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PDF LTC3806 Data sheet ( Hoja de datos )
| Número de pieza | LTC3806 | |
| Descripción | Synchronous Flyback DC/DC Controller | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LTC3806
Synchronous
Flyback DC/DC Controller
FEATURES
s High Efficiency at Full Load
s Better Cross Regulation Than Nonsynchronous
Converters (Multiple Outputs)
s Soft-Start Minimizes Inrush Current
s Current Mode Control Provides Excellent
Transient Response
s High Maximum Duty Cycle: 89% Typical
s ±2% Programmable Undervoltage Lockout Threshold
s ±1% Internal Voltage Reference
s Micropower Start-Up
s Constant Frequency Operation (Never Audible)
s 3mm × 4mm 12-Pin DFN Package
U
APPLICATIO S
s 48V Telecom Supplies
s 12V/42V Automotive
s 24V Industrial
s VoIP Phone
s Power Over Ethernet
DESCRIPTIO
The LTC®3806 is a current mode synchronous flyback
controller that drives N-channel power MOSFETs and
requires very few external components. It is intended for
medium power applications where multiple outputs are
required. Synchronous rectification provides higher effi-
ciency and improved output cross regulation than
nonsynchronous converters.
The IC contains all the necessary control circuitry includ-
ing a 250kHz oscillator, precision undervoltage lockout
circuit with hysteresis, gate drivers for primary and syn-
chronous switches, current mode control circuitry and
soft-start circuitry.
Programmable soft-start reduces inrush currents. This
makes it easier to design compliant Power Over Ethernet
supplies.
Low start-up current reduces power dissipation in the
start-up resistor and reduces the size of the external start-
up capacitor.
The LTC3806 is available in a 12-pin, exposed pad DFN
package.
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATIO
VIN
36V TO 72V
R1
51k
R2
604k
R8
100Ω
C1
100µF
R3
26.7k
C2
1nF
R4
3.4k
C7
4.7µF
LTC3806
RUN SENSE
ITH SS
FB G2
VIN G1
INTVCC GND
C3
4.7µF
D1 T1
•
•
M1 M2
C4 R5
0.47µF 0.056Ω
C5
470µF
R7
12.4k
R6
21k
Figure 1. Multiple Output Flyback Converter for Telecom
VOUT2
2.5V
3A
VOUT1
3.3V
3A
•
M3
C6
470µF
3806 F01
3806f
1
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
LTC3806
SENSE Pin Current
vs Temperature
32.0
31.5
INTVCC Load Regulation
7.020
TA = 25°C
7.015
7.010
INTVCC Line Regulation
7.020
7.015
7.010
31.0
7.005
7.005
7.000
7.000
30.5
6.995
6.995
30.0
–40
–15 10 35 60
TEMPERATURE (°C)
85
3806 G13
6.990
0
10
INTVCC Dropout Voltage
vs Current, Temperature
2.8
2.7 TA = –40°C
2.6
2.5 TA = 0°C
2.4 TA = 25°C
2.3
TA = 55°C
2.2
2.1 TA = 85°C
2.0
1.9
0
10 20 30
INTVCC LOAD (mA)
40 50
3806 G16
20 30 40
INTVCC LOAD (mA)
50
3806 G14
6.990
10
12
14 16
VIN (V)
Efficiency vs Output Power
90
FIGURE 8 CIRCUIT
85
80
75
10 20 30 40 50 60 70 80 90 100
% OF MAXIMUM OUTPUT POWER
3806 G17
18 20
3806 G15
3806f
5
5 Page 
LTC3806
APPLICATIO S I FOR ATIO
The rising threshold voltage on the RUN pin is equal to the
internal reference voltage of 1.230V. The comparator has
91mV of hysteresis to increase noise immunity.
The turn-on and turn-off input voltage thresholds are
programmed using a resistor divider according to the
following formulas:
VIN(OFF) = 1.139V • 1+ RR21
VIN(ON) = 1.230V • 1+ RR21
The resistor R1 is typically chosen to be less than 1M. For
applications where the RUN pin is only to be used as a
logic input, the user should be aware of the 7V Absolute
Maximum Rating for this pin! The RUN pin can be
connected to the input voltage through an external 1M
resistor, as shown in Figure 3c, for “always on” operation.
Application Circuits
A basic LTC3806 application circuit is shown in Figure 1.
External component selection is driven by the character-
istics of the load and the input supply.
Duty Cycle Considerations
Current and voltage stress on the power switch and
synchronous rectifiers, input and output capacitor RMS
currents and transformer utilization (size vs power) are
impacted by duty factor. Unfortunately duty factor cannot
be adjusted to simultaneously optimize all of these re-
quirements. In general, avoid extreme duty factors since
this severely impacts the current stress on most of the
components. A reasonable target for duty factor is 50% at
nominal input voltage. Using this rule of thumb, calculate
the ideal transformer turns ratio:
NIDEAL
=
VOUT1
VIN
•
1–DD
For a 50% duty factor, this reduces to:
NIDEAL
=
VOUT1
VIN
If NIDEAL is integer, use this for your turns ratio. If not, find
a ratio of small integers that comes close to NIDEAL. If these
conditions are met, bifilar winding techniques can be used
that will improve coupling coefficient. Cross regulation
will be better and primary-side snubbing may be reduced
or eliminated.
The selected turns ratio doesn’t have to be perfectly equal
to NIDEAL because a flyback converter’s output voltage is
not set through transformer action. Instead, the trans-
former stores energy when the primary-side switch turns
on and transfers this energy to the output(s) by flyback
action when the primary-side switch turns off.
Cross regulation may be improved by using a target duty
factor which is less than 50%. This improves cross
regulation because the secondary-side MOSFETs (syn-
chronous rectifiers) will be on a larger percentage of the
time (thereby increasing the average coupling between the
outputs). Duty factor is reduced by proportionately in-
creasing all turns ratios.
Reduced duty factor has the following effect on MOSFET
stresses:
LOCATION
Primary
Secondary
MOSFET
CURRENT STRESS
Increased
Reduced
MOSFET
VOLTAGE STRESS
Reduced
Increased
The duty factor with the selected turns ratio will equal:
( )D = VOUT1
VOUT1 + N • VIN
While the output(s)/input turns ratio are not critical, the
turns ratio between outputs are critical and affect the
accuracy of the slave output voltages.
3806f
11
11 Page | ||
| Páginas | Total 20 Páginas | |
| PDF Descargar | [ Datasheet LTC3806.PDF ] | |
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