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PDF LT1683 Data sheet ( Hoja de datos )
| Número de pieza | LT1683 | |
| Descripción | Ultralow Noise Push-Pull DC/DC Controller | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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FEATURES
s Greatly Reduced Conducted and Radiated EMI
s Low Switching Harmonic Content
s Independent Control of Output Switch Voltage and
Current Slew Rates
s Greatly Reduced Need for External Filters
s Dual N-Channel MOSFET Drivers
s 20kHz to 250kHz Oscillator Frequency
s Easily Synchronized to External Clock
s Regulates Positive and Negative Voltages
s Easier Layout Than with Conventional Switchers
U
APPLICATIO S
s Power Supplies for Noise Sensitive Communication
Equipment
s EMI Compliant Offline Power Supplies
s Precision Instrumentation Systems
s Isolated Supplies for Industrial Automation
s Medical Instruments
s Data Acquisition Systems
, LTC and LT are registered trademarks of Linear Technology Corporation.
LT1683
Ultralow Noise
Push-Pull DC/DC Controller
DESCRIPTIO
The LT®1683 is a switching regulator controller designed
to lower conducted and radiated electromagnetic interfer-
ence (EMI). Ultralow noise and EMI are achieved by
controlling the voltage and current slew rates of external
N-channel MOSFET switches. Current and voltage slew
rates can be independently set to optimize harmonic
content of the switching waveforms vs efficiency. The
LT1683 can reduce high frequency harmonic power by as
much as 40dB with only minor losses in efficiency.
The LT1683 utilizes a dual output (push-pull) current
mode architecture optimized for low noise topologies. The
IC includes gate drivers and all necessary oscillator,
control and protection circuitry. Unique error amp cir-
cuitry can regulate both positive and negative voltages.
The oscillator may be synchronized to an external clock for
more accurate placement of switching harmonics.
Protection features include gate drive lockout for low VIN,
opposite gate lockout, soft-start, output current limit,
short-circuit current limiting, gate drive overvoltage clamp
and input supply undervoltage lockout.
TYPICAL APPLICATIO
Ultralow Noise 48V to 5V DC/DC Converter
51k
1N4148
510Ω
0.5W
FZT853
10µF
20V
2N3904
8.2V
48V
39µF
63V
MIDCOM 31244
MBR0530
MBRS340
11V 68µF
20V 17 3
23.2k
976Ω
1.2nF
16.9k
25k 3.3k
25k 3.3k
1.5k
0.22µF 22nF
VIN GCL
5pF
14
SHDN
5
V5
6
SYNC
2
CAP A
1
GATE A
5pF
7
CT
18
CAP B
8
RT
LT1683
19
GATE B
16
RVSL
4 Si9422
CS
15
RCSL
20
PGND
12
VC
9
FB
SS GND NFB
13 11 10
10nF
10pF
200V
Si9422
MBRS340
10pF
200V
30pF
0.1Ω
30pF
22µH
150µF
OS-CON
OPTIONAL
B 22µH
A 5V/2A
2×100µF
POSCAP
7.50k
2.49k
A
200µV/DIV
B
20mV/DIV
1683 TA01
5V Output Noise
(Bandwidth = 100MHz)
200µVP-P
5µs/DIV
1683 TA01a
1683f
1
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
LT1683
SHDN Pin Hysteresis Current vs
Temperature
27
25
23
21
19
17
15
–50 –25
0 25 50 75 100 125 150
TEMPERATURE (°C)
1683 G09
Slope Compensation
110
VC PIN = 0.9V
100 TA = 25°C
90
80
70
60
50
0
10 20 30 40
DUTY CYCLE (%)
50
1683 G12
Gate Drive Undervoltage Lockout
Voltage vs Temperature
7.3
7.2 GCL = 6V
7.1
7.0
6.9
6.8
6.7
6.6
6.5
6.4
6.3
–50 –25 0 25 50 75 100 125 150
TEMPERATURE (°C)
1683 G15
VIN Current vs Temperature
24 WITH NO EXTERNAL MOSFETs
22
20
18 VIN = 12 RCSL, RVSL = 5.7k
16 VIN = 20 RCSL, RVSL = 17k
14 VIN = 12 RCSL, RVSL = 17k
12
10
–50 –25
0 25 50 75 100 125 150
TEMPERATURE (°C)
1683 G10
Gate Drive A/B High Voltage vs
Temperature
10.7
10.6
10.5
10.4
10.3
10.2
10.1
10.0
9.90
9.80
9.70
–50 –25
6.5
GCL = 12V
6.4
6.3
6.2
VIN = 12V
NO LOAD
6.1
6.0
5.9
GCL = 6V
5.8
5.7
5.6
5.5
0 25 50 75 100 125 150
TEMPERATURE (°C)
1683 G13
CS Pin to VC Pin Transfer
Function
1.6
TA = 25°C
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
0 20 40 60 80 100 120
CS PIN VOLTAGE (mV)
1683 G11
Gate Drive A/B Low Voltage vs
Temperature
0.50
0.45 VIN = 12V
NO LOAD
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0
–50 –25 0 25 50 75 100 125 150
TEMPERATURE (°C)
1683 G14
Soft-Start Current vs Temperature
9.5
9.3 SS VOLTAGE = 0.9V
9.1
8.9
8.7
8.5
8.3
8.1
7.9
7.7
7.5
–50 –25 0 25 50 75 100 125 150
TEMPERATURE (°C)
1683 G16
V5 Voltage vs Load Current
5.08
5.06 T = 125°C
5.04
5.02 T = 25°C
5.00
T = –40°C
4.98
4.96
–15
–10 –5 0
5
LOAD CURRENT (mA)
10 15
1683 G17
1683f
5
5 Page 
U
OPERATIO
The voltage slew rate is thus inversely proportional to both
the value of capacitor and RVSL.
