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PDF LT1425 Data sheet ( Hoja de datos )
| Número de pieza | LT1425 | |
| Descripción | Isolated Flyback Switching Regulator | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LT1425
Isolated Flyback
Switching Regulator
FEATURES
s No Transformer “Third Winding” or Optoisolator
Required
s ±5% Accurate Output Voltage Without User Trims
(See Circuit Below)
s Resistor Programmable Output Voltage
s Regulation Maintained Well Into Discontinuous
Mode (Light Load)
s Optional Load Compensation
s Operating Frequency: 285kHz
s Easily Synchronized to External Clock
s Available in 16-Pin Narrow SO Package
U
APPLICATIONS
s Isolated Flyback Switching Regulators
s Ethernet Isolated 5V to – 9V Converters
s Medical Instruments
s Isolated Telecom Supplies
, LTC and LT are registered trademarks of Linear Technology Corporation.
DESCRIPTION
The LT®1425 is a monolithic high power switching regu-
lator specifically designed for the isolated flyback topol-
ogy. No “third winding” or optoisolator is required; the
integrated circuit senses the isolated output voltage
directly from the primary side flyback waveform. A high
current, high efficiency switch is included on the die along
with all oscillator, control and protection circuitry.
The LT1425 operates with input supply voltages from 3V
to 20V and draws only 7mA quiescent current. It can
deliver output power up to 6W with no external power
devices. By utilizing current mode switching techniques, it
provides excellent AC and DC line regulation.
The LT1425 has a number of features not found on other
switching regulator ICs. Its unique control circuitry can
maintain regulation well into discontinuous mode in most
applications. Optional load compensation circuitry allows
for improved load regulation. An externally activated shut-
down mode reduces total supply current to 15µA for
standby operation.
TYPICAL APPLICATION
5V to Isolated – 9VOUT
500V
ISOLATION BARRIER
5V
+ C1
100µF
10V
T1*
12
VIN 11 •
VSW
R1
D1
1N5819
•
C2
47µF
16V
V–
ISOLATED
–9V ± 5% AT
20mA TO 200mA
LT1425
22.6k
F
15
3 1%
*DALE LPE 4841-330MB
SHDN
6
RFB
R2
SYNC
3.01k
4
C3
1000pF
VC RREF
ROCOMP
RCCOMP
SGND PGND
5
14
13
1%
C4
0.1µF
R3
15k
7 10
1425 TA01
9.5
9.4
9.3
9.2
9.1
9.0
8.9
8.8
8.7
8.6
8.5
0
Load Regulation
50 100 150
OUTPUT CURRENT (mA)
200
1425 TA02
1
1 page  
LT1425
PIN FUNCTIONS
GND (Pins 1, 8, 9, 16): Ground. These pins connect to the
substrate of the die and are separate from the power
ground and signal ground. They should connect directly to
a good quality ground plane.
RFB (Pin 3): Input Pin for External “Feedback” Resistor
Connected to Transformer Primary (VSW). The ratio of this
resistor to the RREF resistor, times the internal bandgap
(VBG) reference, is the primary determinant of the output
voltage (plus the effect of any nonunity transformer turns
ratio). The average current through this resistor during the
flyback period should be approximately 400µA. See Appli-
cations Information for more details.
VC (Pin 4): Control Voltage. This pin is the output of the
feedback amplifier and the input of the current compara-
tor. Frequency compensation of the overall loop is effected
by placing a capacitor between this node and ground.
RREF (Pin 5): Input Pin for External Ground-Referred
“Reference” Resistor. This resistor should be in the range
of 3k, but for convenience, need not be this value precisely.
See Applications Information for more details.
SYNC (Pin 6): Pin to Synchronize Internal Oscillator to
External Frequency Reference. It is directly logic compat-
ible and can be driven with any signal between 10% and
90% duty cycle. If unused, this pin can be left floating;
however, for best noise immunity the pin should be
grounded.
SGND (Pin 7): Signal Ground. This pin is a clean ground.
The internal reference and feedback amplifier are referred
to it. Keep the ground path connection to RREF and the VC
compensation capacitor free of large ground currents.
PGND (Pin 10): Power Ground. This pin is the emitter of
the power switch device and has large currents flowing
through it. It should be connected directly to a good quality
ground plane.
VSW (Pin 11): This is the collector node of the output
switch and has large currents flowing through it. Keep the
traces to the switching components as short as possible
to minimize electromagnetic radiation and voltage spikes.
