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PDF LTC3867 Data sheet ( Hoja de datos )
| Número de pieza | LTC3867 | |
| Descripción | Synchronous Step-Down DC/DC Controller | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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FEATURES
n VIN Range: 4V to 38V
n VOUT Range: 0.6V to 14V
n Nonlinear Control Architecture Minimizes Output
Transient Excursions (Optional)
n Programmable DCR Temperature Compensation
n ±0.75% 0.6V Voltage Reference
n Fixed Frequency Range of 200kHz to 1.2MHz
n PLL Frequency Synchronization
n RSENSE or DCR Current Sensing
n Differential Remote Output Voltage Sense
n Supports Smooth Start-Up into Pre-Biased Outputs
n Programmable Soft-Start or VOUT Tracking
n Hiccup Mode/Soft Recovery from Output Overcurrent
n 24-Lead (4mm × 4mm) QFN Package
APPLICATIONS
n Automotive Systems
n Telecom Systems
n Industrial Equipment
n Distributed DC Power Systems
L, LT, LTC, LTM, Burst Mode, OPTI-LOOP, Linear Technology 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, 6177787,
6580258, 6498466, 6611131.
LTC3867
Synchronous Step-Down
DC/DC Controller with
Differential Remote Sense
and Nonlinear Control
DESCRIPTION
The LTC®3867 is a current mode synchronous step-down
switching regulator controller that drives all N-channel
power MOSFET stages. Output voltage transient excursions
are minimized by use of a nonlinear control architecture
to eliminate clock latency issues.
The maximum current sense voltage is programmable
from 30mV to 75mV, allowing the use of either a discrete
sense resistor or the inductor DCR as the sensing element.
Programmable DCR temperature compensation allows
constant current limit regardless of inductor tempera-
ture. Programmable inductor temperature-based thermal
shutdown protects the power components from thermal
stress. Soft recovery from output shorts or overcurrent
minimizes output overshoot.
The LTC3867 features a precision 0.6V reference and
can regulate output voltages up to 14V from a wide
4V to 38V input supply range. The LTC3867 includes
a high speed differential remote sense amplifier. Burst
Mode® operation, continuous and pulse-skipping modes
are supported. The LTC3867 is available in a 24-lead
(4mm × 4mm) QFN package.
TYPICAL APPLICATION
High Efficiency Synchronous Step-Down Controller
0.1µF
37.4k
INTVCC
330pF
49.9k
100pF
DIFF+
DIFF–
ITEMP
ITSD
DIFFOUT
ILIM
TK/SS
FREQ
PGOOD
RUN
EXTVCC
VIN
MODE/PLLIN TG
IFAST
LTC3867
ITH BOOST
SW
49.9k
SGND INTVCC
VFB
75k
SENSE–
BG
SENSE+ PGND
SGND
INTVCC
0.1µF
DB
+
4.7µF
22µF
50V
VIN
5V TO 18V
0.4µH
3.3k 0.22µF
VOUT
1.5V
10A
330µF ×2
+100µF
3867 TA01a
Efficiency and Power Loss
100
90
80
70
60
50
40
30
20
10
0
0.01
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.1 1 10
LOAD CURRENT (A)
0
100
3867 TA01b
3867f
1
1 page  
LTC3867
E LECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the specified operating
junction temperature range, otherwise specifications are at TA = 25°C (Note 2). VIN = 15V, VRUN = 5V unless otherwise specified.
SYMBOL
PARAMETER
CONDITIONS
MIN TYP MAX UNITS
TG
TG
ttrf
BG tr
BG tf
TG/BG t1D
TG Transition Time
Rise Time
Fall Time
BG Transition Time
Rise Time
Fall Time
Top Gate Off to Bottom Gate On Delay,
Synchronous Switch-On Delay Time
CCLLOOAADD
=
=
3300pF
3300pF
(Note
5)
CLOAD = 3300pF
CLOAD = 3300pF (Note 5)
CLOAD = 3300pF Each Driver
25 ns
25 ns
25 ns
25 ns
30 ns
BG/TG t2D
Bottom Gate Off to Top Gate On Delay, CLOAD = 3300pF Each Driver
Top Switch-On Delay Time
30 ns
Thermal Shutdown
IITSD
VITSD
Source Current
Comparator Trip Point
IITSD = 500mV
20 µA
950 mV
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: The LTC3867 is tested under pulsed load conditions such that
TJ ≈ TA. The LTC3867E is guaranteed to meet performance specifications
from 0°C to 85°C operating junction temperature. Specifications over
the –40°C to 125°C operating junction temperature range are assured by
design, characterization and correlation with statistical process controls.
