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参数资料
| 型号： | LTC3805MPMSE-5#PBF |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING REGULATOR, 700 kHz SWITCHING FREQ-MAX, PDSO10 |
| 封装： | LEAD FREE, PLASTIC, MSOP-10 |
| 文件页数： | 3/20页 |
| 文件大小： | 254K |
| 代理商： | LTC3805MPMSE-5#PBF |
LTC3805-5
11
38055fe
APPLICATIONS INFORMATION
Many LTC3805-5 application circuits can be derived from
the topologies shown on the first page or in the Typical
Applications section of this data sheet.
The LTC3805-5 itself imposes no limits on allowed input
voltageVINoroutputvoltageVOUT.Thesearealldetermined
bytheratingsoftheexternalpowercomponents.InFigure8,
thefactorsare:Q1maximumdrain-sourcevoltage(BVDSS),
on-resistance (RDS(ON)) and maximum drain current, T1
saturation flux level and winding insulation breakdown
voltages,CINandCOUTmaximumworkingvoltage,equiva-
lent series resistance (ESR), and maximum ripple current
ratings, and D1 and RSENSE power ratings.
VCC Bias Power
The VCC pin must be bypassed to the GND pin with a
minimum 1F ceramic or tantalum capacitor located im-
mediatelyadjacenttothetwopins.Propersupplybypassing
is necessary to supply the high transient currents required
by the MOSFET gate driver.
For maximum flexibility, the LTC3805-5 is designed so
that it can be operated from voltages well beyond the
LTC3805-5’s absolute maximum ratings. Figure 2 shows
the simplest case, in which the LTC3805-5 is powered
with a resistor RVCC connected between the input voltage
and VCC. The built-in shunt regulator limits the voltage on
the VCC pin to around 9.5V as long as the internal shunt
regulator is not forced to sink more than 25mA. This pow-
ering scheme has the drawback that the power loss in the
resistor reduces converter efficiency and the 25mA shunt
regulator maximum may limit the maximum-to-minimum
range of input voltage.
The typical application circuit in Figure 9 shows a different
flyback converter bias power strategy for a case in which
neither the input or output voltage is suitable for providing
bias power to the LTC3805-5. A small NPN preregulator
transistor and a Zener diode are used to accelerate the
rise of VCC and reduce the value of the VCC bias capacitor.
The flyback transformer has an additional bias winding to
provide bias power. Note that this topology is very power-
ful because, by appropriate choice of transformer turns
ratio, the output voltage can be chosen without regard to
the value of the input voltage or the VCC bias power for
the LTC3805-5. The number of turns in the bias winding
is chosen according to
NBIAS =NSEC
VCC + VD2
VOUT + VD1
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 LTC3805-5, VOUT
is the converter output voltage, VD1 is the forward voltage
drop of D1 and VD2 is the forward voltage drop of D2.
Note that since VOUT is regulated by the converter control
loop, VCC is also regulated although not as precisely. If an
“off-the-shelf” transformer with excessive bias windings
is used, the resistor, RBIAS in Figure 9, can be added to
limit the current.
Transformer Design Considerations
Transformer specification and design is perhaps the most
critical part of applying the LTC3805-5 successfully. In
addition to the usual list of caveats dealing with high fre-
quency power transformer design, the following should
prove useful.
Turns Ratios
Due to the use of the external feedback resistor divider
ratio to set output voltage, the user has relative freedom
in selecting transformer turns ratio to suit a given ap-
plication. Simple ratios of small integers, e.g., 1:1, 2:1,
3:2, etc. can be employed which yield more freedom in
setting total turns and transformer inductance. Simple
integer turns ratios also facilitate the use of “off-the-shelf”
configurable transformers. Turns ratio can be chosen on
Figure 2. Powering the LTC3805-5 via the
Internal Shunt Regulator
LTC3805-5
VCC
RVCC
CVCC
38055 F02
VIN
GND
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