VIN (Pin 12): Supply Voltage. Bypass input supply pin with
10µF or more. The part goes into undervoltage lockout
when VIN drops below 2.8V. Undervoltage lockout stops
switching and pulls the VC pin low.
RCCOMP (Pin 13): Pin for the External Filter Capacitor for
Load Compensation Function. A common 0.1µF
ceramic capacitor will suffice for most applications. See
Applications Information for further details.
ROCOMP (Pin 14): Input Pin for Optional External Load
Compensation Resistor. Use of this pin allows nominal
compensation for nonzero output impedance in the power
transformer secondary circuit, including secondary wind-
ing impedance, output Schottky diode impedance and
output capacitor ESR. In less demanding applications this
resistor is not needed. See Applications Information for
more details.
SHDN (Pin 15): Shutdown. This pin is used to turn off the
regulator and reduce VIN input current to a few tens of
microamperes. The SHDN pin can be left floating when
unused.
5
5 Page 
LT1425
APPLICATIONS INFORMATION
SELECTING RFB AND RREF RESISTOR VALUES
The expression for VOUT developed in the Operation
section can be rearranged to yield the following expres-
sion for RFB:
) )) )RFB = RREF
VOUT + VF + ISEC(ESR)
VBG
α
NSP
The unknown parameter α, which represents the fraction
of RFB current flowing into the RREF node, can be repre-
sented instead by specified data sheet values as follows:
(IREF)(α)(3k) = VBG
) )α =
VBG
(IREF)(3k)
Allowing the expression for RFB to be rewritten as:
) )RFB = RREF
VOUT + VF + ISEC(ESR)
IREF(3k)NSP
where,
VOUT = Desired output voltage
VF = Switching diode forward voltage
(ISEC)(ESR) = Secondary resistive losses
IREF = Data sheet reference current value
NSP = Effective secondary-to-primary turns ratio
Strictly speaking, the above equation defines RFB not as an
absolute value, but as a ratio of RREF. So the next question
is, “What is the proper value for RREF?” The answer is that
RREF should be approximately 3k. This is because the
LT1425 is trimmed and specified using this value of RREF.
If the impedance of RREF varies considerably from 3k,
additional errors will result. However, a variation in RREF
of several percent or so is perfectly acceptable. This yields
a bit of freedom in selecting standard 1% resistor values
to yield nominal RFB/RREF ratios.
SELECTING ROCOMP RESISTOR VALUE
The Operation section previously derived the following
expressions for ROUT, i.e., effective output impedance and
ROCOMP, the external resistor value required for its nomi-
nal compensation:
) )ROUT = ESR
1
1 – DC
) )) )ROCOMP
=
K1
∆VRCCOMP
∆ISW
RFB
ROUT
While the value for ROCOMP may therefore be theoretically
determined, it is usually better in practice to employ
empirical methods. This is because several of the required
input variables are difficult to estimate precisely. For
instance, the ESR term above includes that of the trans-
former secondary, but its effective ESR value depends on
high frequency behavior, not simply DC winding resis-
tance. Similarly, K1 appears to be a simple ratio of VIN to
VOUT times (differential) efficiency, but theoretically esti-
mating efficiency is not a simple calculation. The sug-
gested empirical method is as follows:
Build a prototype of the desired supply using the
eventual secondary components. Temporarily ground
the RCCOMP pin to disable the load compensation func-
tion. Operate the supply over the expected range of
output current loading while measuring the output
voltage deviation. Approximate this variation as a single
value of ROUT (straight line approximation). Calculate a
value for the K1 constant based on VIN, VOUT and the
measured (differential) efficiency. They are then com-
bined with the data sheet typical value for (∆VRCCOMP/
∆ISW ) to yield a value for ROCOMP.
Verify this result by connecting a resistor of roughly this
value from the ROCOMP pin to ground. (Disconnect the
ground short to RCCOMP and connect the requisite
0.1µF filter capacitor to ground.) Measure the output
impedance with the new compensation in place. Modify
the original ROCOMP value if necessary to increase or
decrease the effective compensation.
Once the proper load compensation resistor has been
chosen, it may be necessary to adjust the value of the
RFB resistor. This is because the load compensation
system exhibits some nonlinearity. In particular, the
circuit can shift the reference current by a noticeable
11
11 Page | ||
| Páginas | Total 20 Páginas | |
| PDF Descargar | [ Datasheet LT1425.PDF ] | |
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