LTC3703IGN-5 Linear Technology, LTC3703IGN-5 Datasheet - Page 24

• Manufacturer Part Number: LTC3703IGN-5
• Description: IC,SMPS CONTROLLER,VOLTAGE-MODE,CMOS,SSOP,16PIN,PLASTIC
• Manufacturer: Linear Technology

LTC3703
APPLICATIO S I FOR ATIO
If RUN/SS starts at 0V, the delay before starting is approxi-
mately:
plus an additional delay, before the output will reach its
regulated value, of:
The start delay can be reduced by using diode D1 in
Figure 18.
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
the main output voltage and the turns ratio of the extra
winding to the primary winding as follows:
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
24
V
t
t
DELAY START
DELAY REG
SEC
≈ (N + 1)V
OR 5V
3.3V
,
,
Figure 18. RUN/SS Pin Interfacing
≥
D1
3
=
V
4
OUT
4
U
µ
RUN/SS
1
–
µ
V
A
1
A
V
C
C
C
SS
U
SS
SS
=
=
( .
0 25
( .
0 5
W
s
s
/
/
µ
µ
F C
F C
)
)
RUN/SS
SS
SS
U
3703 F18
C
SS
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:
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
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
MODE/SYNC Pin (External Synchronization)
The internal LTC3703 oscillator can be synchronized to an
external oscillator by applying and clocking the MODE/
SYNC pin with a signal above 2V
locks to the external clock after the second clock transition
is received. When external synchronization is detected,
LTC3703 will operate in forced continuous mode. If an
external clock transition is not detected for three succes-
sive periods, the internal oscillator will revert to the
frequency programmed by the R
V
SEC MIN
(
)
≈
0 8 1
.
V
⎛
⎜
⎝
+
Condition
Current Reversal Enabled
Pulse Skip Mode Operation
No Current Reversal
Regulating a Secondary Winding
Forced Continuous
Current Reversal Enabled
Forced Continuous
R
R
2
1
⎞
⎟
⎠
P-P
SET
. The internal oscillator
resistor. The internal
SEC(MIN)
, the MODE/
3703fa