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PDF LT8311 Data sheet ( Hoja de datos )
| Número de pieza | LT8311 | |
| Descripción | Synchronous Rectifier Controller | |
| Fabricantes | Linear | |
| Logotipo |  |
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FEATURES
LT8311
Synchronous Rectifier
Controller with Opto-Coupler
Driver for Forward Converters
DESCRIPTION
n Wide Input Supply Range: 3.7V to 30V
n Preactive Mode:
n No Pulse Transformer Required
n DCM Operation at Light Load
n SYNC Mode:
n FCM or DCM Operation at Light Load
n Achieves Highest Efficiency
n 1.5% Feedback Voltage Reference
n 10mA Opto-Coupler Driver
n Output Power Good Indicator
n Integrated Soft-Start Function
APPLICATIONS
n Offline and HV Car Battery Isolated Power Supplies
n 48V Isolated Power Supplies
n Industrial, Automotive and Military Systems
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear
Technology Corporation. All other trademarks are the property of their respective owners.
The LT®8311 is used on the secondary side of a forward
converter to provide synchronous MOSFET control and
output voltage feedback through an opto-coupler. The
LT8311’s unique preactive mode allows control of the
secondary-side MOSFETs without requiring a traditional
pulse transformer for primary- to secondary-side com-
munication. In preactive mode, the output inductor current
operates in discontinuous conduction mode (DCM) at
light load. If forced continuous mode (FCM) operation is
desired at light load, the LT8311 can, alternatively, be used
in SYNC mode, where a pulse transformer is required to
send synchronous control signals from the primary-side
IC to the LT8311.
The LT8311 offers a full featured opto-coupler controller,
incorporating a 1.5% reference, a transconductance error
amplifier and a 10mA opto-driver. Power good monitoring
and output soft-start/overshoot control are also included.
The LT8311 is available in a 16-lead FE package with pins
removed for high voltage spacing requirements.
TYPICAL APPLICATION
18V to 72V, 12V/8A Active Clamp Isolated Forward Converter
VIN
18V to
72V
4.7µF
× 3 100k
UVLO_VSEC
5.9k
VIN
100nF
2.2µF
100V
4:4
••
100nF
6.8µH
2k
FSW
OVLO
AOUT
1.82k
LT3753
10k
2k
1.78k
FG
CSW LT8311
CSP
FB
71.5k
49.9k
240kHz
IVSEC
RT
31.6k
SYNC
SOUT
TAO
TAS
TOS
TBLNK
GND SS1
OUT
ISENSEP
OC
ISENSEN
INTVCC
COMP
SS2 FB
34k 0.47µF
1µF
2k
100k
10pF
100k
6mΩ
4.7µF 100Ω
1µF
1.78k
1.5k
15nF
2.94k
CG VIN
CSN PGOOD
OPTO
COMP
INTVCC
SYNC
PMODE
SS TIMER GND
124k
1µF
1k
2.2nF
For more information www.linear.com/LT8311
20k
2.2nF
100k
68pF
22µF
×2
VOUT
+ 12V
470µF
11.3k
100k
VOUT
4.7µF
2.2µF
8311 TA01
8311f
1
1 page  
LT8311
E LECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating
temperature range, otherwise
otherwise noted. (Note 2)
specifications
are
at
TA
=
25°C,
VIN
=
12V,
VINTVCC
=
8V,
PMODE
=
5V,
CCG
=
CFG
=
100pF,
unless
PARAMETER