The current slew feedback loop consists of the voltage
across the external sense resistor, which is internally
amplified and differentiated. The derivative is limited to a
value set by RCSL. The current slew rate is thus inversely
proportional to both the value of sense resistor and RCSL.
The two control loops are combined internally so that a
smooth transition from current slew control to voltage
slew control is obtained. When turning on, the driver
current will slew before voltage. When turning off, voltage
will slew before current. In general it is desirable to have
RVSL and RCSL of similar value.
Internal Regulator
Most of the control circuitry operates from an internal 2.4V
low dropout regulator that is powered from VIN. The
internal low dropout design allows VIN to vary from 2.7V
to 20V with stable operation of the controller. When SHDN
< 1.3V the internal regulator is completely disabled.
5V Regulator
A 5V regulator is provided for powering external circuitry.
This regulator draws current from VIN and requires VIN to
be greater than 6.5V to be in regulation. It can sink or
source 10mA. The output is current limited to prevent
against destruction from accidental short circuits.
Safety and Protection Features
There are several safety and protection features on the
chip. The first is overcurrent limit. Normally the gate
drivers will go low when the output of the internal sense
amplifier exceeds the voltage on the VC pin. The VC pin is
clamped such that maximum output current is attained
when the CS pin voltage is 0.1V. At that level the outputs
will be immediately turned off (no slew). The effect of this
control is that the output voltage will foldback with
overcurrent.
In addition, if the CS voltage exceeds 0.22V, the VC and SS
pins will be discharged to ground also, resetting the soft-
start function. Thus if a short is present this will allow for
faster MOSFET turnoff and less MOSFET stress.
LT1683
If the voltage on the FB pin exceeds regulation by approxi-
mately 0.22V, the outputs will immediately go low. The
implication is that there is an overvoltage fault.
The voltage on GCL determines two features. The first is
the maximum gate drive voltage. This will protect the
MOSFET gate from overvoltage.
With GCL tied to a Zener or an external voltage source then
the maximum gate driver voltage is approximately
VGCL␣ – 0.2V. If GCL is tied to VIN, then the maximum gate
voltage is determined by VIN and is approximately
VIN – 1.6V. There is an internal 19V Zener on the GCL pin
that prevents the gate driver pin from exceeding approxi-
mately 19V.
In addition, the GCL voltage determines undervoltage
lockout of the gate drives. This feature disables the gate
drivers if VIN is too low to provide adequate voltage to turn
on the MOSFETs. This is helpful during start up to insure
the MOSFETs have sufficient gate drive to saturate.
If GCL is tied to a voltage source or Zener less than 6.8V,
the gate drivers will not turn on until VIN exceeds GCL
voltage by 0.8V. For VGCL above 6.5V, the gate drives are
insured to be off for VIN < 7.3V and they will be turned on
by VGCL + 0.8V.
If GCL is tied to VIN, the gate drivers are always enabled
(undervoltage lockout is disabled).
When driving a push pull transformer, it is important to
make sure that both drivers are not on at the same time.
Even though runaway cannot occur under such cross
conduction with this chip because current slew is regu-
lated, increased current would be possible. This chip has
opposite gate lockout whereby when one MOSFET is on
the other MOSFET cannot be turned on until the gate of the
first drops below 1V. This insures that cross conduction
will not occur.
The gate drives have current limits for the drive currents.
If the sink or source current is greater than 300mA then the
current will be limited.
The V5 regulator also has internal current limiting that will
only guarantee ±10mA output current.
1683f
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11 Page | ||
| Páginas | Total 24 Páginas | |
| PDF Descargar | [ Datasheet LT1683.PDF ] | |
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