LTC1735 Linear Technology, LTC1735 Datasheet - Page 19

• Manufacturer Part Number: LTC1735
• Description: High Efficiency Synchronous Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
FCB Pin Operation
When the FCB pin drops below its 0.8V threshold, continu-
ous mode operation is forced. In this case, the top and
bottom MOSFETs continue to be driven synchronously
regardless of the load on the main output. Burst Mode
operation is disabled and current reversal is allowed in the
inductor.
In addition to providing a logic input to force continuous
synchronous operation and external synchronization, the
FCB pin provides a means to regulate a flyback winding
output. During continuous mode, current flows continu-
ously in the transformer primary. The secondary winding(s)
draw current only when the bottom, synchronous switch
is on. When primary load currents are low and/or the V
V
for a sufficient amount of time to transfer power from the
output capacitor to the secondary load. Forced continuous
operation will support secondary windings providing there
is sufficient synchronous switch duty factor. Thus, the
FCB input pin removes the requirement that power must
be drawn from the inductor primary in order to extract
power from the auxiliary windings. With the loop in
continuous mode, the auxiliary outputs may nominally be
loaded without regard to the primary output load.
The secondary output voltage V
shown in Figure 3a by the turns ratio N of the transformer:
However, if the controller goes into Burst Mode operation
and halts switching due to a light primary load current,
then V
V
If V
continuous switching operation until V
its minimum.
In order to prevent erratic operation if no external connec-
tions are made to the FCB pin, the FCB pin has a 0.17 A
OUT
SEC
V
SEC
SEC MIN
to the FCB pin sets a minimum voltage V
ratio is low, the synchronous switch may not be on
SEC
(
V
drops below this level, the FCB voltage forces
SEC
will droop. An external resistive divider from
)
(N + 1)V
. 0 8
V
U
1
OUT
R
R
U
4
3
SEC
W
is normally set as
SEC
is again above
SEC(MIN)
U
IN
:
/
internal current source pulling the pin high. Remember to
include this current when choosing resistor values R3 and
R4.
The internal LTC1735 oscillator can be synchronized to an
external oscillator by applying and clocking the FCB pin
with a signal above 1.5V
external frequency, Burst Mode operation is disabled but
cycle skipping is allowed at low load currents since current
reversal is inhibited. The bottom gate will come on every
10 clock cycles to assure the bootstrap cap is kept re-
freshed. The rising edge of an external clock applied to the
FCB pin starts a new cycle.
The range of synchronization is from 0.9f
f
frequency than 1.3f
pensation and cause loop instability with high duty cycles
(duty cycle > 50%). If loop instability is observed while
synchronized, additional slope compensation can be ob-
tained by simply decreasing C
The following table summarizes the possible states avail-
able on the FCB pin:
Table 1
FCB Pin
DC Voltage: 0V to 0.7V
DC Voltage: 0.9V
Feedback Resistors
Ext Clock: (0V to V
Efficiency Considerations
The percent efficiency of a switching regulator is equal to
the output power divided by the input power times 100%.
It is often useful to analyze individual losses to determine
what is limiting the efficiency and which change would
produce the most improvement. Percent efficiency can be
expressed as:
where L1, L2, etc. are the individual losses as a percentage
of input power.
O
%Efficiency = 100% – (L1 + L2 + L3 + …)
set by C
(V
FCBSYNC
OSC
FCBSYNC
. Attempting to synchronize to a higher
> 1.5V)
O
)
can result in inadequate slope com-
P–P
Condition
Burst Disabled/Forced Continuous
Current Reversal Enabled
Burst Mode Operation,
No Current Reversal
Regulating a Secondary Winding
Burst Mode Operation Disabled
No Current Reversal
. When synchronized to an
OSC
.
LTC1735
O
to 1.3f
19
O
, with