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PDF LTC3786 Data sheet ( Hoja de datos )
| Número de pieza | LTC3786 | |
| Descripción | Synchronous Boost Controller | |
| Fabricantes | Linear | |
| Logotipo |  |
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Features
n Synchronous Operation For Highest Efficiency and
Reduced Heat Dissipation
n Wide VIN Range: 4.5V to 38V (40V Abs Max) and
Operates Down to 2.5V After Start-Up
n Output Voltages Up to 60V
n ±1% 1.2V Reference Voltage
n RSENSE or Inductor DCR Current Sensing
n 100% Duty Cycle Capability for Synchronous MOSFET
n Low Quiescent Current: 55µA
n Phase-Lockable Frequency (75kHz to 850kHz)
n Programmable Fixed Frequency (50kHz to 900kHz)
n Adjustable Output Voltage Soft-Start
n Power Good Output Voltage Monitor
n Low Shutdown Current IQ: <8µA
n Internal 5.4V LDO for Gate Drive Supply
n Thermally Enhanced 16-Pin 3mm × 3mm QFN and
MSOP Packages
Applications
n Industrial and Automotive Power Supplies
n Automotive Start-Stop Systems
n Medical Devices
n High Voltage Battery-Powered Systems
LTC3786
Low IQ Synchronous
Boost Controller
Description
The LTC®3786 is a high performance synchronous boost
converter controller that drives all N-channel power
MOSFETs. Synchronous rectification increases efficiency,
reduces power losses and eases thermal requirements,
allowing the LTC3786 to be used in high power boost
applications.
A 4.5V to 38V input supply range encompasses a wide
range of system architectures and battery chemistries.
When biased from the output of the boost converter or
another auxiliary supply, the LTC3786 can operate from
an input supply as low as 2.5V after start-up. The 55µA
no-load quiescent current extends operating run time in
battery-powered systems.
The operating frequency can be set for a 50kHz to 900kHz
range or synchronized to an external clock using the
internal PLL. The LTC3786 also features a precision 1.2V
reference and a power good output indicator. The SS pin
ramps the output voltage during start-up. The PLLIN/MODE
pin selects among Burst Mode® operation, pulse-skipping
mode or continuous inductor current mode at light loads.
L, LT, LTC, LTM, Burst Mode, OPTI-LOOP, Linear Technology and the Linear logo are
registered trademarks and No RSENSE is a trademark of Linear Technology Corporation. All other
trademarks are the property of their respective owners. Protected by U. S. Patents, including
5408150, 5481178, 5705919, 5929620, 6177787, 6498466, 6580258, 6611131.
Typical Application
12V to 24V/5A Synchronous Boost Converter
VBIAS
SENSE+
0.1µF
LTC3786
PGOOD
SENSE–
PLLIN/MODE
RUN
FREQ
TG
15nF 8.66k
220pF
SS
ITH
SW
BOOST
BG
12.1k
VFB INTVCC
232k
GND
VIN 4.5V TO 24V
0.1µF
4mΩ
220µF
3.3µH
220µF
VOUT
24V
5A
4.7µF
3786 TA01a
Efficiency and Power Loss
vs Load Current
100 10000
90
80
BURST
EFFICIENCY
70
60
BURST
LOSS
1000
100
50
40 10
30
20
10
0
0.00001 0.0001 0.001
VIN = 12V
VOUT = 24V
1
Burst Mode OPERATION
FIGURE 8 CIRCUIT
0.1
0.01 0.1 1 10
OUTPUT CURRENT (A)
3786 TA01b
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1
1 page  
Typical Performance Characteristics
LTC3786
Efficiency and Power Loss
vs Output Current
100
10000
90
80 1000
70
60
50
40
30
20
10
0
0.01
VIN = 12V
VOUT = 24V
FIGURE 8 CIRCUIT
100
10
CCM EFFICIENCY
CMM LOSS
BURST EFFICIENCY
BURST LOSS
1
PULSE-SKIPPING EFFICIENCY
PULSE-SKIPPING LOSS
0.1
0.1 1 10
OUTPUT CURRENT (A)
3786 G01
Efficiency and Power Loss
vs Output Current
100
10000
90
80
BURST
EFFICIENCY
70
60
BURST
LOSS
1000
100
50
40 10
30
20
10
0
0.00001 0.0001 0.001
VIN = 12V
VOUT = 24V
1
Burst Mode OPERATION
FIGURE 8 CIRCUIT
0.1
0.01 0.1 1 10
OUTPUT CURRENT (A)
3786 G02
Efficiency vs Input Voltage
100
ILOAD = 2A
99 FIGURE 8 CIRCUIT
98
VOUT = 12V
97
VOUT = 24V
96
Load Step
Forced Continuous Mode
LOAD STEP
2A/DIV
INDUCTOR
CURRENT
5A/DIV
VOUT
500mV/DIV
95
94
93
0 5 10 15 20 25
INPUT VOLTAGE (V)
3786 G03
VIN = 12V
200µs/DIV
VOUT = 24V
LOAD STEP FROM 200mA TO 2.5A
FIGURE 8 CIRCUIT
Load Step
Burst Mode Operation
LOAD STEP
2A/DIV
INDUCTOR
CURRENT
5A/DIV
VOUT
500mV/DIV
3786 G04
VIN = 12V
200µs/DIV
VOUT = 24V
LOAD STEP FROM 200mA TO 2.5A
FIGURE 8 CIRCUIT
Load Step
Pulse-Skipping Mode
Inductor Current at Light Load
Soft Start-Up
LOAD STEP
2A/DIV
INDUCTOR
CURRENT
5A/DIV
VOUT
500mV/DIV
VIN = 12V
200µs/DIV
VOUT = 24V
LOAD STEP FROM 200mA TO 2.5A
FIGURE 8 CIRCUIT
FORCED
CONTINUOUS
MODE
Burst Mode
OPERATION
5A/DIV
PULSE-
SKIPPING MODE
3786 G06
VIN = 12V
5µs/DIV
VOUT = 24V
ILOAD = 200µA
FIGURE 8 CIRCUIT
VOUT
5V/DIV
3786 G07
0V
VIN = 12V
20ms/DIV
VOUT = 24V
FIGURE 8 CIRCUIT
