ltc3873 Linear Technology Corporation, ltc3873 Datasheet - Page 8

• Manufacturer Part Number: ltc3873
• Description: No Rsense Constant Frequency Current Mode Boost/flyback/sepic Dc/dc Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LTC3873
V
The V
minimum 10μF 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
regulator limits the voltage on the V
V 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
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
D1 powers V
normal operation. Note that R
very small 55μA micropower start-up current while C
is charged to V
the converter begins switching the external MOSFET and
ramps up the converter output voltage at a rate set by the
capacitor C
supply enough current to operate the external MOSFET,
C
be fast enough so that the output voltage reaches its target
value of 5V before V
will fail to start. Otherwise more C
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
8
VCC
CC
Bias Power
begins discharging and V
CC
START
pin must be bypassed to the GND pin with a
RUN/SS
CC
and C
TURNON
from the output once the converter is in
on the RUN/SS pin. Since R
VCC
CC
drops to V
form a start-up trickle charger while
. At this point, V
CC
for the LTC3873. In this case,
CC
START
drops. The soft-start must
TURNOFF
VCC
CC
need only supply the
CC
. The built-in shunt
capacitor is needed
pin to around 9.3
RUN/SS
or the converter
START
> V
cannot
SHDN
VCC
,
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
turns in the bias winding is chosen according to:
where N
N
V
is the converter output voltage, V
voltage of D1 and V
Note that since V
loop, V
value of V
often a limited range of small interger numbers. For proper
operation, the value of V
V
two to one, the requirement is relatively 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
CC
TURNOFF
VCC
SEC
N
BIAS
is the desired voltage to power the LTC3873, V
R3
down to V
is the number of turns in the secondary winding,
D2
C
C
VCC
VIN
CC
CC
BIAS
Figure 5. Typical LTC3873 Application Circuit
. Since the ratio of V
=
CC
bias power for the LTC3873. The number of the
is also regulated although not precisely. The
C
N
C
is often constrained since N
SEC
is the number of turns in the bias winding,
R
I
GND
RUN/SS
TH
START
TURNOFF
R1
OUT
V
LTC3873
V
OUT
CC
V
V
V
D2
CC
FB
IN
is regulated by the converter control
NGATE
+
R2
+
is the forward drop voltage of D2.
SW
.
CC
V
V
D
C
D
2
must be between V
IN
1
N
R
BIAS
N
SL
PRI
TURNON
•
•
T1
Q1
R
SENSE
D1
•
N
SEC
is the forward drop
to V
D1
BIAS
TURNOFF
C
and N
OUT
TURNON
3873 F05
SEC
is over
V
OUT
and
OUT
3873f
are