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PDF LTC3823 Data sheet ( Hoja de datos )
| Número de pieza | LTC3823 | |
| Descripción | Step-Down Synchronous DC/DC Controller | |
| Fabricantes | Linear | |
| Logotipo |  |
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Hay una vista previa y un enlace de descarga de LTC3823 (archivo pdf) en la parte inferior de esta página. Total 26 Páginas | ||
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LTC3823
Fast No RSENSETM Step-Down
Synchronous DC/DC Controller
with Differential Output Sensing,
Tracking and PLL
FEATURES
DESCRIPTION
n Wide VIN Range: 4.5V to 36V
n ±0.67%, 0.6V Reference Voltage
n Output Voltage Tracking Capability
n True Remote Sensing Differential Amplifier
n Sense Resistor Optional
n True Current Mode Control
n 2% to 90% Duty Cycle at 200kHz
n tON(MIN) < 100ns
n Phase-Locked Loop Frequency Synchronization
n Powerful Dual N-Channel MOSFET Driver
n Adjustable Cycle-by-Cycle Current Limit
n Adjustable Switching Frequency
n Programmable Soft-Start
n Current Foldback Protection (Disabled at Start-Up)
n Output Overvoltage Protection
n Micropower Shutdown: 30μA
n Power Good Output Voltage Monitor Tracks the
Reference Input Pin
n Available in (5mm × 5mm) 32-Lead QFN and
28-Lead SSOP Narrow Packages
APPLICATIONS
n Distributed Power Systems
n Server Power Supplies
The LTC®3823 is a synchronous step-down switching regu-
lator controller with true remote differential output sensing
and output voltage up/down tracking capability. Its advanced
functions and high accuracy reference are ideal for powering
high performance server, ASIC and computer memory
systems.
The LTC3823 uses a constant on-time, valley current
mode control architecture to deliver very low duty fac-
tors without requiring a sense resistor. The operating
frequency is selected by an external resistor and is com-
pensated for variations in input supply voltage. An internal
phase-locked loop allows the IC to be synchronized to an
external clock.
Fault protection is provided by an overvoltage compara-
tor and input undervoltage lockout. The regulator current
limit is user programmable. A wide supply range allows
voltages as high as 36V to be stepped down to as low as
a 0.6V output. When using remote sense, output voltages
up to 3.3V can be developed, and up to 90% of VIN without
remote sense. Power supply sequencing is accomplished
using an external soft-start timing capacitor.
L, LT, LTC, LTM, 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 5481178, 5847554,
6580258, 6304066, 6476589, 6774611.
TYPICAL APPLICATION
0.01μF
10k
0.1μF
1000pF 10k
VOUT
9.5k
3k
High Efficiency Step-Down Converter
PGOOD
ION
68k
VIN
PLLFLTR
PLLIN
TRACK/SS
ITH
LTC3823
TG
SW
BOOST
SGND
RUN
VON
VRNG
VDIFFOUT
INTVCC
DRVCC
BG
SENSE+
SENSE–
0.22μF
CMDSH-3
10μF
PGND
VFB VOUTSENSE+
VOUTSENSE–
3823 TA01a
Si4884
1.8μH
Si4874
B340A
+
VIN
10μF 5V TO 28V
35V
s3 VOUT
2.5V
10A
180μF
4V
s2
Efficiency and Power Loss
vs Load Current
97
96
VIN = 5V
VOUT = 2.5V
95 FIGURE 12 CIRCUIT
10
94
EFFICIENCY
93
1
92
POWER LOSS
91
90 0.1
89
88
87
0.1
1
LOAD CURRENT (A)
0.01
10
3823 TA01b
3823fd
1
1 page  
TYPICAL PERFORMANCE CHARACTERISTICS
LTC3823
On-Time vs Temperature
250
245
IION = 30μA
VON = 0V
240
235
230
225
220
215
210
205
200
–50 –25
0 25 50 75 100 125
TEMPERATURE (°C)
3823 G04
Error Amplifier gm vs Temperature
1.8
1.7
1.6
1.5
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3823 G07
INTVCC Load Regulation
0
–0.1
–0.2
–0.3
–0.4
–0.5
–0.6
–0.7
–0.8
–0.9
–1.0
0
5 10 15 20 25 30 35 40 45 50
INTVCC LOAD CURRENT (mA)
3823 G10
Maximum Current Sense
Threshold vs VRNG Voltage
300
250
200
150
100
50
0.5 0.75 1 1.25 1.5 1.75 2
VRNG VOLTAGE (V)
3823 G05
Input Current vs Input Voltage
2.5
Maximum Current Sense
Threshold vs Temperature
150
VRNG = 1V
145
140
135
130
125
120
115
110
105
100
–50 –25
0 25 50 75 100 125
TEMPERATURE (°C)
3823 G06
Shutdown Current vs Input Voltage
60
50
2.0
1.5
1.0
0
5 10 15 20 25
INPUT VOLTAGE, VIN (V)
30
3823 G08
40
30
20
10
0
0 5 10 15 20 25 30
INPUT VOLTAGE, VIN (V)
3823 G09
FCB Pin Current vs Temperature
–1.00
–1.05
–1.10