The LTC3867I is guaranteed to meet performance specifications over the
full –40°C to 125°C operating junction temperature range. The maximum
ambient temperature consistent with these specifications is determined
by specific operating conditions in conjunction with board layout, the
package thermal impedance and other environmental factors.
TJ is calculated from the ambient temperature, TA, and power dissipation,
PD, according to the following formula:
LTC3867UF: TJ = TA + (PD • 47°C/W)
Note 3: The LTC3867 is tested in a feedback loop that servos VITH to a
specified voltage and measures the resultant VFB.
Note 4: Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency. See Applications Information.
Note 5: Rise and fall times are measured using 10% and 90% levels. Delay
times are measured using 50% levels.
Note 6: The minimum on-time condition corresponds to the on inductor
peak-to-peak ripple current ≥40% of IMAX (see Minimum On-Time
Considerations in the Applications Information section).
Note 7: Guaranteed by design.
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5 Page 
LTC3867
OPERATION
The VFB pin receives this feedback signal and compares
it to the internal 0.6V reference. When the load current
increases, it causes a slight decrease in the VFB pin voltage
relative to the 0.6V reference, which in turn causes the
ITH voltage to increase until the inductor’s average current
equals 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 the
reverse current comparator, IREV , or the beginning of the
next cycle.
The main control loop is shut down by pulling the RUN
pin low. Releasing RUN allows an internal 1.0µA current
source to pull up the RUN pin. When the RUN pin reaches
1.22V, the main control loop is enabled and the IC is pow-
ered up. When the RUN pin is low, all functions are kept
in a controlled state.
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 5.3V linear regulator supplies INTVCC
power from VIN. If EXTVCC is taken above 4.7V, the 5.3V
regulator is turned off and an internal switch is turned on
connecting EXTVCC to INTVCC. Using the EXTVCC pin al-
lows the INTVCC power to be derived from a high efficiency
external source such as a switching regulator output. The
top MOSFET driver is biased from the floating bootstrap
capacitor, CB, which normally recharges during the 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 plus 50ns every fifth
cycle to allow CB to recharge. However, it is recommended
that a load be present or the IC operates at low frequency
during the dropout transition to ensure CB is recharged.
Internal Soft-Start
By default, the start-up of the output voltage is normally
controlled by an internal soft-start ramp. The internal
soft-start ramp represents a noninverting input to the
error amplifier. The FB pin is regulated to the lower of the
error amplifier’s three noninverting inputs (the internal
soft-start ramp, the TK/SS pin or the internal 600mV ref-
erence). As the ramp voltage rises from 0V to 0.6V over
approximately 600µs, the output voltage rises smoothly
from its prebiased value to its final set value.
Certain applications can result in the start-up of the con-
verter into a non-zero load voltage, where residual charge
is stored on the output capacitor at the onset of converter
switching. In order to prevent the output from discharging
under these conditions, the bottom MOSFET is disabled
until soft-start is greater than VFB.
Shutdown and Start-Up (RUN and TK/SS Pins)
The LTC3867 can be shut down using the RUN pin. Pulling
the RUN pin below 1.22V shuts down the main control loop
for the controller and most internal circuits, including the
INTVCC regulator. Releasing the RUN pin allows an internal
1.0µA current to pull up the pin and enable the controller.
Alternatively, the RUN pin may be externally pulled up or
driven directly by logic. Be careful not to exceed the ab-
solute maximum rating of 6V on this pin. The start-up of
the controller’s output voltage, VOUT , is controlled by the
voltage on the TK/SS pin. When the voltage on the TK/SS
pin is less than the 0.6V internal reference, the LTC3867
regulates the VFB voltage to the TK/SS pin voltage instead
of the 0.6V reference. This allows the TK/SS pin to be used
to program a soft-start by connecting an external capacitor
from the TK/SS pin to SGND. An internal 1.25µA pull-up
current charges this capacitor, creating a voltage ramp on
the TK/SS pin. As the TK/SS voltage rises linearly from
0V to 0.6V (and beyond), the output voltage, VOUT , rises
smoothly from zero to its final value. Alternatively, the TK/
SS pin can be used to cause the start-up of VOUT to track
that of another supply. Typically, this requires connect-
ing to the TK/SS pin an external resistor divider from the
other supply to ground (see the Applications Information
section). When the RUN pin is pulled low to disable the
controller, or when INTVCC drops below its undervoltage
lockout threshold of 3.2V, the TK/SS pin is pulled low by
an internal MOSFET. When in undervoltage lockout, the
controller is disabled and the external MOSFETs are held off.
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11 Page | ||
| Páginas | Total 30 Páginas | |
| PDF Descargar | [ Datasheet LTC3867.PDF ] | |
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