LTC1627 Linear Technology, LTC1627 Datasheet - Page 10

• Manufacturer Part Number: LTC1627
• Description: Monolithic Synchronous Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIONS
LTC1627
Auxiliary Winding Control Using SYNC/FCB Pin
The SYNC/FCB pin can be used as a secondary feedback
input to provide a means of regulating a flyback winding
output. When this pin drops below its ground referenced
0.8V threshold, continuous mode operation is forced. In
continuous mode, the main and synchronous MOSFETs
are switched continuously regardless of the load on the
main output.
Synchronous switching removes the normal limitation
that power must be drawn from the inductor primary
winding in order to extract power from auxiliary windings.
With continuous synchronous operation power can be
drawn from the auxiliary windings without regard to the
primary output load.
The secondary output voltage is set by the turns ratio of the
transformer in conjunction with a pair of external resistors
returned to the SYNC/FCB pin as shown in Figure 6. The
secondary regulated voltage V
where N is the turns ratio of the transformer, V
main output voltage sensed by V
voltage drop across the Schottky diode.
10
Efficiency Considerations
The 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. Efficiency can be expressed as:
V
SEC
Figure 6. Secondary Output Loop Connection
LTC1627
N
SYNC/FCB
1
SW
V
U
OUT
R3
R4
INFORMATION
U
V
DIODE
SEC
1:N
L1
in Figure 6 is given by:
FB
W
0 8
and V
V
SEC
V
+
+
1
1 F
C
DIODE
OUT
R
R
V
1627 F06
U
OUT
OUT
4
3
is the
is the
where L1, L2, etc. are the individual losses as a percentage
of input power.
Although all dissipative elements in the circuit produce
losses, two main sources usually account for most of the
losses in LTC1627 circuits: V
losses.
1. The V
2. I
Other losses including C
MOSFET switching losses and inductor core losses generally
account for less than 2% total additional loss.
Efficiency = 100% – (L1 + L2 + L3 + ...)
the DC bias current as given in the electrical character-
istics and the internal main switch and synchronous
switch gate charge currents. The gate charge current
results from switching the gate capacitance of the
internal power MOSFET switches. Each time the gate is
switched from high to low to high again, a packet of
charge dQ moves from V
dQ/dt is the current out of V
than the DC bias current. In continuous mode, I
= f(Q
the internal top and bottom switches. Both the DC bias
and gate charge losses are proportional to V
their effects will be more pronounced at higher supply
voltages.
internal switches R
continuous mode the average output current flowing
through inductor L is “chopped” between the main
switch and the synchronous switch. Thus, the series
resistance looking into SW pin from L is a function of
both top and bottom MOSFET R
cycle (DC) as follows:
The R
be obtained from the Typical Performance Characteris-
tics curves. Thus, to obtain I
to R
current.
2
R losses are calculated from the resistances of the
R
L
SW
T
DS(ON)
and multiply by the square of the average output
IN
+ Q
= (R
quiescent current is due to two components:
B
) where Q
DS(ON)TOP
for both the top and bottom MOSFETs can
SW
IN
T
)(DC) + (R
and C
and Q
and external inductor R
IN
IN
2
OUT
quiescent current and I
to ground. The resulting
B
R losses, simply add R
IN
are the gate charges of
that is typically larger
ESR dissipative losses,
DS(ON)BOT
DS(ON)
and the duty
)(1 – DC)
IN
and thus
GATECHG
L
. In
SW
2
R