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PDF LTC3868 Data sheet ( Hoja de datos )
| Número de pieza | LTC3868 | |
| Descripción | Low IQ Dual 2-Phase Synchronous Step-Down Controller | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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Features
n Low Operating IQ: 170µA (One Channel On)
n Wide Output Voltage Range: 0.8V ≤ VOUT ≤ 14V
n Wide VIN Range: 4V to 24V
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 to 850kHz)
n Programmable Fixed Frequency (50kHz to 900kHz)
n Selectable Continuous, Pulse-Skipping or
Burst Mode® Operation at Light Loads
n Very Low Dropout Operation: 99% Duty Cycle
n Adjustable Output Voltage Soft-Start
n Power Good Output Voltage Monitor
n Output Overvoltage Protection
n Output Latchoff Protection During Short Circuit
n Low Shutdown IQ: 8µA
n Internal LDO Powers Gate Drive from VIN or EXTVCC
n No Current Foldback During Start-Up
n Small 5mm × 5mm QFN Package
Applications
n Notebook and Palmtop Computers
n Portable Instruments
n Battery Operated Digital Devices
n Distributed DC Power Systems
LTC3868www.DataSheet4U.com
Low IQ, Dual
2-Phase Synchronous
Step-Down Controller
Description
The LTC®3868 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
input capacitor ESR are minimized by operating the two
controller outputs out of phase.
The 170μA 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
LTC3868 features a precision 0.8V reference and a power
good output indicator. A wide 4V to 24V input supply range
encompasses a wide range of intermediate bus voltages
and battery chemistries.
Independent soft-start pins for each controller ramp the
output voltages during start-up. Current foldback limits
MOSFET heat dissipation during short-circuit conditions.
The output short-circuit latchoff feature further protects
the circuit in short-circuit conditions.
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 LTC3868-1 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, 5705919, 5929620, 6100678, 6144194, 6177787, 6304066, 6580258.
Typical Application
VOUT1
3.3V
5A
High Efficiency Dual 8.5V/3.3V Step-Down Converter
3.3µH
0.1µF
VIN
TG1
BOOST1
SW1
INTVCC
TG2
BOOST2
SW2
4.7µF
22µF
50V
VIN
9V TO 24V
0.1µF
7.2µH
BG1 BG2
LTC3868
PGND
SENSE1+
SENSE2+
0.007Ω
62.5k
150µF
680pF
SENSE1–
VFB1
ITH1
SS1
SGND
SENSE2–
VFB2
ITH2
SS2
20k 15k
0.1µF
0.1µF
680pF
15k
0.01Ω
193k
20k
VOUT2
8.5V
3.5A
150µF
3868 TA01
Efficiency and Power Loss
vs Load Current
100 10000
90
80 EFFICIENCY
70
1000
60 100
50 POWER LOSS
40
30
20
10
0
0.0001
10
0.001 0.01
VIN = 12V
1
VOUT = 3.3V
FIGURE 12 CIRCUIT 0.1
0.1 1
10
OUTPUT CURRENT (A)
3868 TA01b
3868fb
1 page  
Typical Performance Characteristics
LTC3868www.DataSheet4U.com
Efficiency and Power Loss
vs Output Current
100
FIGURE 12 CIRCUIT
90 VIN = 12V
80 VOUT = 3.3V
10000
1000
70
60 100
50
40 10
30
20
10
0
0.0001
0.001 0.01
Burst Mode
OPERATION
PULSE-
1
SKIPPING
FCM
0.1
0.1 1 10
OUTPUT CURRENT (A)
3868 G01
Efficiency vs Output Current
100
90
80
VIN = 5V
70 VIN = 12V
60
50
40
30
20
10
0
0.0001
VOUT = 3.3V
FIGURE 12 CIRCUIT
0.001 0.01 0.1
1
OUTPUT CURRENT (A)
10
3868 G02
Efficiency vs Input Voltage
98 FIGURE 12 CIRCUIT
96 VOUT = 3.3V
94 IOUT = 4A
92
Load Step (Burst Mode Operation)
VOUT
100mV/DIV
AC-
COUPLED
90
88
86
IL
2A/DIV
84
82
80
0 5 10 15 20 25 28
INPUT VOLTAGE (V)
VOUT = 3.3V
20µs/DIV
FIGURE 12 CIRCUIT
3868 G04
3868 G03
Load Step
(Forced Continuous Mode)
VOUT
100mV/DIV
AC-
COUPLED
IL
2A/DIV
VOUT = 3.3V
20µs/DIV
FIGURE 12 CIRCUIT
Load Step (Pulse-Skipping Mode)
Inductor Current at Light Load
Soft Start-Up
3868 G05
VOUT
100mV/DIV
AC-
COUPLED
IL
2A/DIV
VOUT = 3.3V
20µs/DIV
FIGURE 12 CIRCUIT
FORCED
CONTINUOUS
MODE
Burst Mode
OPERATION
2A/DIV
3868 G06
PULSE-
SKIPPING
MODE
VOUT = 3.3V
2µs/DIV
ILOAD = 200µA
FIGURE 12 CIRCUIT
VOUT2
2V/DIV
VOUT1
2V/DIV
3868 G07
20ms/DIV
FIGURE 12 CIRCUIT
3868 G08
3868fb
5 Page 
Operation (Refer to the Functional Diagram)
The LTC3868 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 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 LTC3868 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.
LTC3868www.DataSheet4U.com
Shutdown and Start-Up (RUN1, RUN2
and SS1, SS2 Pins)
The two channels of the LTC3868 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 LTC3868 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 SS pin for that channel.
When the voltage on the SS pin is less than the 0.8V internal
reference, the LTC3868 regulates the VFB voltage to the SS
pin voltage instead of the 0.8V reference. This allows the
SS pin to be used to program a soft-start by connecting
an external capacitor from the SS pin to SGND. An internal
1µA pull-up current charges this capacitor creating a volt-
age ramp on the SS pin. As the 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.
Short-Circuit Latchoff
After the controller has been started and been given ad-
equate time to ramp up the output voltage, the SS capaci-
tor is used in a short-circuit timeout circuit. Specifically,
once the voltage on the SS pin rises above 2V (the arming
threshold), the short-circuit timeout circuit is enabled (see
Figure 1). If the output voltage falls below 70% of its nomi-
nal regulated voltage, the SS capacitor begins discharg-
ing with a net 9µA pulldown current on the assumption
that the output is in an overcurrent and/or short-circuit
condition. If the condition lasts long enough to allow the
SS pin voltage to fall below 1.5V (the latchoff threshold),
the controller will shut down (latch off) until the RUN pin
voltage or the VIN voltage is recycled.
3868fb
11
11 Page | ||
| Páginas | Total 30 Páginas | |
| PDF Descargar | [ Datasheet LTC3868.PDF ] | |
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