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PDF LTC3819 Data sheet ( Hoja de datos )
| Número de pieza | LTC3819 | |
| Descripción | Step-Down Controller | |
| Fabricantes | Linear Technology | |
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LTC3819
2-Phase, High Efficiency,
Step-Down Controller for
Sun Server CPUs
FEATURES
■ Output Stages Operate Antiphase Reducing Input
and Output Capacitance Requirements and Power
Supply Induced Noise
■ Dual Input Supply Capability for Load Sharing
■ 5-Bit SUN CPU VID Code:
VOUT = 1.025V to 1.4125V
■ ±1% Output Voltage Accuracy
■ True Remote Sensing Differential Amplifier
■ Power Good Output Voltage Monitor
■ Supports Active Voltage Positioning
■ Current Mode Control Ensures Current Sharing
■ OPTI-LOOP® Compensation Minimizes COUT
■ Three Operational Modes: PWM, Burst and Cycle Skip
■ Programmable Fixed Frequency: 150kHz to 300kHz
■ Wide VIN Range: 4V to 36V Operation
■ Adjustable Soft-Start Current Ramping
■ Internal Current Foldback and Short-Circuit Shutdown
■ Overvoltage Soft Latch Eliminates Nuisance Trips
■ Available in 36-LUead Narrow (0.209) SSOP Package
APPLICATIO S
■ Sun™ Server CPU Supply
, LTC and LT are registered trademarks of Linear Technology Corporation. OPTI-LOOP
and Burst Mode 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, 6100678, 5408150, 6580258, 6304066, 5705919.
DESCRIPTIO
The LTC®3819 is a 2-phase, VID programmable, synchro-
nous step-down switching regulator controller that drives
two N-channel external power MOSFET stages in a fixed fre-
quency architecture. The 2-phase controller drives its two
output stages out of phase at frequencies up to 300kHz to
minimize the RMS ripple currents in both input and output
capacitors. The 2-phase technique effectively multiplies the
fundamental frequency by two, improving transient re-
sponse while operating each channel at an optimum fre-
quency for efficiency. Thermal design is also simplified.
An operating mode select pin (FCB) can be used to select
among three modes including Burst Mode® operation for
highest efficiency. An internal differential amplifier provides
true remote sensing of the regulated supply’s positive and
negative output terminals as required in high current ap-
plications.
The RUN/SS pin provides soft-start and optional timed,
short-circuit shutdown. Current foldback limits MOSFET
dissipation during short-circuit conditions when the
overcurrent latchoff is disabled. OPTI-LOOP compensation
allows the transient response to be optimized for a wide
range of output capacitors and ESR values.
TYPICAL APPLICATIO
0.1µF FCB
VIN TG1
S
220pF
3.3k
RUN/SS
BOOST1
SW1
LTC3819
ITH BG1
0.47µF
S
SGND
PGND
PGOOD
5 VID BITS VID0–VID4
SENSE1+
SENSE1–
TG2
EAIN
ATTENOUT
BOOST2
SW2
0.47µF
ATTENIN
BG2
VDIFFOUT
VOS–
VOS+
INTVCC
SENSE2+
SENSE2–
10µF
10µF
35V
×6
1µH
D1
0.002Ω
1µH
D2
0.002Ω
Figure 1. High Current Dual Phase Step-Down Converter
VIN
5V TO 28V
VDD_CORE
1.025V TO 1.4125V
45A
+ COUT
270µF
2V
×4
3716 F01
3819f
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TYPICAL PERFOR A CE CHARACTERISTICS
Efficiency vs Load Current
(3 Operating Modes) (Figure 12)
100
Burst Mode
90 OPERATION
80
70
FORCED
60 CONTINUOUS
50 MODE
40
30
20
10
0
0.01
CONSTANT
FREQUENCY
(BURST DISABLED)
VIN = 5V
VOUT = 1.4125V
FREQ = 200kHz
0.1 1 10
LOAD CURRENT (A)
100
3819 G01
Supply Current vs Input Voltage
and Mode
1000
ON
800
600
400
200
SHUTDOWN
0
0 5 10 15 20 25 30 35
INPUT VOLTAGE (V)
3819 G04
Efficiency vs Load Current
(Figure 12)
100
80
60
40
20
0
0.1
VIN = 5V
VIN = 8V
VIN = 12V
VIN = 20V
VOUT = 1.4125V
VEXTVCC = 0V
FREQ = 200kHz
VFCB = 0V
1 10
LOAD CURRENT (A)
100
3819 G02
EXTVCC Voltage Drop
250
200
150
100
50
0
0 10 20 30 40 50
CURRENT (mA)
3819 G05
Internal 5V LDO Line Reg
5.1
ILOAD = 1mA
5.0
4.9
4.8
4.7
4.6
4.5
4.4
0
5 10 15 20 25 30 35
INPUT VOLTAGE (V)
3819 G07
Maximum Current Sense Threshold
vs Duty Factor
75
50
25
0
0 20 40 60 80 100
DUTY FACTOR (%)
3819 G08
LTC3819
Efficiency vs Input Voltage
(Figure 12)
100
IOUT = 20A
VOUT = 1.4125V
90
80
70
60
50
5
10 15
INPUT VOLTAGE (V)
20
3819 G03
INTVCC and EXTVCC Switch
Voltage vs Temperature
5.05
INTVCC VOLTAGE
5.00
4.95
4.90
4.85
4.80
EXTVCC SWITCHOVER THRESHOLD
4.75
4.70
– 50 – 25
0 25 50 75
TEMPERATURE (°C)
100 125
3819 G06
Maximum Current Sense Threshold
vs Percent of Nominal Output