3786 G05
3786 G08
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5 Page 
LTC3786
Operation (Refer to the Block Diagram)
Main Control Loop
The LTC3786 uses a constant-frequency, current mode
step-up control architecture. During normal operation,
the external bottom MOSFET is turned on when the clock
sets the RS latch, and is turned off when the main current
comparator, ICMP , resets the RS latch. The peak inductor
current at which ICMP trips and resets the latch is con-
trolled by the voltage on the ITH pin, which is the output
of the error amplifier, EA. The error amplifier compares
the output voltage feedback signal at the VFB pin, (which
is generated with an external resistor divider connected
across the output voltage, VOUT , to ground) to the internal
1.200V reference voltage. In a boost converter, the required
inductor current is determined by the load current, VIN and
VOUT . When the load current increases, it causes a slight
decrease in VFB relative to the reference, which causes the
EA to increase the ITH voltage until the average inductor
current in each channel matches the new requirement
based on the new load current.
After the bottom MOSFET is turned off each cycle, the
top MOSFET is turned on until either the inductor current
starts to reverse, as indicated by the current comparator
IR, or the beginning of the next clock cycle.
INTVCC Power
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTVCC pin. The
VBIAS LDO (low dropout linear regulator) supplies 5.4V
from VBIAS to INTVCC.
Shutdown and Start-Up (RUN and SS Pins)
The LTC3786 can be shut down using the RUN pin. Pulling
this pin below 1.28V shuts down the main control loop.
Pulling this pin below 0.7V disables the controller and
most internal circuits, including the INTVCC LDOs. In this
state, the LTC3786 draws only 8µA of quiescent current.
Note: Do not apply load while the chip is in shutdown. The
output MOSFET will be turned off during shutdown and
the output load may cause excessive power dissipation
in the body diode.
The RUN pin may be externally pulled up or driven directly
by logic. When driving the RUN pin with a low imped-
ance source, do not exceed the absolute maximum rating
of 8V. The RUN pin has an internal 11V voltage clamp
that allows the RUN pin to be connected through a resis-
tor to a higher voltage (for example, VIN), as long as the
maximum current into the RUN pin does not exceed 100µA.
An external resistor divider connected to VIN can set the
threshold for converter operation. Once running, a 4.5µA
current is sourced from the RUN pin allowing the user to
program hysteresis using the resistor values.
The start-up of the controller’s output voltage, VOUT , is
controlled by the voltage on the SS pin. When the voltage
on the SS pin is less than the 1.2V internal reference, the
LTC3786 regulates the VFB voltage to the SS pin voltage
instead of the 1.2V reference. This allows the SS pin to
be used to program a soft-start by connecting an external
capacitor from the SS pin to GND. An internal 10µA pull-
up current charges this capacitor creating a voltage ramp
on the SS pin. As the SS voltage rises linearly from 0V to
1.2V, the output voltage rises smoothly to its final value.
Light Load Current Operation—Burst Mode Operation,
Pulse-Skipping or Continuous Conduction
(PLLIN/MODE Pin)
The LTC3786 can be enabled to enter high efficiency Burst
Mode operation, constant-frequency pulse-skipping mode
or forced continuous conduction mode at low load cur-
rents. To select Burst Mode operation, tie the PLLIN/MODE
pin to ground. To select forced continuous operation, tie
the PLLIN/MODE pin to INTVCC. To select pulse-skipping
mode, tie the PLLIN/MODE pin to a DC voltage greater
than 1.2V and less than INTVCC – 1.3V.
When the controller is enabled for Burst Mode opera-
tion, the minimum peak current in the inductor is set to
approximately 30% of the maximum sense voltage even
though the voltage on the ITH pin indicates a lower value.
If the average inductor current is higher than the required
current, the error amplifier, EA, will decrease the voltage
on the ITH pin. When the ITH voltage drops below 0.425V,
the internal sleep signal goes high (enabling sleep mode)
and both external MOSFETs are turned off. The ITH pin is
then disconnected from the output of the EA and parked
at 0.450V.
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11 Page | ||
| Páginas | Total 30 Páginas | |
| PDF Descargar | [ Datasheet LTC3786.PDF ] | |
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