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參數(shù)資料
| 型號(hào)： | LTC3873EDDB#TRM |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING CONTROLLER, 240 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封裝： | 3 X 2 MM, 0.75 MM HEIGHT, PLASTIC, MO-229WECD-1, DFN-8 |
| 文件頁數(shù)： | 16/16頁 |
| 文件大?。?/td> | 185K |
| 代理商： | LTC3873EDDB#TRM |
LTC3873
9
3873fa
VCC Bias Power
The VCC 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 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 VCC. The built-in shunt
regulator limits the voltage on the VCC 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 efciency
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 VCC for the LTC3873. In this case,
the LTC3873 is operated directly from either the input or
output voltage. The typical application circuit on the rst
page of this data sheet shows a 5V output converter in
which RSTARTandCVCCformastart-uptricklechargerwhile
D1 powers VCC from the output once the converter is in
normal operation. Note that RSTART need only supply the
very small 55μA micropower start-up current while CVCC
is charged to VTURNON. At this point, VRUN/SS > VSHDN,
the converter begins switching the external MOSFET and
ramps up the converter output voltage at a rate set by the
capacitor CRUN/SS on the RUN/SS pin. Since RSTART cannot
supply enough current to operate the external MOSFET, CVCC
begins discharging and VCC drops. The soft-start must be
fast enough so that the output voltage reaches its target
value of 5V before VCC drops to VTURNOFF or the converter
will fail to start. Otherwise more CVCC capacitor is needed
to hold the input voltage when soft-start is too long.
Figure 5 shows a different 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 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 VCC bias power for the LTC3873. The number of the
turns in the bias winding is chosen according to:
NN
VV
BIAS
SEC
CC
D
OUT
D
=
+
2
1
where NBIAS is the number of turns in the bias winding,
NSEC is the number of turns in the secondary winding,
VCC is the desired voltage to power the LTC3873, VOUT
is the converter output voltage, VD1 is the forward drop
voltage of D1 and VD2 is the forward drop voltage of D2.
Note that since VOUT is regulated by the converter control
loop, VCC is also regulated although not precisely. The
value of VCC is often constrained since NBIAS and NSEC are
often a limited range of small integer numbers. For proper
operation, the value of VCC must be between VTURNON and
VTURNOFF. Since the ratio of VTURNON to VTURNOFF 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
CVCC down to VTURNOFF.
APPLICATIONS INFORMATION
VCC
RUN/SS NGATE
LTC3873
ITH
GND
CC
COUT
NSEC
NPRI
NBIAS
VOUT
CVIN
CIN
CVCC
R3
RSTART
D2
T1
D1
VIN
RSENSE
3873 F05
Q1
SW
VFB
R1
R2
RSL
Figure 5. Typical LTC3873 Application Circuit
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