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PDF LTC3872 Data sheet ( Hoja de datos )
| Número de pieza | LTC3872 | |
| Descripción | No RSENSE Current Mode Boost DC/DC Controller | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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FEATURES
■ No Current Sense Resistor Required
■ VOUT up to 60V
■ Constant Frequency 550kHz Operation
■ Internal Soft-Start and Optional External Soft-Start
■ Adjustable Current Limit
■ Pulse Skipping at Light Load
■ VIN Range: 2.75V to 9.8V
■ ±1.5% Voltage Reference Accuracy
■ Current Mode Operation for Excellent Line and Load
Transient Response
■ Low Profile (1mm) SOT-23 and 3mm × 2mm DFN
Packages
U
APPLICATIO S
■ Telecom Power Supplies
■ 42V Automotive Systems
■ 24V Industrial Controls
■ IP Phone Power Supplies
LTC3872
No RSENSE
Current Mode Boost
DC/DC Controller
DESCRIPTIO
The LTC®3872 is a constant frequency current mode
boost DC/DC controller that drives an N-channel power
MOSFET and requires very few external components. The
No RSENSETM architecture eliminates the need for a sense
resistor, improves efficiency and saves board space.
The LTC3872 provides excellent AC and DC load and line
regulation with ±1.5% output voltage accuracy. It incor-
porates an undervoltage lockout feature that shuts down
the device when the input voltage falls below 2.3V.
High switching frequency of 550kHz allows the use of a
small inductor. The LTC3872 is available in an 8-lead low
profile (1mm) ThinSOTTM package and 8-pin 3mm × 2mm
DFN package.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
No RSENSE and ThinSOT are trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
Protected by U.S. Patents including 6498466, 6611131, 5731694.
TYPICAL APPLICATIO
High Efficiency 3.3V Input, 5V Output Boost Converter
1.8nF
17.4k
47pF VIN
ITH
IPRG
VIN
LTC3872
GND SW
VFB RUN/SS NGATE
11k
1%
34.8k
1nF
1%
1μH
D1
M1
100μF
×2
VIN
3.3V
10μF
VOUT
5V
2A
3872 TA01
Efficiency and Power Loss vs Load Current
100
90
80
70
60
50
40
30
20
10
0
1
10
1
0.1
0.01
10 100 1000
LOAD CURRENT (mA)
0.001
10000
3872 TA01b
3872fa
1
1 page  
LTC3872
PI FU CTIO S (TS8/DD8)
IPRG (Pin 1/Pin 4): Current Sense Limit Select Pin.
ITH (Pin 2/Pin 3): It serves as the error amplifier com-
wwwp.deantassahetieot4nu.pcoomint. Nominal voltage range for this pin is
0.7V to 1.9V.
VFB (Pin 3/Pin 2): Receives the feedback voltage from an
external resistor divider across the output.
GND (Pin 4/Pin 1): Ground Pin.
NGATE (Pin 5/Pin 8): Gate Drive for the External N-Channel
MOSFET. This pin swings from 0V to VIN.
VIN (Pin 6/Pin 7): Supply Pin. This pin must be closely
decoupled to GND (Pin 4).
RUN/SS (Pin 7/Pin 6): Shutdown and external soft-start
pin. In shutdown, all functions are disabled and the NGATE
pin is held low.
SW (Pin 8/Pin 5): Switch node connection to inductor and
current sense input pin through external slope compensa-
tion resistor. Normally, the external N-channel MOSFET’s
drain is connected to this pin.
Exposed Pad (NA/Pin 9): Ground. Must be soldered to PCB
for electrical contact and rated thermal performance.
FUNCTIONAL DIAGRAM
VIN GND SW
UNDERVOLTAGE UV
LOCKOUT
VOLTAGE
REFERENCE
1.2V
SLOPE
COMPENSATION
SHUTDOWN
COMPARATOR
0.7μA
RUN/SS
SHDN
INTERNAL
SOFT-START
RAMP
+
ITH
BUFFER
–
–+
CURRENT
COMPARATOR
ILIM
IPRG
RS R S
LATCH Q
550kHz
OSCILLATOR
CURRENT LIMIT
CLAMP
ERROR
AMPLIFIER
SWITCHING
LOGIC CIRCUIT
VIN
NGATE
VFB
1.2V
ITH
3872 FD
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5 Page 
LTC3872
APPLICATIO S I FOR ATIO
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•
IO(MAX)
1– DMAX
For the bulk C component, which also contributes 1% to
the total ripple:
COUT
≥
IO(MAX)
0.01• VO •
f
For many designs it is possible to choose a single capacitor
type that satisfies both the ESR and bulk C requirements
for the design. In certain demanding applications, however,
the ripple voltage can be improved significantly by con-
necting two or more types of capacitors in parallel. For
example, using a low ESR ceramic capacitor can minimize
the ESR step, while an electrolytic capacitor can be used
to supply the required bulk C.
Once the output capacitor ESR and bulk capacitance have
been determined, the overall ripple voltage waveform
should be verified on a dedicated PC board (see Board
Layout section for more information on component place-
ment). Lab breadboards generally suffer from excessive
series inductance (due to inter-component wiring), and
these parasitics can make the switching waveforms look
significantly worse than they would be on a properly
designed PC board.
The output capacitor in a boost regulator experiences
high RMS ripple currents, as shown in Figure 6. The RMS
output capacitor ripple current is:
IRMS(COUT) ≈ IO(MAX) •
VO – VIN(MIN)
VIN(MIN)
Note that the ripple current ratings from capacitor manu-
facturers are often based on only 2000 hours of life. This
makes it advisable to further derate the capacitor or to
choose a capacitor rated at a higher temperature than
required. Several capacitors may also be placed in parallel
to meet size or height requirements in the design.
Manufacturers such as Nichicon, United Chemicon and
Sanyo should be considered for high performance through-
hole capacitors. The OS-CON semiconductor dielectric
capacitor available from Sanyo has the lowest product of
ESR and size of any aluminum electrolytic, at a somewhat
higher price.
In surface mount applications, multiple capacitors may
have to be placed in parallel in order to meet the ESR or
RMS current handling requirements of the application.
Aluminum electrolytic and dry tantalum capacitors are
both available in surface mount packages. In the case of
tantalum, it is critical that the capacitors have been surge
tested for use in switching power supplies. An excellent
choice is AVX TPS series of surface mount tantalum. Also,
ceramic capacitors are now available with extremely low
ESR, ESL and high ripple current ratings.
LD
VOUT
VIN SW
COUT RL
5a. Circuit Diagram
IL IIN
5b. Inductor and Input Currents
ISW
tON
5c. Switch Current
ID tOFF
IO
5d. Diode and Output Currents
VOUT
(AC)
ΔVCOUT
ΔVESR
RINGING DUE TO
TOTAL INDUCTANCE
(BOARD + CAP)
5e. Output Voltage Ripple Waveform
Figure 5. Switching Waveforms for a Boost Converter
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11
11 Page | ||
| Páginas | Total 20 Páginas | |
| PDF Descargar | [ Datasheet LTC3872.PDF ] | |
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