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PDF LTC3890-3 Data sheet ( Hoja de datos )
| Número de pieza | LTC3890-3 | |
| Descripción | 2-Phase Synchronous Step-Down DC/DC Controller | |
| Fabricantes | Linear | |
| Logotipo |  |
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Features
n Wide VIN Range: 4V to 60V (65V Abs Max)
n Low Operating IQ: 50µA (One Channel On)
n Wide Output Voltage Range: 0.8V ≤ VOUT ≤ 24V
n RSENSE or DCR Current Sensing
n Out-of-Phase Controllers Reduce Input Capacitance
and Power Supply Induced Noise
n Phase-Lockable Frequency (75kHz to 850kHz)
n Programmable Fixed Frequency (50kHz to 900kHz)
n Selectable Continuous, Pulse-Skipping or Low Ripple
Burst Mode® Operation at Light Loads
n Very Low Dropout Operation: 99% Duty Cycle
n Adjustable Output Voltage Soft-Start or Tracking
n Power Good Output Voltage Monitor
n Low Shutdown IQ: <14µA
n Internal LDO Powers Gate Drive from VIN or EXTVCC
n Narrow SSOP Package
Applications
n Automotive Always-On Systems
n Battery Operated Digital Devices
n Distributed DC Power Systems
L, LT, LTC, LTM, Linear Technology, Burst Mode, OPTI-LOOP, PolyPhase and the Linear logo
are registered trademarks of Linear Technology Corporation. All other trademarks are the
property of their respective owners. Protected by U.S. Patents including 5481178, 5705919,
5929620, 6100678, 6144194, 6177787, 6304066, 6580258, 7230497.
LTC3890-3
60V Low IQ, Dual, 2-Phase
Synchronous Step-Down
DC/DC Controller
Description
The LTC®3890-3 is a high performance dual step-down
switching regulator DC/DC controller that drives all
N-channel synchronous power MOSFET stages. A constant
frequency current mode architecture allows a phase-
lockable frequency of up to 850kHz. Power loss and supply
noise are minimized by operating the two controller output
stages out-of-phase.
The 50μA no-load quiescent current extends operating life
in battery-powered systems. OPTI-LOOP® compensation
allows the transient response to be optimized over a wide
range of output capacitance and ESR values. A wide 4V
to 60V 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 mode control limits
the inductor current during short-circuit conditions. The
PLLIN/MODE pin selects among Burst Mode operation,
pulse-skipping mode, or continuous conduction mode at
light loads.
For versions with different and/or additional features, see
the LTC3890 family summary, Table 1, in the Pin Functions
section of this data sheet.
Typical Application
High Efficiency Dual 8.5V/3.3V Output Step-Down Converter
VOUT1
3.3V
5A
4.7µH
0.008Ω
100k
470µF
0.1µF
VIN
TG1
INTVCC
TG2
BOOST1
BOOST2
SW1 SW2
BG1 BG2
LTC3890-3 PGND
SENSE1+
SENSE2+
31.6k
1000pF
SENSE1–
VFB1
ITH1
SENSE2–
VFB2
ITH2
TRACK/SS1 SGND TRACK/SS2
34.8k 0.1µF
0.1µF
4.7µF
VIN
9V TO 60V
22µF
0.1µF
8µH
1000pF
34.8k
0.01Ω
100k
10.5k
VOUT2
8.5V
3A
330µF
38903 TA01a
For more information www.linear.com/3890-3
Efficiency and Power Loss
vs Output Current
100 VIN = 12V
90 VOUT = 3.3V
80
10000
1000
70
60 100
50
40 10
30
20 1
10
0
0.0001
0.001 0.01 0.1
1
OUTPUT CURRENT (A)
0.1
10
38903 TA01b
38903f
1
1 page  
Typical Performance Characteristics
Efficiency and Power Loss
vs Output Current
100 VIN = 12V
BURST EFFICIENCY
90 VOUT = 3.3V
80
10000
1000
70 FCM LOSS
60
100
50
40
PULSE-SKIPPING
LOSS
BURST LOSS
10
30
20
FCM EFFICIENCY
1
10
0
0.0001
PULSE-SKIPPING
EFFICIENCY
0.001 0.01 0.1
1
OUTPUT CURRENT (A)
0.1
10
FIGURE 13 CIRCUIT
38903 G01
Efficiency vs Output Current
100
90 VOUT = 8.5V
80
VOUT = 3.3V
70
60
50
40
30
20
Burst Mode OPERATION
10
0
VIN = 12V
0.0001 0.001 0.01 0.1
OUTPUT CURRENT (A)
1
10
FIGURE 13 CIRCUIT
38903 G02
LTC3890-3
Efficiency vs Input Voltage
100
98
96 VOUT2 = 8.5V
94
92
90
88 VOUT1 = 3.3V
86
84
82
80
ILOAD = 2A
0 5 10 15 20 25 30 35 40 45 50 55 60
INPUT VOLTAGE (V)
FIGURE 13 CIRCUIT
38903 G03
Load Step
Burst Mode Operation
VOUT
100mV/DIV
AC-
COUPLED
Load Step
Pulse-Skipping Mode
VOUT
100mV/DIV
AC-
COUPLED
Load Step
Forced Continuous Mode
VOUT
100mV/DIV
AC-
COUPLED
IL
2A/DIV
50µs/DIV
VIN = 12V
VOUT = 3.3V
FIGURE 13 CIRCUIT
38903 G04
IL
2A/DIV
50µs/DIV
VIN = 12V
VOUT = 3.3V
FIGURE 13 CIRCUIT
38903 G05
IL
2A/DIV
50µs/DIV
VIN = 12V
VOUT = 3.3V
FIGURE 13 CIRCUIT
38903 G06
Inductor Current at Light Load
FORCED
CONTINUOUS
MODE
Burst Mode
OPERATION
1A/DIV
PULSE-SKIPPING
MODE
VIN = 12V
VOUT = 3.3V
ILOAD = 200µA
5µs/DIV
38903 G07
Soft Start-Up
VOUT2
2V/DIV
VOUT1
2V/DIV
2ms/DIV
FIGURE 13 CIRCUIT
38903 G08
Tracking Start-Up
VOUT2
2V/DIV
VOUT1
2V/DIV
2ms/DIV
FIGURE 13 CIRCUIT
38903 G09
For more information www.linear.com/3890-3
38903f
5
5 Page 
LTC3890-3
Operation (Refer to the Functional Diagram)
Main Control Loop
The LTC3890-3 uses a constant frequency, current mode
step-down architecture 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 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 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 LTC3890-3 switching regulator outputs.
Each top MOSFET driver is biased from the floating boot-
strap 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 LTC3890-3 can be independently
shut down using the RUN1 and RUN2 pins. Pulling either of
these pins below 1.15V 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 LTC3890-3 draws only
14µA of quiescent current.
Releasing either RUN pin allows a small internal current to
pull up the pin to enable that controller. The RUN1 pin has a
7µA pull-up current while the RUN2 pin has a smaller 0.5µA.
The 7µA current on RUN1 is designed to be large enough
so that the RUN1 pin can be safely floated (to always en-
able the controller) without worry of condensation or other
small board leakage pulling the pin down. This is ideal for
always-on applications where one or both controllers are
enabled continuously and never shut down.
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 LTC3890-3 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 5V), 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 the
Applications Information section).
For more information www.linear.com/3890-3
38903f
11
11 Page | ||
| Páginas | Total 30 Páginas | |
| PDF Descargar | [ Datasheet LTC3890-3.PDF ] | |
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