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PDF LT3845 Data sheet ( Hoja de datos )
| Número de pieza | LT3845 | |
| Descripción | High Voltage Synchronous Current Mode Step-Down Controller | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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FEATURES
■ High Voltage Operation: Up to 60V
■ Synchronizable Up to 600kHz
■ Adjustable Constant Frequency: 100kHz to 500kHz
■ Output Voltages Up to 36V
■ Adaptive Nonoverlap Circuitry Prevents Switch
Shoot-Through
■ Reverse Inductor Current Inhibit for Discontinuous
Operation Improves Efficiency with Light Loads
■ Programmable Soft-Start
■ 120μA No Load Quiescent Current
■ 10μA Shutdown Supply Current
■ 1% Regulation Accuracy
■ Standard Gate N-Channel Power MOSFETs
■ Current Limit Unaffected by Duty Cycle
■ Reverse Overcurrent Protection
■ 16-Lead Thermally Enhanced TSSOP Package,
16-Pin PDIP
APPLICATIONS
■ 12V and 42V Automotive and Heavy Equipment
■ 48V Telecom Power Supplies
■ Avionics and Industrial Control Systems
■ Distributed Power Converters
LT3845www.DataSheet4U.com
High Voltage Synchronous
Current Mode Step-Down
Controller with Adjustable
Operating Frequency
DESCRIPTION
The LT®3845 is a high voltage, synchronous, current
mode controller used for medium to high power, high
efficiency supplies. It offers a wide 4V to 60V input range
(7.5V minimum start-up voltage). An onboard regulator
simplifies the biasing requirements by providing IC power
directly from VIN.
Burst Mode® operation maintains high efficiency at light
loads by reducing IC quiescent current to 120μA. Light
load efficiency is also improved with the reverse inductor
current inhibit function which supports discontinuous
operation.
Additional features include adjustable fixed operating
frequency that can be synchronized to an external clock
for noise sensitive applications, gate drivers capable of
driving large N-channel MOSFETs, a precision undervoltage
lockout, 10μA shutdown current, short-circuit protection
and a programmable soft-start.
The LT3845 is available in a 16-lead thermally enhanced
TSSOP package and 16-pin through hole N package.
L, LT, LTC, LTM, Burst Mode, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their respective
owners.. Protected by U.S. Patents, including 5481178, 6611131, 6304066, 6498466, 6580258.
TYPICAL APPLICATION
High Voltage Step-Down Regulator 48V to 12V at 75W
VIN
20V TO 55V
2.2μF
100V
47μF
63V
82.5k
1M
1500pF
VIN BOOST
SHDN
LT3845
CSS
TG
SW
BURST_EN
VCC
16.2k
143k
20k
2200pF
100pF
49.9k
VFB
VC
SYNC
fSET
BG
PGND
SENSE+
SENSE–
SGND
0.1μF
BAS521
Si7370DP
Si7370DP
15μH
B160
0.01Ω
VOUT
12V
75W
33μF
×3
1μF
1N4148
3845 TA01a
Efficiency and Power Loss vs
Load Current
100
VIN = 48V
95
7
6
90 5
85 4
80 3
75 2
LOSS
70 1
65
0.1
1
LOAD CURENT (A)
0
10
3845 TA01b
3845fd
1
1 page  
TYPICAL PERFORMANCE CHARACTERISTICS
Shutdown Threshold (Rising)
vs Temperature
1.38
Shutdown Threshold (Falling)
vs Temperature
1.26
1.37 1.25
1.36 1.24
1.35 1.23
1.34 1.22
1.33 1.21
1.32
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3845 G01
1.20
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3845 G02
VCC vs ICC(LOAD)
8.05
TA = 25°C
8.00
7.95
7.90
7.85
0
5 10 15 20 25 30 35 40
ICC(LOAD) (mA)
3845 G04
VCC UVLO Threshold (Rising)
vs Temperature
6.5
6.4
6.3
6.2
6.1
6.0
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3845 G07
VCC vs VIN
9 ICC = 20mA
TA = 25°C
8
7
6
5
4
3
4 5 6 7 8 9 10 11 12
VIN (V)
3845 G05
ICC vs VCC (SHDN = 0V)
25
TA = 25°C
20
15
10
5
0
0 2 4 6 8 10 12 14 16 18 20
VCC (V)
3845 G08
LT3845www.DataSheet4U.com
VCC vs Temperature
8.2
ICC = 20mA
8.1
8.0
7.9
7.8
7.7
7.6
7.5
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3845 G03
ICC Current Limit vs Temperature
225
200
175
150
125
100
75
50
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3845 G06
Error Amp Transconductance
vs Temperature
350
345
340
335
330
325
320
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3845 G09
3845fd
5
5 Page 
LT3845www.DataSheet4U.com
APPLICATIONS INFORMATION
where QG(TOTAL) is the total MOSFET gate charge of the
TG and BG.
In applications where these conditions are exceeded, VCC
must be derived from an external source after start-up.
Maximum continuous regulator power dissipation may be
exceeded for short duration VIN transients.
In LT3845 converter applications with output voltages in
the 9V to 20V range, back-feeding VCC and VBOOST from
the converter output is trivial, accomplished by connect-
ing diodes from the output to these supply pins. Deriving
these supplies from output voltages greater than 20V will
require additional regulation to reduce the feedback voltage.
Outputs lower than 9V will require step-up techniques to
increase the feedback voltage to something greater than
the 8V VCC regulated output. Low power boost switchers
are sometimes used to provide the step-up function, but
a simple charge-pump can perform this function in many
instances.
Charge Pump Doubler
VCC
LT3845
B0520
1μF
VOUT
B0520
1μF
BG Si1555DL
Charge Pump Tripler
B0520
B0520 1μF
VOUT
VCC
LT3845
BG
B0520 1μF
1μF Si1555DL
Si1555DL
3845 AI01
Inductor Auxiliary Winding
TG
SW
LT3845
VCC
BG
•N
VOUT
•
3845 AI04
Burst Mode
The LT3845 employs low current Burst Mode functional-
ity to maximize efficiency during no load and low load
conditions. Burst Mode operation is enabled by shorting
the BURST_EN pin to SGND. Burst Mode operation can be
disabled by shorting BURST_EN to either VFB or VCC.
When the required switch current, sensed via the VC
pin voltage, is below 15% of maximum, the Burst Mode
operation is employed and that level of sense current is
latched onto the IC control path. If the output load requires
less than this latched current level, the converter will
overdrive the output slightly during each switch cycle.
This overdrive condition is sensed internally and forces
the voltage on the VC pin to continue to drop. When the
voltage on VC drops 150mV below the 15% load level,
switching is disabled and the LT3845 shuts down most
of its internal circuitry, reducing total quiescent current
to 120μA. When the converter output begins to fall, the
VC pin voltage begins to climb. When the voltage on the
VC pin climbs back to the 15% load level, the IC returns
to normal operation and switching resumes. An internal
clamp on the VC pin is set at 100mV below the switch
disable threshold, which limits the negative excursion of
the pin voltage, minimizing the converter output ripple
during Burst Mode operation.
During Burst Mode operation, VIN pin current is 20μA and
VCC current is reduced to 100μA. If no external drive is
provided for VCC, all VCC bias currents originate from the
3845fd
11
11 Page | ||
| Páginas | Total 26 Páginas | |
| PDF Descargar | [ Datasheet LT3845.PDF ] | |
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