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PDF LTC3879 Data sheet ( Hoja de datos )
| Número de pieza | LTC3879 | |
| Descripción | Wide Operating Range No RSENSE Step-Down Controller | |
| Fabricantes | Linear Technology Corporation | |
| Logotipo |  |
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Hay una vista previa y un enlace de descarga de LTC3879 (archivo pdf) en la parte inferior de esta página. Total 28 Páginas | ||
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FEATURESwww.datasheet4u.com
n Wide VIN Range: 4V to 38V
n ±1% 0.6V Voltage Reference
n Extremely Fast Transient Response
n tON(MIN): 43ns
n No RSENSE™ Valley Current Mode Control
n Stable with Low ESR Ceramic COUT
n Supports Smooth Start-Up in Pre-Biased Output
n Optimized for High Step-Down Ratios
n Adjustable Output Voltage Soft-Start or Tracking
n Power Good Output Voltage Monitor
n Dual N-Channel MOSFET Synchronous Drive
n Adjustable Switching Frequency
n Programmable Current Limit with Foldback
n Output Overvoltage Protection
n 16-Pin MSOP and 3mm × 3mm QFN Packages
APPLICATIONS
n Distributed Power Systems
n Embedded Computing
n Communications Infrastructure
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
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, 6100678, 6580258, 5847554, 6304066.
LTC3879
Fast, Wide Operating
Range No RSENSE
Step-Down Controller
DESCRIPTION
The LTC®3879 is a synchronous step-down switching
regulator controller optimized for high switching frequency
and fast transient response. The constant on-time valley
current mode architecture allows for a wide input range,
including very low duty factor operation. No external sense
resistor or slope compensation is required. System flex-
ibility is offered through a user-programmable soft-start pin
and independent RUN pin. The voltage ramping soft-start
function can be programmed with a capacitor or made to
track an external reference source.
Operating frequency is set by an external resistor and
compensated for variations in VIN to offer excellent line
stability. Discontinuous mode operation provides high
efficiency during light load conditions. A forced continu-
ous control pin allows the user to reduce noise and RF
interference. Safety features include output overvoltage
protection and programmable current limit with foldback.
The current limit is user programmable.
The LTC3879 allows operation from 4V to 38V at the input
and from 0.6V to 90% VIN at the output. The LTC3879 is
available in a small 16-pin thermally enhanced MSOP or
3mm × 3mm QFN packages.
TYPICAL APPLICATION
High Efficiency Step-Down Converter
0.1μF
RUN ION
TRACK/SS VIN
220pF
27k
TG
LTC3879
ITH SW
SGND BOOST
MODE
INTVCC
BG
PGOOD PGND
VRNG
VFB
432k
RJK0305
0.56μH
0.22μF
4.7μF
RJK0330
10k
VIN
4.5V TO 28V
10μF
VOUT
1.2V
15A
330μF
s2
10k
3879 TA01a
Efficiency vs Load Current
100
90 DISCONTINUOUS
MODE
80
70
60 CONTINUOUS
MODE
50
40
30
20
10
0
0.01
VIN = 12V
VOUT = 1.2V
SW FREQ = 400kHz
FIGURE 10 CIRCUIT
0.1 1
10
LOAD CURRENT (A)
100
3879 TA01b
3879f
1
1 page  
TYPICAL PERFORMANCE CHARACTERISTICS
LTC3879
Transient Response FCM
(Forced Continuous Mode)
www.datasheet4u.com
VOUT (AC)
50mV/DIV
IL
10A/DIV
ILOAD
10A/DIV
50μs/DIV
LOAD STEP 0A TO 10A TO 0A
VIN = 12V
VOUT = 1.2V
MODE = 0V
SW FREQ = 400kHz
FIGURE 10 CIRCUIT
3879 G01
Transient Response FCM
Positive Load Step
VSW
20V/DIV
VOUT (AC)
50mV/DIV
IL
10A/DIV
ILOAD
10A/DIV
5μs/DIV
LOAD STEP 0A TO 10A
VIN = 12V
VOUT = 1.2V
MODE = 0V
SW FREQ = 400kHz
FIGURE 10 CIRCUIT
3879 G02
Transient Response FCM
Negative Load Step
VSW
20V/DIV
VOUT (AC)
50mV/DIV
IL
10A/DIV
ILOAD
10A/DIV
5μs/DIV
LOAD STEP 10A TO 0A
VIN = 12V
VOUT = 1.2V
MODE = 0V
SW FREQ = 400kHz
FIGURE 10 CIRCUIT
3879 G03
Transient Response DCM
(Discontinuous Mode)
VOUT (AC)
50mV/DIV
IL
10A/DIV
ILOAD
10A/DIV
50μs/DIV
LOAD STEP 1A TO 11A TO 1A
VIN = 12V
VOUT = 1.2V
MODE = INTVCC
SW FREQ = 400kHz