Voltage (Foldback)
80
70
60
50
40
30
20
10
0
0 25 50 75 100
PERCENT OF NOMINAL OUTPUT VOLTAGE (%)
3819 G09
3819f
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OPERATIO (Refer to Functional Diagram)
Main Control Loop
The LTC3819 uses a constant frequency, current mode
step-down architecture with the two output stages oper-
ating 180 degrees out of phase. During normal operation,
each top MOSFET is turned on when the clock for that
channel sets the RS latch, and turned off when the main
current comparator, I1, resets the RS latch. The peak
inductor current at which I1 resets the RS latch is con-
trolled by the voltage on the ITH pin, which is the output of
error amplifier EA. The EAIN pin receives the voltage
feedback signal, which is compared to the internal refer-
ence voltage by the EA. When the load current increases,
it causes a slight decrease in VEAIN relative to the 0.6V
reference, which in turn causes the ITH voltage to increase
until the average inductor current matches the new load
current. After the top MOSFET has turned off, the bottom
MOSFET is turned on until either the inductor current
starts to reverse, as indicated by current comparator I2, or
the beginning of the next cycle.
The top MOSFET drivers are biased from floating boot-
strap capacitor CB, which normally is recharged during
each off cycle through an external diode when the top
MOSFET turns off. As 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 de-
tects this and forces the top MOSFET off for about 500ns
every tenth cycle to allow CB to recharge.
The main control loop is shut down by pulling the RUN/
SS pin low. Releasing RUN/SS allows an internal 1.2µA
current source to charge soft-start capacitor CSS. When
CSS reaches 1.5V, the main control loop is enabled with
the ITH voltage clamped at approximately 30% of its
maximum value. As CSS continues to charge, the ITH pin
voltage is gradually released allowing normal, full-current
operation.
Low Current Operation
The FCB pin selects between two modes of low current
operation. When the FCB pin voltage is below 0.6V, the
LTC3819
controller forces continuous PWM current mode opera-
tion. In this mode, the top and bottom MOSFETs are
alternately turned on to maintain the output voltage inde-
pendent of direction of inductor current. When the FCB pin
is below VINTVCC – 2V but greater than 0.6V, the controller
enters Burst Mode operation. Burst Mode operation sets
a minimum output current level before inhibiting the top
switch and turns off the synchronous MOSFET(s) when
the inductor current goes negative. This combination of
requirements will, at low currents, force the ITH pin below
a voltage threshold that will temporarily inhibit turn-on of
both output MOSFETs until the output voltage drops.
There is 60mV of hysteresis in the burst comparator B tied
to the ITH pin. This hysteresis produces output signals to
the MOSFETs that turn them on for several cycles, fol-
lowed by a variable “sleep” interval depending upon the
load current. The resultant output voltage ripple is held to
a very small value by having the hysteretic comparator
after the error amplifier gain block.
Constant Frequency Operation
When the FCB pin is tied to INTVCC, Burst Mode operation
is disabled and a forced minimum peak output current
requirement is removed. This provides constant frequency,
discontinuous (preventing reverse inductor current) cur-
rent operation over the widest possible output current
range. This constant frequency operation is not as efficient
as Burst Mode operation, but does provide a lower noise,
constant frequency operating mode down to approxi-
mately 1% of designed maximum output current.
Continuous Current (PWM) Operation
Tying the FCB pin to ground will force continuous current
operation. This is the least efficient operating mode, but
may be desirable in certain applications. The output can
source or sink current in this mode. When sinking current
while in forced continuous operation, current will be
forced back into the main power supply potentially boost-
ing the input supply to dangerous voltage levels—
BEWARE!
3819f
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| Páginas | Total 30 Páginas | |
| PDF Descargar | [ Datasheet LTC3819.PDF ] | |
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