Datasheet

LTC3873
9
3873fa
V
CC
Bias Power
The V
CC
pin must be bypassed to the GND pin with a
minimum 10F ceramic or tantalum capacitor located
immediately adjacent to the two pins. Proper supply by-
passing is necessary to supply the high transient currents
required by the MOSFET gate driver.
For maximum fl exibility, the LTC3873 is designed so
that it can be operated from voltages well beyond the
LTC3873’s absolute maximum ratings. In the simplest case,
the LTC3873 can be powered with a resistor connected
between the input voltage and V
CC
. The built-in shunt
regulator limits the voltage on the V
CC
pin to around 9.3V
as long as the shunt regulator is not forced to sink more
than 25mA. This powering scheme has the drawback that
the power loss in the resistor reduces converter effi ciency
and the 25mA shunt regulator maximum may limit the
maximum-minimum range of input voltage.
In some cases, the input or the output voltage is within
the operational range of V
CC
for the LTC3873. In this case,
the LTC3873 is operated directly from either the input or
output voltage. The typical application circuit on the fi rst
page of this data sheet shows a 5V output converter in
which R
START
and C
VCC
form a start-up trickle charger while
D1 powers V
CC
from the output once the converter is in
normal operation. Note that R
START
need only supply the
very small 55µA micropower start-up current while C
VCC
is charged to V
TURNON
. At this point, V
RUN/SS
> V
SHDN
,
the converter begins switching the external MOSFET and
ramps up the converter output voltage at a rate set by the
capacitor C
RUN/SS
on the RUN/SS pin. Since R
START
cannot
supply enough current to operate the external MOSFET, C
VCC
begins discharging and V
CC
drops. The soft-start must be
fast enough so that the output voltage reaches its target
value of 5V before V
CC
drops to V
TURNOFF
or the converter
will fail to start. Otherwise more C
VCC
capacitor is needed
to hold the input voltage when soft-start is too long.
Figure 5 shows a different fl yback converter bias power
strategy for a case in which neither the input or the output
is suitable for providing the bias power to the LTC3873.
The trickle charger is identical to that described in the
prior paragraph. However, the fl yback transformer has an
additional bias winding to provide bias power. Note that this
topology is very powerful because, by appropriate choice
of the transformer turn ratio, the output voltage can be
chosen without regard to the value of the input voltage or
the V
CC
bias power for the LTC3873. The number of the
turns in the bias winding is chosen according to:
NN
VV
VV
BIAS SEC
CC D
OUT D
=
+
+
2
1
where N
BIAS
is the number of turns in the bias winding,
N
SEC
is the number of turns in the secondary winding,
V
CC
is the desired voltage to power the LTC3873, V
OUT
is the converter output voltage, V
D1
is the forward drop
voltage of D1 and V
D2
is the forward drop voltage of D2.
Note that since V
OUT
is regulated by the converter control
loop, V
CC
is also regulated although not precisely. The
value of V
CC
is often constrained since N
BIAS
and N
SEC
are
often a limited range of small integer numbers. For proper
operation, the value of V
CC
must be between V
TURNON
and
V
TURNOFF
. Since the ratio of V
TURNON
to V
TURNOFF
is over
two to one, the requirement is relative easy to satisfy.
Finally, as with all trickle charger start-up schemes, the
soft-start must be fast enough so that the power supplied
by the bias winding is available before the discharge of
C
VCC
down to V
TURNOFF
.
APPLICATIONS INFORMATION
V
CC
RUN/SS NGATE
LTC3873
I
TH
GND
C
C
C
OUT
N
SEC
N
PRI
N
BIAS
V
OUT
C
VIN
C
IN
C
VCC
R3
R
START
D2
T1
D1
V
IN
R
SENSE
3873 F05
Q1
•
•
SW
V
FB
R1 R2
R
SL
•
Figure 5. Typical LTC3873 Application Circuit