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PDF LTC3873-5 Data sheet ( Hoja de datos )
| Número de pieza | LTC3873-5 | |
| Descripción | No RSENSETM Constant Frequency Current Mode Boost/Flyback/SEPIC DC/DC Controller | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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Hay una vista previa y un enlace de descarga de LTC3873-5 (archivo pdf) en la parte inferior de esta página. Total 16 Páginas | ||
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LTC3873-5
No RSENSETM Constant
Frequency Current Mode
Boost/Flyback/SEPIC
DC/DC Controller
FEATURES
n VIN and VOUT Limited Only by External Components
n Internal or Programmable External Soft-Start
n Constant Frequency 200kHz Operation
n Adjustable Current Limit
n Current Sense Resistor Optional
n ±1.5% Voltage Reference Accuracy
n Current Mode Operation for Excellent Line and Load
Transient Response
n 4.1V Undervoltage Threshold for Logic Level
MOSFET Applications
n Low Quiescent Current: 300μA
n Low Profile (1mm) ThinSOTTM and (0.75mm)
2mm × 3mm DFN Package
APPLICATIONS
n Telecom Power Supplies
n Automotive Power Supplies
n PoE Applications
DESCRIPTION
The LTC®3873-5 is a constant frequency current mode
controller for boost, flyback or SEPIC DC/DC converters
designed to drive an N-channel MOSFET for high input
and output voltage converter applications.
The LTC3873-5 provides ±1.5% output voltage accuracy
and consumes only 300μA quiescent current during nor-
mal operation and only 50μA during micropower start-up.
Using a 9.3V internal shunt regulator, the LTC3873-5 can
be powered from a high input voltage through a resistor
or it can be powered directly from a low impedance DC
voltage of 9V or less. Soft-start can be programmed using
an external capacitor.
The LTC3873-5 is available in 8-lead ThinSOT and 2mm
× 3mm DFN packages.
PARAMETER
VCC UV+
VCC UV–
LTC3873
8.4V
4V
LTC3873-5
4.1V
2.9V
L, 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.
TYPICAL APPLICATION
High Efficiency 5V Input, 12V Output Boost Converter
4.7nF
11.8k
47pF
12k
1nF
RUN/SS
ITH VCC
LTC3873-5
IPRG
GND SW
VFB NGATE
108k
10μH
10μF
22μF
VIN
5V
VOUT
12V
2A
38735 TA01a
Efficiency and Power Loss vs Load Current
100
90
80
70
60
50
40
30
20
10
0
10
10000
1000
100
100
IOUT (mA)
1000
10
3873 TA01b
38735fb
1
1 page  
TYPICAL PERFORMANCE CHARACTERISTICS
LTC3873-5
Frequency vs Temperature
www.datasheet4u.com
250
VIN = 5V
230
210
190
170
150
–60 –40 –20 0 20 40 60 80 100 120
TEMPERATURE (°C)
3873-5 G09
Maximum Sense Threshold
vs Temperature
300
IPRG = VIN
250
200 IPRG = FLOAT
150
IPRG = GND
100
50
0
–60 –40 –20 0 20 40 60 80 100 120
TEMPERATURE (°C)
38735 G10
PIN FUNCTIONS (TS8/DD8)
IPRG (Pin 1/Pin 4): Current Sense Limit Select Pin.
ITH (Pin 2/Pin 3): This pin serves as the error amplifier
compensation point. Nominal voltage range for this pin
is 0.7V to 1.9V.
VFB (Pin 3/Pin 2): This pin 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.
VCC (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.
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5 Page 
LTC3873-5
APPLICATIONS INFORMATION
Input Capacitors
The input capacitor of a boost converter is less critical due
wwwt.doatthasehfeaect4tuth.caotmthe input current waveform is triangular, and
does not contain large square wave currents as found in
the output capacitor. The input voltage source impedance
determines the size of the capacitor that is typically 10μF to
100μF. A low ESR is recommended although not as critical
as the output capacitor can be on the order of 0.3Ω.
The RMS input ripple current for a boost converter is:
IRMS(CIN)
=
0.3
•
VIN(MIN)
L•f
• DMAX
Please note that the input capacitor can see a very high
surge current when a battery is suddenly connected to the
input of the converter and solid tantalum capacitors can
fail catastrophically under these conditions.
In a flyback converter, the input flows in pulses placing
severe demands on the input capacitors. Select an input
capacitor with a ripple current rating greater than:
IRMS
=
PIN
VIN(MIN)
1– DMAX
DMAX
Duty Cycle Considerations
The LTC3873-5 imposes a maximum duty cycle limit of
80% typical. For a flyback converter, the maximum duty
cycle prevents the transformer core from saturation. In
a boost converter application, however, it sets a limit on
the maximum step-up ratio or maximum output voltage
with the given input voltage of:
VOUT(MAX)
=
VIN(MIN)
1– 0.8%
–
VD
Current and voltage stress on the power switch and
synchronous rectifiers, input and output capacitor RMS
currents and transformer utilization (size vs power) are
impacted by duty factor. Unfortunately duty factor can-
not be adjusted to simultaneously optimize all of these
requirements. In general, avoid extreme duty factors since
this severely impacts the current stress on most of the
components. A reasonable target for duty factor is 50% at
nominal input voltage. Using this rule of thumb, the ideal
transformer turns ratio is:
NIDEAL
=
VOUT
VIN
•
1– D
D
=
VOUT
VIN
Output Diode Selection
To maximize efficiency, a fast switching diode with low
forward drop and low reverse leakage is desired. The output
diode in a boost converter conducts current during the
switch off-time. The peak reverse voltage that the diode
must withstand is equal to the regulator output voltage.
The average forward current in normal operation is equal
to the output current, and the peak current is equal to the
peak inductor current.
38735fb
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| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LTC3873-5.PDF ] | |
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