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PDF LTC3805-5 Data sheet ( Hoja de datos )
| Número de pieza | LTC3805-5 | |
| Descripción | Adjustable Frequency Current Mode Flyback DC/DC Controller | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LTC3805-5
Adjustable Frequency
Current Mode Flyback/
Boost/SEPIC DC/DC Controller
www.datasheet4u.com
FEATURES
DESCRIPTION
■ VIN and VOUT Limited Only by External Components
■ 4.5V Undervoltage Lockout Threshold
■ Adjustable Slope Compensation
■ Adjustable Overcurrent Protection with Automatic
Restart
■ Adjustable Operating Frequency (70kHz to 700kHz)
with One External Resistor
■ Synchronizable to an External Clock
■ ±1.5% Reference Accuracy
■ Only 100mV Current Sense Voltage Drop
■ RUN Pin with Precision Threshold and Adjustable
Hysteresis
■ Programmable Soft-Start with One External Capacitor
■ Low Quiescent Current: 360μA
■ Small 10-Lead MSOP and 3mm × 3mm DFN
APPLICATIONS
■ Automotive Power Supplies
■ Telecom Power Supplies
■ Isolated Electronic Equipment
■ Auxiliary/Housekeeping Power Supplies
■ Power over Ethernet
The LTC®3805-5 is a current mode DC/DC controller de-
signed to drive an N-channel MOSFET in flyback, boost
and SEPIC converter applications. Operating frequency
and slope compensation can be programmed by external
resistors. Programmable overcurrent sensing protects
the converter from overload and short-circuit conditions.
Soft-start can be programmed using an external capacitor
and the soft-start capacitor also programs an automatic
restart feature.
The LTC3805-5 provides ±1.5% output voltage accuracy
and consumes only 360μA of quiescent current during
normal operation and only 40μA during micropower start-
up. Using a 9.5V internal shunt regulator, the LTC3805-5
can be powered from a high VIN through a resistor or it
can be powered directly from a low impedance DC voltage
from 4.7V to 8.8V.
The LTC3805-5 is available in the 10-lead MSOP package
and the 3mm × 3mm DFN package.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
TYPICAL APPLICATION
5V to 12V/1A Boost Converter
VIN
5V
20k
470pF
4.3μH
22μF
×2
0.1μF
VCC
RUN GATE
LTC3805-5
ITH OC
SSFLT
ISENSE
FS FB
118k SYNC GND
1.33k
3k
UPS840
VOUT
12V
100μF 1A
191k
8mΩ
13.7k
38055 TA01
Efficiency and Power Loss vs Load Current
100 2.5
95 5V EFFICIENCY
8.5V EFFICIENCY
90
8.5V LOSS
5V LOSS
85
2.0
1.5
1
80 0.5
75
0.01
0.1 1
LOAD CURRENT (A)
0
10
38055TA01b
38055fa
1
1 page  
TYPICAL PERFORMANCE CHARACTERISTICS
LTC3805-5
Start-Up ICC Supply Current
vs Temperature
www.datas5h0 eet4u.com
48
46
44
42
40
38
36
34
32
30
–50 –25
0 25 50 75 100 125
TEMPERATURE (°C)
38055 G09
ICC Supply Current
vs Temperature
350
340
330
320
310
300
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
38055 G10
VCC Shunt Regulator Voltage
vs Temperature
9.60
9.55
VCLAMP25mA
9.50
9.45
9.40
9.35
9.30 VCLAMP1mA
9.25
9.20
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
38055 G11
Peak Current Sense Voltage
vs Temperature
100.8
100.6
100.4
100.2
100.0
99.8
99.6
99.4
99.2
99.0
–50 –25
0 25 50 75 100 125
TEMPERATURE (°C)
38055 G12
Overcurrent Threshold
vs Temperature
104
102
100
98
96
94
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
38055 G13
Internal Soft-Start Time
vs Temperature
1.75
1.70
1.65
1.60
1.55
1.50
1.45
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
38055 G14
External Soft-Start Current
vs Temperature
5.3
5.2
5.1
5.0
4.9
4.8
4.7
4.6
4.5
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
38055 G15
External Timeout Current
vs Temperature
2.1
2.0
1.9
1.8
1.7
1.6
1.5
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
38055 G16
38055fa
5
5 Page 
LTC3805-5
APPLICATIONS INFORMATION
Many LTC3805-5 application circuits can be derived from
the topologies shown on the first page or in the Typical
wwwA.dpatpalsihceaetti4oun.csomsection of this data sheet.
The LTC3805-5 itself imposes no limits on allowed input
voltage VIN or output voltage VOUT. These are all determined
by the ratings of the external power components. In Figure 8,
the factors are: Q1 maximum drain-source voltage (BVDSS),
on-resistance (RDS(ON)) and maximum drain current, T1
saturation flux level and winding insulation breakdown
voltages, CIN and COUT maximum working voltage, 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 1μF ceramic or tantalum capacitor located im-
mediately adjacent to the two pins. Proper supply bypassing
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 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-
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.
VIN
RVCC
CVCC
LTC3805-5
VCC
GND
38055 F02
Figure 2. Powering the LTC3805-5 via the
Internal Shunt Regulator
38055fa
11
11 Page | ||
| Páginas | Total 20 Páginas | |
| PDF Descargar | [ Datasheet LTC3805-5.PDF ] | |
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