LTC3703-5
24
37035fa
the main output voltage and the turns ratio of the extra
winding to the primary winding as follows:
V
SEC
≈
(N + 1)V
OUT
Since the secondary winding only draws current when the
synchronous switch is on, load regulation at the auxiliary
output will be relatively good as long as the main output is
running in continuous mode. As the load on the primary
output drops and the LTC3703-5 switches to Pulse Skip
Mode operation, the auxiliary output may not be able to
maintain regulation, especially if the load on the auxiliary
output remains heavy. To avoid this, the auxiliary output
voltage can be divided down with a conventional feedback
resistor string with the divided auxiliary output voltage fed
back to the MODE/SYNC pin. The MODE/SYNC threshold
is trimmed to 800mV with 20mV of hysteresis, allowing
precise control of the auxiliary voltage and is set as
follows:
V
V
R
R
SEC MIN
(
)
.
≈
+
⎛
⎝⎜
⎞
⎠⎟
0 8
1
1
2
where R1 and R2 are shown in Figure 9c.
If the LTC3703-5 is operating in Pulse Skip Mode and the
auxiliary output voltage drops below V
SEC(MIN)
, the MODE/
SYNC pin will trip and the LTC3703-5 will resume continu-
ous operation regardless of the load on the main output.
Thus, the MODE/SYNC pin removes the requirement that
power must be drawn from the inductor primary in order
to extract power from the auxiliary winding. With the loop
in continuous mode (MODE/SYNC < 0.8V), the auxiliary
outputs may nominally be loaded without regard to the
primary output load.
The following table summarizes the possible states avail-
able on the MODE/SYNC pin:
Table 1.
MODE/SYNC Pin
Condition
DC Voltage: 0V to 0.75V
Forced Continuous
Current Reversal Enabled
DC Voltage:
≥
0.87V
Pulse Skip Mode Operation
No Current Reversal
Feedback Resistors
Regulating a Secondary Winding
Ext. Clock: 0V to
≥
2V
Forced Continuous
No Current Reversal
the RUN/SS pin allows an internal 4
µ
A current source to
charge up the soft-start capacitor C
SS
. When the voltage
on RUN/SS reaches 1V, the LTC3703-5 begins operating
at its minimum on-time. As the RUN/SS voltage increases
from 1V to 3V, the duty cycle is allowed to increase from
0% to 100%. The duty cycle control minimizes input
supply inrush current and elimates output voltage over-
shoot at start-up and ensures current limit protection even
with a hard short. The RUN/SS voltage is internally clamped
at 4V.
If RUN/SS starts at 0V, the delay before starting is
approximately:
t
V
A
C
s
F C
DELAY START
SS
SS
,
( .
/
)
=
µ
=
µ
1
4
0 25
plus an additional delay, before the output will reach its
regulated value, of:
t
V
V
A
C
s
F C
DELAY REG
SS
SS
,
–
( .
/
)
≥
µ
=
µ
3
1
4
0 5
The start delay can be reduced by using diode D1 in
Figure 18.
Figure 18. RUN/SS Pin Interfacing
APPLICATIO S I FOR ATIO
W
U
U
U
3.3V
OR 5V
RUN/SS
D1
C
SS
37035 F18
RUN/SS
C
SS
MODE/SYNC Pin (Operating Mode and Secondary
Winding Control)
The MODE/SYNC pin is a dual function pin that can be used
for enabling or disabling Pulse Skip Mode operation and
also as an external clock input for synchronizing the inter-
nal oscillator (see next section). Pulse Skip Mode is enabled
when the MODE/SYNC pin is above 0.8V and is disabled,
i.e. forced continuous, when the pin is below 0.8V.
In addition to providing a logic input to force continuous
operation and external synchronization, the MODE/SYNC
pin provides a means to regulate a flyback winding output
as shown in Figure 9c. The auxiliary output is taken from
a second winding on the core of the inductor, converting
it to a transformer. The auxiliary output voltage is set by
