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PDF LTC3827 Data sheet ( Hoja de datos )
| Número de pieza | LTC3827 | |
| Descripción | 2-Phase Synchronous Step-Down Controller | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LTC3827
Low IQ, Dual, 2-Phase
Synchronous Step-Down Controller
FEATURES
wwwn.daWtasihdeeet4Ouu.ctopmut Voltage Range: 0.8V ≤ VOUT ≤ 10V
n Low Operating IQ: 80μA (One Channel On)
n Out-of-Phase Controllers Reduce Required Input
Capacitance and Power Supply Induced Noise
n OPTI-LOOP® Compensation Minimizes COUT
n ±1% Output Voltage Accuracy
n Wide VIN Range: 4V to 36V Operation
n Phase-Lockable Fixed Frequency 140kHz to 650kHz
n Selectable Continuous, Pulse Skipping or Low Ripple
Burst Mode® Operation at Light Loads
n Dual N-Channel MOSFET Synchronous Drive
n Very Low Dropout Operation: 99% Duty Cycle
n Adjustable Output Voltage Soft-Start or Tracking
n Output Current Foldback Limiting
n Power Good Output Voltage Monitor
n Output Overvoltage Protection
n Low Shutdown IQ: 8μA
n Internal LDO Powers Gate Drive from VIN or VOUT
n Small 5mm × 5mm QFN Package
APPLICATIONS
n Automotive Systems
n Battery-Operated Digital Devices
n Distributed DC Power Systems
DESCRIPTION
The LTC®3827 is a high performance dual step-down
switching regulator controller that drives all N-channel
synchronous power MOSFET stages. A constant frequency
current mode architecture allows a phase-lockable fre-
quency of up to 650kHz. Power loss and noise due to the
ESR of the input capacitor ESR are minimized by operating
the two controller output stages out of phase.
The 80μA no-load quiescent current extends operating
life in battery-powered systems. OPTI-LOOP compensa-
tion allows the transient response to be optimized over
a wide range of output capacitance and ESR values. The
LTC3827 features a precision 0.8V reference and a power
good output indicator. A wide 4V to 36V input supply range
encompasses all battery chemistries.
Independent TRACK/SS pins for each controller ramp the
output voltage during start-up. Current foldback limits
MOSFET heat dissipation during short-circuit conditions.
The PLLIN/MODE pin selects among Burst Mode opera-
tion, pulse skipping mode, or continuous inductor cur-
rent mode at light loads. For a leaded package version
(28-lead SSOP), see the LTC3827-1 datasheet.
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
Burst Mode and OPTI-LOOP are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners. Protected by U.S. Patents
including 5481178, 5929620, 6177787, 6144194, 5408150, 6580258, 6304066, 570919.
TYPICAL APPLICATION
VOUT1
3.3V
5A
3.3μH
0.015Ω
62.5k
150μF
High Efficiency Dual 8.5V/3.3V Step-Down Converter
+
4.7μF
0.1μF
VIN
TG1
BOOST1
SW1
INTVCC
TG2
BOOST2
SW2
1μF
0.1μF
22μF
50V
VIN
4V TO 36V
7.2μH
BG1 BG2
LTC3827
PGND
SENSE1+
SENSE2+
0.015Ω
SENSE1–
SENSE2–
VFB1
VFB2
ITH1 ITH2
20k 220pF TRACK/SS1 SGND TRACK/SS2
15k 0.1μF
0.1μF
220pF
15k
192.5k
20k
VOUT2
8.5V
3.5A
150μF
3827 TA01
Efficiency and Power Loss
vs Load Current
100 100000
90 EFFICIENCY
VIN = 12V; VOUT = 3.3V
10000
80
70 1000
60
50 100
40
30 POWER LOSS 10
20
10
0
0.001 0.01
1
FIGURE 13 CIRCUIT
0.1
0.1 1 10 100 1000 10000
LOAD CURRENT (mA)
3827 TA01b
3827ff
1
1 page  
TYPICAL PERFORMANCE CHARACTERISTICS
LTC3827
Efficiency and Power Loss
vs Output Current
www.d1a0ta0 sheet4uB.ucrosmt Mode OPERATION
90 FORCED CONTINUOUS MODE
PULSE SKIPPING MODE
80
10000
1000
70
60 100
50
40
30 VIN = 12V
VOUT = 3.3V
20
10
1
10
0
0.001 0.01
FIGURE 13 CIRCUIT
0.1
0.1 1 10 100 1000 10000
LOAD CURRENT (mA)
3827 G01
Load Step
(Burst Mode Operation)
Efficiency vs Load Current
100
VIN = 12V
VIN = 5V
90 VOUT = 3.3V
80
70
60
50
40
0.001 0.01
FIGURE 13 CIRCUIT
0.1 1 10 100 1000 10000
LOAD CURRENT (mA)
3827 G02
Load Step
(Forced Continuous Mode)
Efficiency vs Input Voltage
98
96
94
92
90
88
86
84 VOUT = 3.3V
FIGURE 13 CIRCUIT
82
0 5 10 15 20 25 30
INPUT VOLTAGE (V)
35 40
3827 G03
Load Step
(Pulse Skip Mode)
VOUT
100mV/DIV
AC
COUPLED
VOUT
100mV/DIV
AC
COUPLED
IL
2A/DIV
IL
2A/DIV
20μs/DIV
VOUT = 3.3V
FIGURE 13 CIRCUIT
3827 G04
20μs/DIV
VOUT = 3.3V
FIGURE 13 CIRCUIT
Inductor Current at Light Load
