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PDF LTC3857 Data sheet ( Hoja de datos )
| Número de pieza | LTC3857 | |
| Descripción | Synchronous Step-Down Controller | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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Features
LTC3857www.DataSheet4U.com
Low IQ, Dual, 2-Phase
Synchronous Step-Down
Description
Controller
n Low Operating IQ: 50µA (One Channel On)
n Wide Output Voltage Range: 0.8V ≤ VOUT ≤ 24V
n Wide VIN Range: 4V to 38V (40V Abs Max)
n RSENSE or DCR Current Sensing
n Out-of-Phase Controllers Reduce Required Input
Capacitance and Power Supply Induced Noise
n OPTI-LOOP® Compensation Minimizes COUT
n Phase-Lockable Frequency (75kHz-850kHz)
n Programmable Fixed Frequency (50kHz-900kHz)
n Selectable Continuous, Pulse-Skipping or Low Ripple
Burst Mode® Operation at Light Loads
n Selectable Current Limit
n Very Low Dropout Operation: 99% Duty Cycle
n Adjustable Output Voltage Soft-Start or Tracking
n Power Good Output Voltage Monitors
n Output Overvoltage Protection
n Low Shutdown IQ : <8µA
n Internal LDO Powers Gate Drive from VIN or EXTVCC
n No Current Foldback During Start-up
n Small Low Profile (0.75mm) 5mm × 5mm QFN Package
Applications
n Automotive Always-On Systems
n Battery Operated Digital Devices
n Distributed DC Power Systems
The LTC®3857 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 850kHz. 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 50μA no-load quiescent current extends operating run
time in battery-powered systems. The LTC3857 features a
precision 0.8V reference and power good output indicators. A
wide 4V to 38V input supply range encompasses a wide range
of intermediate bus voltages and battery chemistries.
Independent TRACK/SS pins for each controller ramp the
output voltages 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 current
mode at light loads.
For a leaded 28-lead SSOP package with a fixed current
limit and one PGOOD output, without phase modulation
or a clock output, see the LTC3857-1 data sheet.
L, LT, LTC, LTM, Burst Mode, OPTI-LOOP, PolyPhase, µModule, Linear Technology and the
Linear logo are registered trademarks and No RSENSE and UltraFast are 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, 5705919, 6100678.
Typical Application
High Efficiency Dual 3.3V/8.5V Step-Down Converter
4.7µF
VIN
22µF 9V TO 38V
50V
VIN
TG1
INTVCC
TG2
3.3µH
0.1µF
BOOST1
BOOST2
0.1µF
7.2µH
SW1 SW2
BG1 BG2
LTC3857
PGND
SENSE1+
SENSE2+
VOUT1
3.3V
5A
0.007Ω
62.5k
150µF
680pF
SENSE1–
VFB1
ITH1
SENSE2–
VFB2
ITH2
TRACK/SS1 SGND TRACK/SS2
20k 15k
0.1µF
0.1µF
680pF
15k
0.010Ω
193k
20k
VOUT2
8.5V
3.5A
150µF
3857 TA01
Efficiency and Power Loss
vs Output Current
100
90
VVIONU=T
12V
= 3.3V
80 FIGURE 13 CIRCUIT
70
10000
1000
60 100
50
40 10
30
20 1
10
0
0.00001 0.0001 0.001 0.01 0.1
OUTPUT CURRENT (A)
0.1
1 10
3857 TA01b
3857fa
1 page  
Typical Performance Characteristics
LTC3857www.DataSheet4U.com
Efficiency and Power Loss
vs Output Current
100
90
VIN = 12V
VOUT = 3.3V
80 FIGURE 13 CIRCUIT
10000
1000
70
60 100
50
40 10
30
20 1
10
0 0.1
0.00001 0.0001 0.001 0.01 0.1 1 10
OUTPUT CURRENT (A)
38 57 G01
BURST EFFICIENCY
PULSE-SKIPPING
EFFICIENCY
CCM EFFICIENCY
BURST LOSS
PULSE-SKIPPING
LOSS
CCM LOSS
Load Step (Burst Mode Operation)
Efficiency vs Output Current
100
90 VIN = 5V
80
70
60
