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PDF LTC3813 Data sheet ( Hoja de datos )
| Número de pieza | LTC3813 | |
| Descripción | 100V Current Mode Synchronous Step-Up Controller | |
| Fabricantes | Linear Technology | |
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LTC3813
100V Current Mode
Synchronous Step-Up Controller
FEATURES
■ High Output Voltages: Up to 100V
■ Large 1Ω Gate Drivers
■ No Current Sense Resistor Required
■ Dual N-Channel MOSFET Synchronous Drive
■ ±0.5% 0.8V Voltage Reference
■ Fast Transient Response
■ Programmable Soft-Start
■ Generates 10V Driver Supply from Input Supply
■ Synchronizable to External Clock
■ Power Good Output Voltage Monitor
■ Adjustable Off-Time/Frequency: tOFF(MIN) < 100ns
■ Adjustable Cycle-by-Cycle Current Limit
■ Programmable Undervoltage Lockout
■ Output Overvoltage Protection
■ 28-Pin SSOP Package
U
APPLICATIO S
■ 24V Fan Supplies
■ 48V Telecom and Base Station Power Supplies
■ Networking Equipment, Servers
■ Automotive and Industrial Control Systems
DESCRIPTIO
The LTC3813 is a synchronous step-up switching regulator
controller that can generate output voltages up to 100V.
The LTC3813 uses a constant off-time peak current control
architecture with accurate cycle-by-cycle current limit,
without requiring a sense resistor.
A precise internal reference provides ±0.5% DC accuracy.
A high bandwidth (25MHz) error amplifier provides very
fast line and load transient response. Large 1Ω gate drivers
allow the LTC3813 to drive multiple MOSFETs for higher
current applications. The operating frequency is selected
by an external resistor and is compensated for variations
in VIN and can also be synchronized to an external clock
for switching-noise sensitive applications. A shutdown pin
allows the LTC3813 to be turned off, reducing the supply
current to 240µA.
PARAMETER
Maximum VOUT
MOSFET Gate Drive
INTVCC UV+
INTVCC UV –
LTC3813
100V
6.35V to 14V
6.2V
6V
LTC3814-5
60V
4.5V to 14V
4.2V
4V
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners. Protected by U.S. Patents,
including 5481178, 5847554, 6304066, 6476589, 6580258, 6677210, 6774611.
TYPICAL APPLICATIO
High Efficiency High Voltage Step-Up Converter
500k
VOUT
1000pF
0.01µF
100k
100pF
IOFF NDRV
PGOOD
BOOST
LTC3813
VRNG
TG
SYNC
SS
SHDN
ITH
VFB
SGND
SW
EXTVCC
DRVCC
INTVCC
SENSE+
BG
SENSE–
BGRTN
50k
0.1µF
10µH
VIN
+ 10V TO 40V
22µF
M1
Si7850DP
VOUT
50V/5A
D1
MBR1100
M2
Si7850DP
×2
1µF
30.9k
+
270µF
×2
499Ω
3813 TA01
Efficiency vs Load Current
100
VIN = 36V
95 VIN = 24V
VIN = 12V
90
85
80
012345
LOAD (A)
3813 TA01b
3813f
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TYPICAL PERFOR A CE CHARACTERISTICS
LTC3813
Load Transient Response
VOUT
200mV/
DIV
IOUT
2A/DIV
100µs/DIV
FIGURE 14 CIRCUIT
VIN = 12V
0A TO 4A LOAD STEP
3813 G01
Start-Up
VOUT
20V/DIV
SS
4V/DIV
IL
5A/DIV
1ms/DIV
FRONT PAGE CIRCUIT
VIN = 24V
ILOAD = 2A
Overcurrent Operation
VOUT
20V/DIV
IL
5A/DIV
3813 G02
500µs/DIV
FRONT PAGE CIRCUIT
VIN = 24V
RSHORT = 1Ω
3813 G03
Efficiency vs Load Current
100
VOUT = 24V
VIN = 12V
95
90 VIN = 5V
85
80
0
123
LOAD (A)
4
3813 G04
Current Sense Threshold
vs ITH Voltage
400
VRNG = 2V
300
1.4V
200
1V
100 0.7V
0.5V
0
–100
–200
–300
–400
0 0.5 1 1.5 2
ITH VOLTAGE (V)
2.5 3
3810 G11
Frequency vs Input Voltage
300
FRONT PAGE CIRCUIT
280
ILOAD = 0A
260
ILOAD = 1A
240
220
200
10
15 20 25 30 35 40
INPUT VOLTAGE (V)
3813 G05
Frequency vs Load Current
280
FRONT PAGE CIRCUIT
270
260
250 VIN = 24V
240
230
220 VIN = 12V
210
200
0
123
LOAD CURRENT (A)
4
3813 G06
Off-Time vs IOFF Current
10000
VOFF = INTVCC
1000
100
10
10
100 1000
IOFF CURRENT (µA)
10000
3813 G08
Off-Time vs VOFF Voltage
700
IOFF = 300µA
600
500
400
300
200
100
0
0 0.5 1 1.5 2 2.5 3
VOFF VOLTAGE (V)
3813 G09
3813f
5
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LTC3813
OPERATION
Main Control Loop
The LTC3813 is a current mode controller for DC/DC step-
