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PDF LTC3805 Data sheet ( Hoja de datos )
| Número de pieza | LTC3805 | |
| Descripción | Adjustable Frequency Current Mode Flyback DC/DC Controller | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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FEATURESwww.datasheet4u.com
■ VIN and VOUT Limited Only by External Components
■ 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
■ Current Mode Operation for Excellent Line and Load
Transient Response
■ 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
■ Telecom Power Supplies
■ 42V and 12V Automotive Power Supplies
■ Isolated Electronic Equipment
LTC3805
Adjustable Frequency
Current Mode Flyback
DC/DC Controller
DESCRIPTION
The LTC®3805 is a current mode controller for flyback
DC/DC converters designed to drive an N-channel MOSFET
in high input and output voltage converter applications.
Operating frequency and slope compensation can be pro-
grammed by external resistors. Programmable overcurrent
sensing protects the converter from short-circuits. Soft-
start can be programmed using an external capacitor
and the soft-start capacitor also programs an automatic
restart feature.
The LTC3805 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 can
be powered from a high VIN through a resistor or it can
be powered directly from a low impedance DC voltage of
9V or less.
The LTC3805 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
36V – 72V to 5V/2A Nonisolated Flyback Converter
VIN
36V TO
72V
22μF
×2
221k
100k
22μF
8.66k
20k
470pF
0.1μF
118k
VCC
RUN GATE
LTC3805
ITH OC
SSFLT
ISENSE
FS FB
1.33k
3k
SYNC GND
3805 TA01a
68mΩ
VOUT
5V
AT 2A
100μF
×2
71.5k
13.7k
Efficiency and Power Loss
vs Load Current; VO = 5V
100
VIN = 48V
90
10
80
70 EFFICIENCY
60
50 1
LOSS
40
30
20
10
0
0.01
0.1 1
LOAD CURRENT (A)
0.1
10
3805 TA01b
3805fb
1
1 page  
TYPICAL PERFORMANCE CHARACTERISTICS
LTC3805
Start-Up ICC Supply Current
vs Temperature
www.datas50heet4u.com
48
46
44
42
40
38
36
34
32
30
–50 –25
0 25 50 75 100 125
TEMPERATURE (°C)
3805 G09
ICC Supply Current
vs Temperature
350
340
330
320
310
300
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3805 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
3805 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)
3805 G12
Overcurrent Threshold
vs Temperature
104
102
100
98
96
94
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3805 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
3805 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
3805 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
3805 G16
3805fb
5
5 Page 
LTC3805
APPLICATIONS INFORMATION
Many LTC3805 application circuits can be derived from
the topology shown on the first page of this data sheet
wwwa.dnadtasfhroeemt4uth.ceomtopology shown in Figure 2.
The LTC3805 itself imposes no limits on allowed input volt-
age VIN or output voltage VOUT. These are all determined
by the ratings of the external power components. 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 is designed so that it
can be operated from voltages well beyond the LTC3805’s
absolute maximum ratings. Figure 2 shows the simplest
case, in which the LTC3805 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 re-
duces converter efficiency and the 25mA shunt regulator
maximum may limit the maximum-to-minimum range of
input voltage.
In some cases, the input or output voltage is within the
operational range of VCC for the LTC3805. In this case,
the LTC3805 is operated directly from either the input or
output voltage. Figure 3 shows a 5V output converter in
which RSTART and CVCC form a start-up trickle charger
while D2 powers VCC from the output once the converter
is in normal operation. Note that RSTART need only supply
the very small 40μA micropower start-up current while
VIN
RVCC
LTC3805
CVCC
VCC
GND
3805 F02
Figure 2. Powering the LTC3805 via the
Internal Shunt Regulator
VIN
36V TO 72V
CIN
2.2μF
×2
R1
221k
R2
8.66k
RITH
20k
CITH
470pF
CSS
0.1μF
RFS
118k
RSTART
100k
CVCC
22μF
D2 PRI
SEC
VCC GATE
RUN
ITH LTC3805
SSFLT
SYNC
OC
FS
FB
ISENSE
GND
ROC
1.33k
RSLOPE
3k
Q1
RSENSE
68mΩ
3805 F02
R3
71.5k
R4
13.7k
VOUT
5V AT 2A
COUT
100μF
×2
Figure 3. Powering the LTC3805 From the Output
3805fb
11
11 Page | ||
| Páginas | Total 20 Páginas | |
| PDF Descargar | [ Datasheet LTC3805.PDF ] | |
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