LTC3703_15 Datasheet, PDF (24/34 Pages) Linear Technology – 100V Synchronous Switching Regulator Controller

LTC3703
Applications Information
charge up the soft-start capacitor C_SS. When the voltage
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% to 100%. The duty cycle control minimizes input supply
inrush current and eliminates output voltage overshoot 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_DELAY,START
=
1V
4µA
C_SS
=
(0.25s/µF)C_SS
plus an additional delay, before the output will reach its
regulated value, of:
t_DELAY,REG
≥
3V – 1V
4µA
C_SS
=
(0.5s/µF)C_SS
The start delay can be reduced by using diode D1 in
Figure 18.
3.3V
OR 5V
RUN/SS
D1
RUN/SS
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., 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 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
24
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 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_SEC(MIN)
≈
0.8V


1+
R1_
R2 ^
where R1 and R2 are shown in Figure 10c.
If the LTC3703 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 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
DC Voltage: 0V to 0.75V
DC Voltage: ≥ 0.87V
Feedback Resistors
Ext. Clock: 0V to ≥ 2V
CONDITION
Forced Continuous
Current Reversal Enabled
Pulse-Skip Mode Operation
No Current Reversal
Regulating a Secondary Winding
Forced Continuous
Current Reversal Enabled
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