up converters. In normal operation, the top MOSFET is
turned on for a fixed interval determined by a one-shot
timer (OST). When the top MOSFET is turned off, the bot-
tom MOSFET is turned on until the current comparator
ICMP trips, restarting the one-shot timer and initiating the
next cycle. Inductor current is determined by sensing the
voltage between the SENSE– and SENSE+ pins using a
sense resistor or the bottom MOSFET on-resistance. The
voltage on the ITH pin sets the comparator threshold cor-
responding to the inductor peak current. The fast 25MHz
error amplifier EA adjusts this voltage by comparing the
feedback signal VFB to the internal 0.8V reference volt-
age. If the load current increases, it causes a drop in the
feedback voltage relative to the reference. The ITH voltage
then rises until the average inductor current again matches
the load current.
The operating frequency is determined implicitly by the
top MOSFET on-time (tOFF) and the duty cycle required to
maintain regulation. The one-shot timer generates a top
MOSFET on-time that is inversely proportional to the IOFF
current and proportional to the VOFF voltage. Connecting
VOUT to IOFF and VIN to VOFF with a resistive divider keeps
the frequency approximately constant with changes in VIN.
The nominal frequency can be adjusted with an external
resistor ROFF.
For applications with stringent constant-frequency require-
ments, the LTC3813 can be synchronized with an external
clock. By programming the nominal frequency the same as
the external clock frequency, the LTC3813 behaves as a con-
stant-frequency part against the load and supply variations.
Pulling the ⎯S⎯H⎯D⎯N pin low forces the controller into its
shutdown state, turning off both M1 and M2. Forcing a
voltage above 1.5V will turn on the device.
Fault Monitoring/Protection
Constant off-time current mode architecture provides ac-
curate cycle-by-cycle current limit protection—a feature
that is very important for protecting the high voltage
power supply from output overcurrent conditions. The
cycle-by-cycle current monitor guarantees that the induc-
tor current will never exceed the value programmed on
the VRNG pin.
Overvoltage and undervoltage comparators OV and UV
pull the PGOOD output low if the output feedback voltage
exits a ±10% window around the regulation point after the
internal 125µs power bad mask timer expires. Furthermore,
in an overvoltage condition, M1 is turned off and M2 is
turned on immediately and held on until the overvoltage
condition clears.
The LTC3813 provides two undervoltage lockout com-
parators—one for the INTVCC/DRVCC supply and one for
the input supply VIN. The INTVCC UV threshold is 6.2V to
guarantee that the MOSFETs have sufficient gate drive volt-
age before turning on. The VIN UV threshold (UVIN pin) is
0.8V with 10% hysteresis which allows programming the
VIN threshold with the appropriate resistor divider con-
nected to VIN. If either comparator inputs are under the
UV threshold, the LTC3813 is shut down and the drivers
are turned off.
Strong Gate Drivers
The LTC3813 contains very low impedance drivers capable
of supplying amps of current to slew large MOSFET gates
quickly. This minimizes transition losses and allows paral-
leling MOSFETs for higher current applications. A 100V
floating high side driver drives the top side MOSFET and
a low side driver drives the bottom side MOSFET (see
Figure 1). The bottom side driver is supplied directly
from the DRVCC pin. The top MOSFET drivers are biased
from floating bootstrap capacitor CB, which normally is
recharged during each off cycle through an external diode
DRVCC
LTC3813
DRVCC
BOOST
TG
SW
BGRTN
0V TO –5V
BG
VIN
+
DB CIN
L
CB
M1 +
M2
VOUT
COUT
3813 F01
Figure 1. Floating TG Driver Supply and Negative BG Return
3813f
11
11 Page | ||
| Páginas | Total 30 Páginas | |
| PDF Descargar | [ Datasheet LTC3813.PDF ] | |
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