LTC3873 Datasheet, PDF (9/16 Pages) Linear Technology – No RSENSETM Constant Frequency Current Mode Boost/Flyback/SEPIC DC/DC Controller

LTC3873
APPLICATIONS INFORMATION
V_CC Bias Power
The V_CC pin must be bypassed to the GND pin with a
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 flexibility, 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 efficiency
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 first
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 flyback 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 flyback 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:
N_BIAS
=
N_SEC
V_CC + V_D2
V_OUT + V_D1
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.
T1
N_BIAS
D2
V_IN
•
D1
R3
C_VCC
C_VIN
C_C
R_START
C_{IN •}
N_PRI
N_SEC
•
V_CC
RUN/SS NGATE
LTC3873
I_TH
R_SL
SW
GND
V_FB
R1
R2
Q1
R_SENSE
C_OUT
V_OUT
3873 F05
Figure 5. Typical LTC3873 Application Circuit
3873fa
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