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PDF LTC3873 Data sheet ( Hoja de datos )
| Número de pieza | LTC3873 | |
| 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 (archivo pdf) en la parte inferior de esta página. Total 16 Páginas | ||
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LTC3873
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 Maximum 60V on SW Node with RDS(ON) Sensing
n ±1.5% Voltage Reference Accuracy
n Current Mode Operation for Excellent Line and Load
Transient Response
n Low Quiescent Current: 300μA
n Low Profile (1mm) ThinSOTTM and (0.75mm)
2mm × 3mm DFN Package
APPLICATIONS
n Telecom Power Supplies
n 42V and 12V Automotive Power Supplies
n Portable Electronic Equipment
DESCRIPTION
The LTC®3873 is a constant frequency current mode,
boost, flyback or SEPIC DC/DC controller that drives an
N-channel power MOSFET in high input and output volt-
age converter applications. Soft-start can be programmed
using an external capacitor.
The LTC3873 provides ±1.5% output voltage accuracy and
consumes only 300μA quiescent current during normal
operation and only 55μA during micropower start-up.
Using a 9.3V internal shunt regulator, the LTC3873 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.
The LTC3873 is available in 8-lead ThinSOT and 2mm ×
3mm DFN packages.
PARAMETER
VCC UV+
VCC UV–
LTC3873
8.4V
4V
LTC3873-5
3.9V
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
4.7μF
100V
X5R
7.5k
2.2nF
5V Output Nonisolated Telecom Power Supply
10μF
10V
X5R
0.1μF
VIN
36V TO 72V
221k
T1
•
D1
D2
•
VCC
ITH NGATE
LTC3873
RUN/SS
GND SW
IPRG
VFB
12.1k
38.3k
M1
68mΩ
100μF
6.3V
X5R
s3
VOUT
5V
2A MAX
3873 TA01a
Efficiency and Power Loss vs Load Current
100 3000
90
EFFICIENCY
80
2500
70 2000
60
50
40
30
20
10
0
10
POWER LOSS
1500
1000
VIN = 72V
VIN = 60V 500
VIN = 48V
VIN = 36V
10
1000
LOAD CURRENT (mA)
3873 TA01b
3873fa
1
1 page  
TYPICAL PERFORMANCE CHARACTERISTICS
LTC3873
Frequency vs Temperature
www.datasheet4u.com 250
230
210
190
170
150
–60 –40 –20 0 20 40 60 80 100 120
TEMPERATURE (°C)
3873 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)
3873 G10
PIN FUNCTIONS (TS8/DDB)
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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LTC3873
APPLICATIONS INFORMATION
plus the secondary-to-primary referred voltage of the
flyback pulse (including leakage spike) must not exceed
wwwt.dhaetaaslhleoewt4eud.coemxternal MOSFET breakdown rating.
Leakage Inductance
Transformer leakage inductance (on either the primary
or secondary) causes a voltage spike to occur after the
output switch (Q1) turn-off. This is increasingly prominent
at higher load currents where more stored energy must
be dissipated. In some cases a “snubber” circuit will be
required to avoid overvoltage breakdown at the MOSFET’s
drain node. Application Note 19 is a good reference on
snubber design. A bifilar or similar winding technique is a
good way to minimize troublesome leakage inductances.
However, remember that this will limit the primary-to-
secondary breakdown voltage, so bifilar winding is not
always practical.
Power MOSFET Selection
The power MOSFET serves two purposes in the LTC3873:
it represents the main switching element in the power path
and its RDS(ON) represents the current sensing element
for the control loop. Important parameters for the power
MOSFET include the drain-to-source breakdown voltage
(BVDSS), the threshold voltage (VGS(TH)), the on-resistance
(RDS(ON)) versus gate-to-source voltage, the gate-to-source
and gate-to-drain charges (QGS and QGD, respectively),
the maximum drain current (ID(MAX)) and the MOSFET’s
thermal resistances (RTH(JC) and RTH(JA)).
For boost applications with RDS(ON) sensing, refer to
the LTC3872 data sheet for the selection of MOSFET
RDS(ON).
MOSFETs have conduction losses (I2R) and switching
losses. For VDS < 20V, high current efficiency generally
improves with large MOSFETs with low RDS(ON), while
for VDS > 20V the transition losses rapidly increase to the
point that the use of a higher RDS(ON) device with lower
reverse transfer capacitance, CRSS, actually provides
higher efficiency.
Output Capacitors
The output capacitor is normally chosen by its effective
series resistance (ESR), which determines output ripple
voltage and affects efficiency. Low ESR ceramic capaci-
tors are often used to minimize the output ripple. Boost
regulators have large RMS ripple current in the output
capacitor that must be rated to handle the current. The
output ripple current (RMS) is:
IRMS(COUT) ≈ IOUT(MAX) •
VOUT – VIN(MIN)
VIN(MIN)
Output ripple is then simply:
VOUT = RESR(ΔIL(RMS))
The output capacitor for flyback converter should have a
ripple current rating greater than:
IRMS = IOUT •
DMAX
1– DMAX
Input Capacitors
The input capacitor of a boost converter is less critical due
to the fact that the 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.
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11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LTC3873.PDF ] | |
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