VIN = 12V
50
40
30
20
10
0
0.00001 0.0001 0.001
VOUT = 3.3V
FIGURE 13 CIRCUIT
0.01 0.1 1 10
OUTPUT CURRENT (A)
3857 G02
Efficiency vs Input Voltage
98 VOUT = 3.3V
96 ILOAD = 5A
94
92
90
88
86
84
82
80
1 5 10 15 20 25 30 35 40
INPUT VOLTAGE (V)
3857 G03
Load Step
(Forced Continuous Mode)
Load Step
(Pulse-Skipping Mode)
VOUT
100mV/DIV
VOUT
100mV/DIV
VOUT
100mV/DIV
INDUCTOR
CURRENT
2A/DIV
INDUCTOR
CURRENT
2A/DIV
VIN = 12V
20µs/DIV
VOUT = 3.3V
FIGURE 13 CIRCUIT
3857 G04
Inductor Current at Light Load
VIN = 12V
20µs/DIV
VOUT = 3.3V
FIGURE 13 CIRCUIT
Soft Start-Up
INDUCTOR
CURRENT
2A/DIV
3857 G05
VIN = 12V
20µs/DIV
VOUT = 3.3V
FIGURE 13 CIRCUIT
Tracking Start-Up
3857 G06
FORCED
CONTINUOUS
MODE
Burst Mode
OPERATION
2A/DIV
PULSE-
SKIPPING MODE
VIN = 12V
5µs/DIV
VOUT = 3.3V
ILOAD = 200µA
FIGURE 13 CIRCUIT
VOUT2
2V/DIV
VOUT1
2V/DIV
VOUT2
2V/DIV
VOUT1
2V/DIV
3857 G07
20ms/DIV
FIGURE 13 CIRCUIT
3857 G08
20ms/DIV
FIGURE 13 CIRCUIT
3857 G09
3857fa
5 Page 
LTC3857www.DataSheet4U.com
Operation (Refer to the Functional Diagram)
Main Control Loop
The LTC3857 uses a constant frequency, current mode
step-down architecture with the two controller channels
operating 180 degrees out of phase. During normal
operation, 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 resistor
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, the VIN LDO (low dropout linear regulator) supplies
5.1V from VIN to INTVCC. If EXTVCC is taken above 4.7V,
the VIN LDO is turned off and an EXTVCC LDO is turned on.
Once enabled, the EXTVCC LDO supplies 5.1V from EXTVCC
to INTVCC. Using the EXTVCC pin allows the INTVCC power
to be derived from a high efficiency external source such
as one of the LTC3857 switching regulator outputs.
Each top MOSFET driver is biased from the floating
bootstrap capacitor, CB, which normally recharges during
each 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 LTC3857 can be independently
shut down using the RUN1 and RUN2 pins. Pulling either of
these pins below 1.26V shuts down the main control loop
for that controller. Pulling both pins below 0.7V disables
both controllers and most internal circuits, including the
INTVCC LDOs. In this state, the LTC3857 draws only 8µA
of quiescent current.
The RUN pin may be externally pulled up or driven directly
by logic. When driving the RUN pin with a low impedance
source, do not exceed the absolute maximum rating of
8V. The RUN pin has an internal 11V voltage clamp that
allows the RUN pin to be connected through a resistor to a
higher voltage (for example, VIN), so long as the maximum
current into the RUN pin does not exceed 100µA.
The start-up of each controller’s output voltage VOUT is
controlled by the voltage on the TRACK/SS pin for that
channel. When the voltage on the TRACK/SS pin is less
than the 0.8V internal reference, the LTC3857 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 up to the absolute maximum
rating of 6V), 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. Typically,
this requires connecting to the TRACK/SS pin an external
resistor divider from the other supply to ground (see
Applications Information section).
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11
11 Page | ||
| Páginas | Total 30 Páginas | |
| PDF Descargar | [ Datasheet LTC3857.PDF ] | |
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