CONDITIONS
Inverting DC Gain From COMP Pin to
OPTO Pin
Opto-Driver –3dB Bandwidth
(∆VOPTO/∆VCOMP), 1.290V ≤ COMP ≤ 1.310V
(∆VOPTO/∆VCOMP), 1.490V ≤ COMP ≤ 1.510V
(∆VOPTO/∆VCOMP), 1.890V ≤ COMP ≤ 1.910V
No Load (Note 5)
Opto-Driver Output Swing Low
FB = 1V, COMP = SS = OPTO = Open
Opto-Driver Output Swing High
VIN = 3.7V, FB = 1.5V, COMP = SS = Open, IOPTO = 10mA
VIN = 30V, FB = 1.5V, COMP = SS = Open, IOPTO = 10mA
Opto-Driver Output Short-Circuit Current VIN = 30V, FB = 1.5V, COMP = SS = Open, OPTO = 0V (Note 6)
Opto-Driver Output Sink Current
FB = 1V, OPTO = 1.2V (Note 7)
Internal Linear Regulator
INTVCC Regulation Voltage
INTVCC Load Regulation
INTVCC UVLO Rising
INTVCC UVLO Falling
INTVCC OVLO Rising
INTVCC OVLO Falling
INTVCC Current Limit
INTVCC Dropout Voltage
CG and FG Gate Drivers
No Load
(∆VINTVCC /∆IINTVCC), 0A ≤ IINTVCC ≤ 20mA
INTVCC > IINTVCC_UVLO_RISING (= 4.6V)
INTVCC < IINTVCC_UVLO_FALLING (= 4.3V)
VIN = 6V, IINTVCC = 10mA, Not Switching
Driver Output Rise Time
Driver Output Fall Time
Driver Output High Voltage
CCG = CFG = 3.3nF, INTVCC = 8V (Note 4)
CCG = CFG = 3.3nF, INTVCC = 8V (Note 4)
Driver Output Low Voltage
PMODE Selection
PMODE Trip Voltage
PMODE Ramp Up
Hysteresis
PMODE Input Current
PMODE = 18V
Preactive Mode (Tie PMODE to 0V)
Preactive Mode Operating
Frequency Range
CSW High Trip Voltage
CSW Ramp Up
CSW High Input Current
CSW = 150V (Note 7)
CSW Low Trip Voltage
CSW Ramp Down
FSW Trip Voltage
FSW High Input Current
FSW = 150V (Note 7)
CG Falling Edge to CSW Rising Edge
Prediction Delay
CSW = 150kHz (Note 10), FSW = 0V, CSP = –500mV
CG Falling Edge Delay to FG Rising Edge CSW = 150kHz (Note 10), FSW = 0V, CSP = –500mV
MIN TYP
–5
–5.9
–6.2
400
l 0.5
l VIN – 1.7 VIN – 1.4
l 5.2
6.5
l 10.5
15
l 200
300
MAX
0.85
18
420
l 6.5
l
l 4.1
l
l 14
l 38
7 7.5
1.8 3
4.6 4.8
4.3
16.5 17.5
15
48 58
20
400
l VINTVCC
– 0.2
l
25
25
0.7
l1
l
1.2 1.4
30
60 90
l 100
l1
l
l –250
l1
l
l5
l 10
1.2
250
–150
1.2
250
100
50
300
1.4
500
–50
1.4
500
300
80
UNITS
V/ V
V/ V
V/ V
kHz
V
V
V
mA
µA
V
mV/mA
V
V
V
V
mA
mA
mV
ns
ns
V
V
V
mV
µA
kHz
V
µA
mV
V
µA
ns
ns
For more information www.linear.com/LT8311
8311f
5
5 Page 
LT8311
PIN FUNCTIONS
CSW (Pin 1): Catch MOSFET Drain Sense Pin. Connect
this pin to the external N-channel catch MOSFET’s drain
through a 2k resistor (typical) in preactive mode. Mini-
mize parasitic capacitance on the pin. Connect to GND in
SYNC mode.
FSW (Pin 3): Forward MOSFET Drain Sense Pin. Con-
nect this pin to the external N-channel forward MOSFET’s
drain through 2k resistor (typical) in preactive mode.
Minimize parasitic capacitance on the pin. Connect to
GND in SYNC mode.
FG (Pin 5): Forward MOSFET Gate Driver Pin. This pin
drives the gate of the external N-channel forward MOSFET.
Minimize trace length between this pin and the forward
MOSFET gate.
INTVCC (Pin 6): Internal Linear Regulator’s Output Pin.
