LTC1775 LINER [Linear Technology], LTC1775 Datasheet - Page 16

• Manufacturer Part Number: LTC1775
• Description: High Power No RSENSE TM Current Mode Synchronous Step-Down Switching Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC1775
Run/Soft Start Function
The RUN/SS pin is a dual purpose pin that provides a soft
start function and a means to shut down the LTC1775. Soft
start reduces surge currents from V
creasing the controller’s current limit I
can also be used for power supply sequencing.
Pulling the RUN/SS pin below 1.4V puts the LTC1775 into
a low quiescent current shutdown (I
be driven directly from logic as shown in Figure 9. Releas-
ing the RUN/SS pin allows an internal 3 A current source
to charge up the soft-start capacitor C
been pulled all the way to ground there is a delay before
starting of approximately:
When the voltage on RUN/SS reaches 1.4V the LTC1775
begins operating with a clamp on I
voltage on RUN/SS increases to approximately 3.1V, the
clamp on I
This takes an additional 0.5s/ F. During this time the load
current will be folded back to approximately 80mV/R
until the output reaches half of its final value.
Diode D1 in Figure 9 reduces the start delay while allowing
C
diode and C
RUN/SS pin has an internal 6V zener clamp (See Func-
tional Diagram).
16
FCB Pin Operation
When the FCB pin drops below its 1.19V threshold,
continuous synchronous operation is forced. In this case,
the top and bottom MOSFETs continue to be driven
regardless of the load on the main output. Burst Mode
operation is disabled and current reversal under light
loads is allowed in the inductor.
SS
t
DELAY
to charge up slowly for the soft start function. This
OR 5V
3.3V
TH
SS
is raised until its full 2.4V range is restored.
can be deleted if soft start is not needed. The
Figure 9. RUN/SS Pin Interfacing
1 4
3
.
D1
A
V
U
RUN/SS
C
SS
C
U
SS
0 5
. /
s
W
Q
F C
< 30 A). This pin can
TH
IN
SS
TH(MAX)
SS
by gradually in-
. If RUN/SS has
at 0.8V. As the
RUN/SS
U
. This pin
1775 F09
C
SS
DS(ON)
In addition to providing a logic input to force continuous
operation, the FCB pin provides a means to regulate a
flyback winding output. It can force continuous synchro-
nous operation when needed by the flyback winding,
regardless of the main output load.
The secondary output voltage V
shown in Figure 5a by the turns ratio N of the transformer:
However, if the controller goes into Burst Mode operation
and halts switching due to a light main load current, then
V
the FCB pin sets a minimum voltage V
If V
continuous operation until V
minimum.
Minimum On-Time Considerations
Minimum on-time t
that the LTC1775 is capable of turning the top MOSFET on
and off again. It is determined by internal timing delays and
the amount of gate charge required to turn on the top
MOSFET. Low duty cycle applications may approach this
minimum on-time limit and care should be taken to ensure
that:
If the duty cycle falls below what can be accommodated by
the minimum on-time, the LTC1775 will begin to skip
cycles. The output voltage will continue to be regulated,
but the ripple current and ripple voltage will increase.
The minimum on-time for the LTC1775 is generally about
0.5 s. However, as the peak sense voltage (I
R
increases up to about 0.7 s. This is of particular concern
in forced continuous applications with low ripple current
at light loads. If the duty cycle drops below the minimum
on-time limit in this situation, a significant amount of
SEC
DS(ON)
V
V
t
SEC
ON MIN
SEC
SEC MIN
will droop. An external resistor divider from V
(
(
) decreases, the minimum on-time gradually
drops below this level, the FCB voltage forces
(N + 1)V
)
)
(
V
. 1 19
V
IN
OUT
)( )
OUT
ON(MIN)
f
V
1
R
R
is the smallest amount of time
4
3
SEC
SEC
is again above its
is normally set as
SEC(MIN)
:
L(PEAK) •
SEC
to