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PDF LT1738 Data sheet ( Hoja de datos )
| Número de pieza | LT1738 | |
| Descripción | Slew Rate Controlled Ultralow Noise DC/DC Controller | |
| Fabricantes | Linear | |
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LT1738
Slew Rate Controlled
Ultralow Noise DC/DC Controller
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 Single N-Channel MOSFET Driver
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
DESCRIPTIO
The LT®1738 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 an exter-
nal N-channel MOSFET switch. Current and voltage slew
rates can be independently set to optimize harmonic
content of the switching waveforms vs efficiency. The
LT1738 can reduce high frequency harmonic power by as
much as 40dB with only minor losses in efficiency.
The LT1738 utilizes a current mode architecture opti-
mized for single switch topologies such as boost, flyback
and Cuk. The IC includes gate drive and all necessary
oscillator, control and protection circuitry. Unique error
amp circuitry can regulate both positive and negative
voltages. The internal oscillator may be synchronized to
an external clock for more accurate placement of switch-
ing harmonics.
Protection features include gate drive lockout for low VIN,
soft-start, output current limit, short-circuit current limit-
ing, gate drive overvoltage clamp and input supply
undervoltage lockout.
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATIO
Ultralow Noise 5V to 12V Converter
5V
VIN
+
100µF
1.3nF
16.9k
25k 3.3k
25k 3.3k
22nF
0.22µF
1.5k
22µH
P3
17 3
VIN GCL 5pF
14
SHDN
5
V5
6
SYNC
2
CAP
7
CT
1
GATE
8
RT
LT1738
16
RVSL
4
CS
15
RCSL
20
PGND
12
VC
9
FB
SS GND NFB
13 11 10
10nF
MBRD620
B
+
4 × 150µF
OSCON
10µH
150µF +
OSCON
OPTIONAL
A 12V
1A
POINT A
ON SCHEMATIC
500µV/DIV
Si9426
25mΩ
21.5k
POINT B
ON SCHEMATIC
50mV/DIV
2.5k
1738 TA01
12V Output Noise
(Bandwidth = 100MHz)
400µVP-P
5µs/DIV
1738 TA01a
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1
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
LT1738
SHDN Pin Hysteresis Current vs
Temperature
27
25
23
21
19
17
15
–50 –25
0 25 50 75 100 125 150
TEMPERATURE (°C)
1738 G09
Slope Compensation
110
VC PIN = 0.9V
100 TA = 25°C
90
80
70
60
50
0
20 40 60 80
DUTY CYCLE (%)
100
1738 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)
1738 G15
VIN Current vs Temperature
18
17 VIN = 12 RVSL, RCSL = 4.85k
16
15 VIN = 20 RVSL, RCSL = 17k
14
13 VIN = 12 RVSL, RCSL = 17k
12
11 USING LOAD OF FIGURE 1A
fOSC = 120kHz
10
–50 –25 0 25 50 75
100 125 150
TEMPERATURE (°C)
1738 G10
Gate Drive High Voltage vs
Temperature
10.7 6.5
10.6
10.5 GCL = 12V
6.4
6.3
10.4 6.2
10.3
VIN = 12V
6.1
10.2
NO LOAD
6.0
10.1 5.9
10.0 GCL = 6V
9.90
5.8
5.7
9.80 5.6
9.70 5.5
–50 –25 0 25 50 75 100 125 150
TEMPERATURE (°C)
1738 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)
1738 G11
Gate Drive 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)
1738 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)
1738 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
1738 G17
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5 Page 
U
OPERATIO
LT1738
Table 1. Safety and Protection Features
FEATURE
FUNCTION
Maximum Current Fault
Short-Circuit Fault
Overvoltage Fault
Turn Off FET at Maximum
Switch Current (VSENSE = 0.1)
Turn Off FET and Reset VC
for Short-Circuit (VSENSE = 0.22)
Turn Off Driver If FB > VREG + 0.22V
(Output Overvoltage)
GCL Clamp
Set Max Gate Voltage to Prevent
FET Gate Breakdown
Gate Drive
Undervoltage Lockout
Disable Gate Drive When VIN
Is Too Low. Set Via GCL Pin
Thermal Shutdown
Turn Off Driver If Chip
Temperature Is Too Hot
VIN Undervoltage Lockout
Gate Drive Source and Sink Current Limit
Disable Part When VIN ≅ 2.55V
Limit Gate Drive Current
V5 Source/Sink Current Limit
Limit Current from V5
Shutdown
Disable Part When SHDN <1.3V
EFFECT on GATE DRIVER
Immediately Goes Low
Immediately Goes Low
Immediately Goes Low
Limits Max Voltage
Immediately Goes Low
Immediately Goes Low
Immediately Goes Low
Limit Drive Current
None
SLEW CONTROL EFFECT on VC, SS
Overridden
None
Overridden
Overridden
Discharge VC, SS
to GND
None
None None
Overridden
None
Overridden
None
Overridden
None
None
None
None
None
APPLICATIO S I FOR ATIO
Reducing EMI from switching power supplies has tradi-
tionally invoked fear in designers. Many switchers are
designed solely on efficiency and as such produce wave-
forms filled with high frequency harmonics that then
propagate through the rest of the system.
The LT1738 provides control over two of the more impor-
tant variables for controlling EMI with switching inductive
loads: switch voltage slew rate and switch current slew
rate. The use of this part will reduce noise and EMI over
conventional switch mode controllers. Because these
variables are under control, a supply built with this part will
exhibit far less tendency to create EMI and less chance of
encountering problems during production.
It is beyond the scope of this data sheet to get into EMI
fundamentals. Application Note 70 contains much infor-
mation concerning noise in switching regulators and
should be consulted.
Oscillator Frequency
The oscillator determines the switching frequency and
therefore the fundamental positioning of all harmonics.
The use of good quality external components is important
to ensure oscillator frequency stability. The oscillator is of
a sawtooth design. A current defined by external resistor
RT is used to charge and discharge the capacitor CT . The
discharge rate is approximately ten times the charge rate.
By allowing the user to have control over both compo-
nents, trimming of oscillator frequency can be more easily
achieved.
The external capacitance CT is chosen by:
C T (nF) =
2180
f(kHz)•RT (kΩ)
where f is the desired oscillator frequency in kHz. For RT
equal to 16.9k, this simplifies to:
C T (nF) =
129
f(kHz)
e.g., CT = 1.29nF for f = 100kHz
Nominally RT should be 16.9k. Since it sets up current, its
temperature coefficient should be selected to compliment
the capacitor. Ideally, both should have low temperature
coefficients.
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11 Page | ||
| Páginas | Total 20 Páginas | |
| PDF Descargar | [ Datasheet LT1738.PDF ] | |
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