INTVCC powers the gate drivers on the LT8311. The volt-
age on this pin is internally regulated to 7V. Alternatively,
the pin can be overdriven externally. A minimum of 4.7µF
(ceramic capacitor) must be placed from this pin to GND.
VIN (Pin 7): Input Supply Pin. This pin must be locally
bypassed.
PMODE (Pin 8): Preactive Mode Select Pin. Tying PMODE
to GND enables preactive mode. Tying PMODE to INTVCC
enables SYNC mode.
OPTO (Pin 9): Opto Driver Output Pin. Tie this pin, through
a series resistor, to the input of the opto-coupler. This
pin can source up to 10mA, sink 300μA typically, and is
short-circuit protected.
COMP (Pin 10): Error Amplifier Output Pin. Tie an ex-
ternal compensation network to this pin when using the
LT8311’s transconductance error amplifier as part of a
voltage feedback loop.
FB (Pin 11): Feedback Pin. This is the inverting input of
the LT8311’s internal error amplifier. The FB pin voltage
tracks the lower of the internal 1.227V reference and the
SS pin voltage. 75nA (bias current) typically flows out of
the pin. Tie this pin to a resistor divider network from the
output to set the desired output voltage.
cycle. If the sum of the on times of the catch and forward
MOSFET, per cycle (including the dead time), exceeds the
timeout period programmed by the TIMER resistor, then
all synchronous conduction will be shut down. Synchro-
nous conduction resumes when the timeout period is
reset again. See the Applications Information section for
more details on programming the TIMER resistor. Keep
the ground return trace of this pin short, and away from
paths with switching noise.
PGOOD (Pin 13): Output Power Good Pin. The open-drain
output will be pulled to ground when the FB pin voltage
stays within ±7% of the internal 1.227V reference for a
period of 175µs. The internal PGOOD comparator has a
hysteresis of ±3%. Therefore, when FB exists outside ±10%
of the 1.227V reference, the PGOOD pin will be pulled high
by an external pull-up resistor or current source.
SS (Pin 14): Soft-Start Pin. A capacitor from the SS pin to
GND will be charged up by SS’s internally trimmed 10µA
current source. Since FB tracks the lower of the SS pin
voltage and the internal reference of 1.227V, the charge
rate of the SS pin can be used to set the slew rate at which
the FB pin charges up to its regulation voltage of 1.227V.
The SS pin typically charges up to 2V. When using the
LT8311 as part of voltage feedback loop, place a ceramic
capacitor of at least 1nF on this pin to GND. For details on
SS start-up and overshoot control functions, please refer
to the Applications Information section.
SYNC (Pin 15): Synchronization Pin. The SYNC pin, used
only in SYNC mode, serves as an edge-sensitive input to
receive timing information for synchronous switching. It
is typically driven with PWM synchronization signals from
the primary-side IC through a pulse transformer. A nega-
tive voltage slew on the SYNC pin (–1.2V threshold) turns
on the forward MOSFET and turns off the catch MOSFET.
Equivalently, a positive voltage slew (1.2V threshold) turns
on the catch MOSFET and turns off the forward MOSFET.
Tie the SYNC pin to GND in preactive mode.
CG (Pin 16): Catch MOSFET Gate Driver Pin. This pin drives
the gate of the external N-channel catch MOSFET. Minimize
trace length between this pin and the catch MOSFET gate.
TIMER (Pin 12): Switching Period Timeout Pin. A resistor
from this pin to ground sets an upper limit on the sum
of the forward and catch MOSFET on times (including
dead time between the two MOSFETs on period), every
CSN, CSP (Pin 18, Pin 20): Current Sense Differential
Inputs. CSP and CSN are the positive and negative inputs,
respectively, of the LT8311’s internal current sense com-
parator. The pins are typically connected across the catch
For more information www.linear.com/LT8311
8311f
11
11 Page | ||
| Páginas | Total 30 Páginas | |
| PDF Descargar | [ Datasheet LT8311.PDF ] | |
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