Soft Start-Up
FORCED
CONTINUOUS
MODE
2A/DIV
BURST MODE
OPERATION
PULSE
SKIPPING
MODE
4μs/DIV
VOUT = 3.3V
ILOAD = 300μA
FIGURE 13 CIRCUIT
3827 G07
20ms/DIV
FIGURE 13 CIRCUIT
VOUT
100mV/DIV
AC
COUPLED
3827 G05
IL
2A/DIV
20μs/DIV
VOUT = 3.3V
FIGURE 13 CIRCUIT
VOUT2
2V/DIV
VOUT1
2V/DIV
Tracking Start-Up
3827 G08
20ms/DIV
FIGURE 13 CIRCUIT
3827 G06
VOUT2
2V/DIV
VOUT1
2V/DIV
3827 G09
3827ff
5
5 Page 
LTC3827
OPERATION (Refer to Functional Diagram)
Main Control Loop
The LTC3827 uses a constant frequency, current mode
wwws.dtaetpas-hdeoewt4nu.caormchitecture with the two controller channels
operating 180 degrees out of phase. During normal op-
eration, each external top MOSFET is turned on when the
clock for that channel 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 controlled 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 resis-
tor divider connected across the output voltage, VOUT, to
ground) to the internal 0.800V reference voltage. 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 matches
the new load current.
After the top MOSFET is turned off each cycle, the bottom
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/EXTVCC Power
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTVCC pin.
When the EXTVCC pin is left open or tied to a voltage less
than 4.7V, an internal 5.25V low dropout linear regulator
supplies INTVCC power from VIN. If EXTVCC is taken above
4.7V, the 5.25V regulator is turned off and a 7.5V low
dropout linear regulator is enabled that supplies INTVCC
power from EXTVCC. If EXTVCC is less than 7.5V (but
greater than 4.7V), the 7.5V regulator is in dropout and
INTVCC is approximately equal to EXTVCC. When EXTVCC
is greater than 7.5V (up to an absolute maximum rating
of 10V), INTVCC is regulated to 7.5V. Using the EXTVCC
pin allows the INTVCC power to be derived from a high
efficiency external source such as one of the LTC3827
switching regulator outputs.
Each top MOSFET driver is biased from the floating boot-
strap capacitor, CB, which normally recharges during each
off cycle through an external diode when the top MOSFET
turns off. If the input voltage VIN decreases to a voltage
close to VOUT, the loop may enter dropout and attempt
to turn on the top MOSFET continuously. The dropout
detector detects this and forces the top MOSFET off for
about one twelfth of the clock period every tenth cycle to
allow CB to recharge.
Shutdown and Start-Up (RUN1, RUN2 and
TRACK/SS1, TRACK/SS2 Pins)
The two channels of the LTC3827 can be independently
shut down using the RUN1 and RUN2 pins. Pulling either
of these pins below 0.7V shuts down the main control
loop for that controller. Pulling both pins low disables
both controllers and most internal circuits, including the
INTVCC regulator, and the LTC3827 draws only 8μA of
quiescent current.
Releasing either RUN pin allows an internal 0.5μA current
to pull up the pin and enable that controller. Alternatively,
the RUN pin may be externally pulled up or driven directly
by logic. Be careful not to exceed the Absolute Maximum
rating of 7V on this pin.
The start-up of each controller’s output voltage VOUT is
controlled by the voltage on the TRACK/SS1 and TRACK/
SS2 pin. When the voltage on the TRACK/SS pin is less
than the 0.8V internal reference, the LTC3827 regulates
the VFB voltage to the TRACK/SS pin voltage instead of the
0.8V reference. This allows the TRACK/SS pin to be used
to program a soft-start by connecting an external capacitor
from the TRACK/SS pin to SGND. An internal 1μA pull-up
current charges this capacitor creating a voltage ramp on
the TRACK/SS pin. As the TRACK/SS voltage rises linearly
from 0V to 0.8V (and beyond), the output voltage VOUT
rises smoothly from zero to its final value.
Alternatively the TRACK/SS pin can be used to cause the
start-up of VOUT to “track” that of another supply. Typi-
cally, this requires connecting to the TRACK/SS pin an
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11
11 Page | ||
| Páginas | Total 30 Páginas | |
| PDF Descargar | [ Datasheet LTC3827.PDF ] | |
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