively, the FS/SYNC pin can be tied to V
CC
, which
sets the switching frequency to a fixed value of 275kHz.
In general, a higher switching frequency will result in a
smaller size for inductors and transformers, but at the cost
of reduced efficiency. Although the LTC3766 can operate
from 75kHz to 500kHz, the best balance between efficiency
and size for a forward converter is found when operating
between 150kHz and 350kHz.
If a clock signal (>2V) is detected at the FS pin, the LTC3766
will automatically synchronize to the falling edge of this sig
-
nal. Table 1 summarizes the operation of the FS/SYNC pin.
Table 1
FS/SYNC PIN SWITCHING FREQUENCY
V
CC
275kHz
R
FS
to GND f
SW
= 4R
FS
Reference Clock f
SW
= f
REF
(100kHz to 500kHz)
In PolyPhase applications, synchronization can be achieved
by tying the PT
–
pin of the master to the FS/SYNC pin of
each slave. The relative phase delay of each slave is set
using the PHASE pin. Any one of five preset values can be
selected as shown in Table 2. Note that the phase delay
is relative to the falling edge of the incoming reference
clock on the FS/SYNC pin, since the falling edge of PT
–
corresponds to the beginning of the PWM cycle.
Table 2
PHASE PIN PHASE DELAY APPLICATION
GND 180° 2-Phase and 4-Phase
25k to GND 240° 3-Phase
50k to GND 120° 3-Phase
100k to GND 90° 4-Phase
100k to V
CC
270° 4-Phase
In some applications, it is desirable to start switching at a
given frequency, and then synchronize to a clock reference
signal at a later time. This can be accomplished by using
the circuit shown in Figure 6.
FS/SYNC
LTC3766
GND
R
FS
BAT54
50k
CLK
2N2222A
10nF10k
3766 F06
Figure 6. Synchronization After Free Running
Once the LTC3766 has been synchronized, do not remove
the external synchronizing clock unless the LTC3766
is also shut down. Removal of the external clock after
synchronization will result in operation at low frequencies
for a period of time, which can lead to very high currents
in external power components.
Setting the Output Voltage
The LTC3766 output voltage is set by an external feedback
resistor divider placed across the output as shown in
Figure 7. The regulated output voltage is determined by:
V
OUT
= 0.6 V • 1+
R
B
R
A
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