–1.15
–1.20
–1.25
–1.30
–1.35
–1.40
–1.45
–1.50
–50 –25
0 25 50 75 100 125
TEMPERATURE (°C)
3823 G11
Track Up
FIGURE 12 CIRCUIT
TRACK/SS
TRACK/SS
AND VFB
500mV/DIV
VOUT
2V/DIV
VFB
VOUT
250ms/DIV
3823 G12
3823fd
5
5 Page 
LTC3823
APPLICATIONS INFORMATION
The basic LTC3823 application circuit is shown in
Figure 12. External component selection is primarily de-
termined by the maximum load current and begins with
the selection of the sense resistance and power MOSFET
switches. The LTC3823 uses either a sense resistor or
the on-resistance of the synchronous power MOSFET for
determining the inductor current. The desired amount of
ripple current and operating frequency largely determines
the inductor value. Finally, CIN is selected for its ability to
handle the large RMS current into the converter and COUT
is chosen with low enough ESR to meet the output voltage
ripple and transient specification.
Maximum Sense Voltage and VRNG Pin
Inductor current is determined by measuring the volt-
age across a sense resistance that appears between the
SENSE– and SENSE+ pins. The maximum sense voltage
is set by the voltage applied to the VRNG pin and is equal
to approximately (0.133)VRNG. The current mode control
loop will not allow the inductor current valleys to exceed
(0.133)VRNG/RSENSE. In practice, one should allow some
margin for variations in the LTC3823 and external com-
ponent values and a good guide for selecting the sense
resistance is:
RSENSE
=
10
VRNG
• IOUT(MAX)
An external resistive divider from INTVCC can be used
to set the voltage of the VRNG pin between 0.5V and 2V
resulting in nominal sense voltages of 50mV to 200mV.
Additionally, the VRNG pin can be tied to SGND or INTVCC in
which case the nominal sense voltage defaults to 50mV or
200mV, respectively. The maximum allowed sense voltage
is about 1.33 times this nominal value.
Connecting the SENSE+ and SENSE– Pins
The IC can be used with or without a sense resistor. When
using a sense resistor, place it between the source of the
bottom MOSFET, M2, and PGND. Connect the SENSE+ and
SENSE– pins to the top and bottom of the sense resistor.
Using a sense resistor provides a well defined current
limit, but adds cost and reduces efficiency. Alternatively,
one can eliminate the sense resistor and use the bottom
MOSFET as the current sense element by simply connecting
the SENSE+ pin to the SW pin and SENSE– pin to PGND.
This improves efficiency, but one must carefully choose
the MOSFET on-resistance as discussed below.
Power MOSFET Selection
The LTC3823 requires two external N-channel power
MOSFETs, one for the top (main) switch and one for the
bottom (synchronous) switch. Important parameters for
the power MOSFETs are the breakdown voltage V(BR)DSS,
threshold voltage V(GS)TH, on-resistance RDS(ON), reverse
transfer capacitance CRSS and maximum current IDS(MAX).
The gate drive voltage is set by the 5V INTVCC supply.
Consequently, logic-level threshold MOSFETs must be used
in LTC3823 applications. If the input voltage is expected
to drop below 5V, then sub-logic level threshold MOSFETs
should be considered.
When the bottom MOSFET is used as the current sense
element, particular attention must be paid to its on-resis-
tance. MOSFET on-resistance is typically specified with
a maximum value RDS(ON)(MAX) at 25°C. In this case,
additional margin is required to accommodate the rise in
MOSFET on-resistance with temperature:
RDS(ON)(MAX)
=
RSENSE
ρT
The ρT term is a normalization factor (unity at 25°C) ac-
counting for the significant variation in on-resistance with
temperature, typically about 0.4%/°C as shown in Figure 1.
For a maximum junction temperature of 100°C, using a
value ρT = 1.3 is reasonable.
The power dissipated by the top and bottom MOSFETs
strongly depends upon their respective duty cycles and the
load current. When the LTC3823 is operating in continuous
mode, the duty cycles for the MOSFETs are:
DTOP
=
VOUT
VIN
DBOT
=
VIN
– VOUT
VIN
3823fd
11
11 Page | ||
| Páginas | Total 26 Páginas | |
| PDF Descargar | [ Datasheet LTC3823.PDF ] | |
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