FIGURE 10 CIRCUIT
3879 G04
Normal Start-Up, RUN Pin
Released from GND
RUN
5V/DIV
VOUT
0.5V/DIV
TRACK/SS
0.5V/DIV
20ms/DIV
VIN = 12V
VOUT = 1.2V
SW FREQ = 400kHz
FIGURE 10 CIRCUIT
3879 G05
Start-Up Into a Pre-Biased Output
RUN Pin Released from GND
IL
5A/DIV
VOUT
0.5V/DIV
TRACK/SS
0.5V/DIV
20ms/DIV
VIN = 12V
VOUT = 1.2V
SW FREQ = 400kHz
FIGURE 10 CIRCUIT
3879 G06
Start-Up After Input Undervoltage
VIN Cycled Low to High
VIN
10V/DIV
TRACK/SS
5V/DIV
VOUT
1V/DIV
100ms/DIV
VIN = 12V
VOUT = 1.2V
SW FREQ = 400kHz
FIGURE 10 CIRCUIT
3879 G07
Coincident Rail Tracking 1.2V
VOUT Tracks External 1.8V Supply
VMASTER
0.5V/DIV
VOUT
0.5V/DIV
5ms/DIV
VIN = 12V
VOUT = 1.2V
SW FREQ = 400kHz
FIGURE 10 CIRCUIT
3879 G08
Efficiency vs Load Current
100
90 DISCONTINUOUS
MODE
80
70
60 CONTINUOUS
MODE
50
40
30
20
10
0
0.01
VIN = 12V
VOUT = 1.2V
SW FREQ = 400kHz
FIGURE 10 CIRCUIT
0.1 1
10
LOAD CURRENT (A)
100
3879 G09
3879f
5
5 Page 
LTC3879
APPLICATIONS INFORMATION
The basic LTC3879 application circuit is shown on the first
page of this data sheet. External component selection is
wwwl.adragtaeslhyeedte4tue.crommined by maximum load current and begins
with the selection of sense resistance and power MOSFET
switches. The LTC3879 uses the on-resistance of the syn-
chronous power MOSFET to determine the inductor current.
The desired 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 output
voltage ripple and transient specifications.
Maximum VDS Sense Voltage and VRNG Pin
Inductor current is measured by sensing the bottom
MOSFET VDS voltage that appears between the PGND
and SW pins. The maximum allowed VDS sense voltage is
set by the voltage applied to the VRNG pin and is approxi-
mately equal to (0.133)VRNG. The current mode control
loop does not allow the inductor current valleys to exceed
(0.133)VRNG. In practice, one should allow margin, to ac-
count for variations in the LTC3879 and external component
values. A good guide for setting VRNG is:
VRNG = 7.5 • (Maximum VDS Sense Voltage)
An external resistive divider from INTVCC can be used
to set the voltage on the VRNG pin between 0.2V and 2V,
resulting in peak sense voltages between 26.6mV and
266mV. The wide peak voltage sense range allows for a
variety of applications and MOSFET choices. The VRNG pin
can also be tied to either SGND or INTVCC to force internal
defaults. When VRNG is tied to SGND, the device operates
at a valley current sense threshld of 30mV typical. If VRNG
is tied to INTVCC, the device operates at a valley current
sense threshold of 75mV typical.
The gate drive voltages are set by the 5.3V INTVCC supply.
Consequently, logic-level threshold MOSFETs must be used
in LTC3879 applications. If the input voltage is expected
to drop below 5V, then sub-logic level threshold MOSFETs
should be considered.
Using the bottom MOSFET as the current sense element
requires particular attention be paid to its on-resistance.
MOSFET on-resistance is typically specified with a maxi-
mum 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)
=
Max VDS Sense Voltage
IO • ρT
The ρT term is a normalization factor (unity at 25°C)
accounting 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 of ρT = 1.3 is reasonable.
The power dissipated by the top and bottom MOSFETs
depends upon their respective duty cycles and the load
current. When the LTC3879 is operating in continuous
mode, the duty cycles for the MOSFETs are:
DTOP
=
VOUT
VIN
DBOT
=
VIN
– VOUT
VIN
2.0
1.5
Power MOSFET Selection
The LTC3879 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 VBR(DSS),
threshold voltage VGS(TH), on-resistance RDS(ON), re-
verse transfer capacitance CRSS and maximum current
IDS(MAX).
1.0
0.5
0
–50 0
50 100 150
JUNCTION TEMPERATURE (°C)
3879 F01
Figure 1. RDS(ON) vs Temperature
3879f
11
11 Page | ||
| Páginas | Total 28 Páginas | |
| PDF Descargar | [ Datasheet LTC3879.PDF ] | |
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