LTC3703
applicaTions inForMaTion
charge up the soft-start capacitor C . When the voltage
the main output voltage and the turns ratio of the extra
winding to the primary winding as follows:
SS
on RUN/SS reaches 0.9V, the LTC3703 begins operating
at its minimum on-time. As the RUN/SS voltage increases
from 1.4V to 3V, the duty cycle is allowed to increase from
0%to100%.Thedutycyclecontrolminimizesinputsupply
inrush current and eliminates output voltage overshoot at
start-up and ensures current limit protection even with a
hardshort.TheRUN/SSvoltageisinternallyclampedat4V.
V
≈ (N + 1)V
OUT
SEC
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 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
precisecontroloftheauxiliaryvoltageandissetasfollows:
If RUN/SS starts at 0V, the delay before starting is
approximately:
1V
4µA
tDELAY,START
=
CSS = (0.25s/µF)CSS
plus an additional delay, before the output will reach its
regulated value, of:
3V –1V
4µA
tDELAY,REG
≥
CSS = (0.5s/µF)CSS
R1
R2
VSEC(MIN) ≈ 0.8V 1+
The start delay can be reduced by using diode D1 in
Figure 18.
where R1 and R2 are shown in Figure 10c.
If the LTC3703 is operating in pulse-skip mode and the
3.3V
OR 5V
RUN/SS
RUN/SS
auxiliaryoutputvoltagedropsbelowV
,theMODE/
SEC(MIN)
D1
SYNC pin will trip and the LTC3703 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.
C
SS
C
SS
3703 F18
Figure 18. RUN/SS Pin Interfacing
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
internal oscillator (see next section). Pulse-skip mode is
enabled when the MODE/SYNC pin is above 0.8V and is
disabled,i.e.,forcedcontinuous,whenthepinisbelow0.8V.
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
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 10c. 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
DC Voltage: ≥ 0.87V
Pulse-Skip Mode Operation
No Current Reversal
Feedback Resistors
Regulating a Secondary Winding
Ext. Clock: 0V to ≥ 2V
Forced Continuous
Current Reversal Enabled
3703fc
24
Linear Systems [ Linear Systems ]
