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PDF LT3844 Data sheet ( Hoja de datos )
| Número de pieza | LT3844 | |
| Descripción | Current Mode Switching Regulator Controller | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LT3844www.DataSheet4U.com
FEATURES
High Voltage, Current Mode
Switching Regulator Controller with
Programmable Operating Frequency
DESCRIPTIO
■ High Voltage Operation: Up to 60V
■ Output Voltages up to 36V (Step-Down)
■ Programmable Constant Frequency: 100kHz to
500kHz
■ Synchronizable up to 600kHz
■ Burst Mode® Operation: 120μA Supply Current
■ 10μA Shutdown Supply Current
■ ±1.3% Reference Accuracy
■ Drives N-Channel MOSFET
■ Programmable Soft-Start
■ Programmable Undervoltage Lockout
■ Internal High Voltage Regulator for Gate Drive
■ Thermal Shutdown
■ Current Limit Unaffected by Duty Cycle
■ 16-Pin Thermally Enhanced TSSOP Package
U
APPLICATIO S
■ Industrial Power Distribution
■ 12V and 42V Automotive and Heavy Equipment
■ High Voltage Single Board Systems
■ Distributed Power Systems
■ Avionics
■ Telecom Power
The LT®3844 is a DC/DC controller used for medium
power, low part count, high efficiency supplies. It offers
a wide 4V-60V input range (7.5V minimum startup volt-
age) and can implement step-down, step-up, inverting
and SEPIC topologies.
The LT3844 includes Burst Mode operation, which re-
duces quiescent current below 120μA and maintains high
efficiency at light loads. An internal high voltage bias
regulator allows for simple biasing.
Additional features include current mode control for fast
line and load transient response; programmable fixed
operating frequency that can be synchronized to an exter-
nal clock for noise sensitive applications; a gate driver
capable of driving large N-channel MOSFETs; a precision
undervoltage lockout function; 10μA shutdown current;
short-circuit protection and a programmable soft-start
function.
The LT3844 is available in a 16-lead thermally enhanced
TSSOP package.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
Burst Mode is a registered trademark of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
Protected by U.S. Patents including 5731694, 6498466, 6611131.
TYPICAL APPLICATIO
High Voltage Step-Down Regulator 48V to 12V, 50W
VIN
36V
TO 68μF
60V
130k 14.7k
22μF 1 MEG
VIN
BOOST
SHDN
1000pF
82.5k
CSS
TG
10Ω
SW
10k
680p
BURST_EN
LT3844
VFB
VC
120pF
SYNC
VCC
1μF
PGND
SENSE+
SENSE–
fSET SGND
R7
49.9k
0.22μF
Si7850DP
10μH 0.02Ω
PDS5100H
68μF
VOUT
12V
50W
33μF
3844 TA01
Efficiency and Power Loss
vs Load Current
90 7
88 6
86
EFFICIENCY
84
5
4
82 3
LOSS
80 2
78
76
0.1
1
1
LOAD CURRENT (A)
VIN = 48V 0
10
3844 TA01b
3844fa
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LT3844www.DataSheet4U.com
TYPICAL PERFOR A CE CHARACTERISTICS
I(SENSE+ + SENSE–) vs
VSENSE (CM)
400
300
TA = 25°C
200
100
0
–100
–200
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
VSENSE (CM) (V)
3844 G10
Maximum Current Sense
Threshold vs Temperature
106
104
102
100
98
96
94
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3844 G16
Operating Frequency
vs Temperature
308
306
304
302
300
298
296
294
292
290
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3844 G17
VIN UVLO Threshold (Rising)
vs Temperature
4.54
4.52
4.50
4.48
4.46
4.44
4.42
4.40
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3844 G14
Error Amp Reference
vs Temperature
1.234
1.233
1.232
1.231
1.230
1.229
1.228
1.227
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3844 G12
VIN UVLO Threshold (Falling)
vs Temperature
3.86
3.84
3.82
3.80
3.78
3.76
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3844 G15
PI FU CTIO S
VIN (Pin 1): The VIN pin is the main supply pin and should
be decoupled to SGND with a low ESR capacitor located
close to the pin.
SHDN (Pin 2): The SHDN pin has a precision IC enable
threshold of 1.35V (rising) with 120mV of hysteresis. It is
used to implement an undervoltage lockout (UVLO) cir-
cuit. See Application Information section for implement-
ing a UVLO function. When the SHDN pin is pulled below
a transistor VBE (0.7V), a low current shutdown mode is
entered, all internal circuitry is disabled and the VIN supply
current is reduced to approximately 9μA. Typical pin input
bias current is <10μA and the pin is internally clamped to
6V.
CSS (Pin 3): The soft-start pin is used to program the
supply soft-start function. Use the following formula to
calculate CSS for a given output voltage slew rate:
CSS = 2μA(tSS/1.231V)
The pin should be left unconnected when not using the
soft-start function.
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APPLICATIO S I FOR ATIO
Therefore, the peak inductor current is 100mV/RSENSE.
The maximum output load current, IOUT(MAX), is the peak
inductor current minus half the peak-to-peak ripple cur-
rent, ΔIL.
Allowing adequate margin for ripple current and external
component tolerances, RSENSE can be calculated as
follows:
RSENSE
=
70mV
IOUT(MAX)
sensitive communications systems, it is often desirable to
keep the switching noise out of a sensitive frequency band.
The LT3844 uses a constant frequency architecture that
can be programmed over a 100kHz to 500kHz range with
a single resistor from the fSET pin to ground, as shown in
Figure 1. The nominal voltage on the fSET pin is 1V and the
current that flows from this pin is used to charge an
internal oscillator capacitor. The value of RSET for a given
operating frequency can be chosen from Figure 4 or from
the following equation:
Typical values for RSENSE are in the range of 0.005Ω
to 0.05Ω.
Operating Frequency
The choice of operating frequency and inductor value is a
trade off between efficiency and component size. Low
frequency operation improves efficiency by reducing
MOSFET switching losses and gate charge losses. How-
ever, lower frequency operation requires more inductance
for a given amount of ripple current, resulting in a larger
inductor size and higher cost. If the ripple current is
allowed to increase, larger output capacitors may be
required to maintain the same output ripple. For convert-
ers with high step-down VIN to VOUT ratios, another
consideration is the minimum on-time of the LT3844 (see
the Minimum On-time Considerations section). A final
consideration for operating frequency is that in noise-
200
180
160
140
120
100
80
60
40
20
0
100 200 300 400 500 600
FREQUENCY (kHz)
3844 G19
RSET(kΩ) = 8.4 • 104 • fSW(–1.31)
The following table lists typical resistor values for com-
mon operating frequencies:
Recommended 1% Standard Values
RSET
191kΩ
118kΩ
80.6kΩ
63.4kΩ
49.9kΩ
40.2kΩ
33.2kΩ
27.4kΩ
23.2kΩ
fSW
100kHz
150kHz
200kHz
250kHz
300kHz
350kHz
400kHz
450kHz
500kHz
Step-Down Converter: Inductor Selection
The critical parameters for selection of an inductor are
minimum inductance value, volt-second product, satura-
tion current and/or RMS current.
For a given ΔI, The minimum inductance value is calcu-
lated as follows:
L
≥
VOUT
•
VIN(MAX) – VOUT
fSW • VIN(MAX) • ΔIL
fSW is the switch frequency.
Figure 1. Timing Resistor (RSET) Value
3844fa
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| Páginas | Total 24 Páginas | |
| PDF Descargar | [ Datasheet LT3844.PDF ] | |
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