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PDF LT1950 Data sheet ( Hoja de datos )
| Número de pieza | LT1950 | |
| Descripción | Single Switch PWM Controller | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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FEATURES
s Wide Input Range: 3V to 25V
s Programmable Volt-Second Clamp
s Output Power Levels from 25W to 500W
s Auxiliary Boost Converter Provides 10V Gate Drive
from VIN as Low as 3V
s Programmable Operating Frequency (100kHz to
500kHz) with One External Resistor
s Programmable Slope Compensation
s Programmable Leading Edge Blanking
s ±2% Internal 1.23V Reference
s Accurate Shutdown Pin Threshold with
Programmable Hysteresis
s 60ns Current Sense Delay
s 2.5V Auxiliary Reference Output
s Synchronizable to an External Clock up to 1.5 • fOSC
s Current Mode Control
s Small 16-Pin SSUOP Package
APPLICATIO S
s Telecom Power Supplies
s Automotive Power Supplies
s Portable Electronic Equipment
s Isolated and Nonisolated DC/DC Converters
LT1950
Single Switch PWM
Controller with Auxiliary
Boost Converter
DESCRIPTIO
The LT®1950 is a wide input range, forward, boost, flyback
and SEPIC controller that drives an N-channel power
MOSFET with few external components required.
A resistor programmable duty cycle clamp can be used to
generate a volt-second clamp for forward converter appli-
cations. An internal boost switcher is available for creating
a separate supply for the output gate driver, allowing 10V
gate drive from input voltages as low as 3V. The LT1950’s
operating frequency can be set with an external resistor
over a 100kHz to 500kHz range and a SYNC pin allows the
part to be synchronized to an external clock. Additional
programmability exists for leading edge blanking and
slope compensation.
A fast current sense comparator achieves 60ns current
sense delay and the error amplifier is a true voltage mode
error amplifier, allowing a wide range of compensation
networks. An accurate shutdown pin with programmable
hysteresis is available for undervoltage lockout and shut-
down. The LT1950 is available in a small 16-Pin SSOP
package.
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATIO
36V to 72V DC to 26V/5A (Single Switch) Forward Converter
2.5V
0.1µF
249k
4.99k
SLOPE
VIN
VREF
VIN2
BOOST VSEC
ROSC
SHDN
LT1950
BLANK GATE
SYNC
GND
ISENSE
PGND
FB COMP
10VBIAS
VIN
470k
MBRB20200 47µH
47µF
1µF 18k
Si7450
0.015Ω
4.7k
0.022µF 100k
1950 TA01a
VOUT
26V
5A
Efficiency vs Load Current
95
VIN = 36V
90 VIN = 48V
VIN = 72V
85
80
75
70
0.5
1.5 2.5 3.5 4.5
LOAD CURRENT (A)
5.5
s19n501T9A5010b 1950fs
1
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TYPICAL PERFOR A CE CHARACTERISTICS
BLANK Override Threshold –
ISENSE Maximum Threshold vs
Temperature
40
Minimum VIN Start-Up Voltage vs
Temperature (VIN2 Boosted)
3.00
35
2.75
30
25 2.50
20
2.25
15
10
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1950 G07
SHDN Input Current *(–1) vs
Temperature
14
12
SHDN = SHDN THRESHOLD + 70mV
10
8
6
4 SHDN = SHDN THRESHOLD – 100mV
2
0
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1950 G10
VIN2: GATE Enable
vs Temperature
9.2
8.7
GATE ENABLE
8.2
HYSTERESIS
7.7
GATE DISABLE
7.2
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1950 G13
2.00
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1950 G08
SHDN Current Hysteresis vs
Temperature
11
10
9
8
7
6
5
4
3
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1950 G11
VIN2: BOOST Disable
vs Temperature
13.0
12.5
12.0
11.5
BOOST DISABLE
11.0
10.5 HYSTERESIS
10.0
BOOST RE-ENABLE
9.5
9.0
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1950 G14
LT1950
VIN IQ vs Temperature
3.1
2.9
2.7
2.5
2.3
2.1
1.9
1.7
1.5
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1950 G09
GATE Rise/Fall Time vs
GATE Capacitance
125
TA = 25°C
100
tr
75
tf
50
25
0
0 1000 2000 3000 4000 5000
GATE CAPACITANCE (pF)
1950 G12
BOOST Switch ILIMIT vs
Temperature
250
200
150
100
50
–50 –25
0 25 50 75 100 125
TEMPERATURE (°C)
1950 G15
sn1950 1950fs
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LT1950
APPLICATIO S I FOR ATIO
Shutdown and Undervoltage Lockout
Figure 4 shows how to program undervoltage lockout
(UVLO) for the VIN supply. Typically, UVLO is used in
situations where the input supply is current limited, or has
a relatively high source resistance. A switching regulator
draws constant power from the source, hence source
current increases as source voltage drops. This looks like
a negative load resistance to the source and can cause the
source to current limit or latch low under low source
voltage conditions. An internally set undervoltage lockout
(UVLO) threshold prevents the regulator from operating at
source voltages where these problems might occur. An
internal comparator will force the part into shutdown
below the minimum VIN of 2.6V. This feature can be used
to prevent excessive discharge of battery-operated sys-
tems. Alternatively, UVLO threshold is adjustable. The
shutdown threshold voltage of the SHDN pin is 1.32V.
Forcing the SHDN pin below this 1.32V threshold causes
VREF to be disabled and stops switching at the GATE pin.
If the SHDN pin is left open circuit, a permanent 3µA flows
out of the pin to ensure that the pin defaults high to allow
normal operation. Voltages above the 1.32V threshold
cause an extra 7µA to be sourced out of the pin, providing
current hysteresis. This can be used to set voltage hyster-
esis of the UVLO threshold using the following equations:
Example: switching should not start until the input is
above 11V and is to stop if the input falls below 9V.
VH = 11V
VL = 9V
R1= 11V – 9V = 286k
7µA
R2 = 1.32V = 36k
(11V – 1.32V) + 3µA
286k
Keep the connections from the resistors to the SHDN pin
short and make sure that the interplane or surface capaci-
tance to the switching nodes are minimized. If high resis-
tance values are used, the SHDN pin should be bypassed
with a 1nF capacitor to prevent coupling problems from
the switch node.
LT1950
VIN
R1 3µA
C1 R2
1.32V
7µA
GND
VREF
+
R1= VH – VL
7µA
R2 = 1.32V
(VH – 1.32V) + 3µA
R1
VH = Turn on threshold
VL = Turn off threshold
Figure 4. Undervoltage Lockout
1950 F04
Generating VIN2 Supply Using BOOST Pin
The LT1950’s BOOST pin is used to provide a “boosted”
11V supply at the VIN2 pin for VIN voltages as low as 3V.
Since VIN2 supplies the output driver for the GATE pin of
the IC, it is advantageous to generate a boosted VIN2. This
architecture achieves high GATE drive for an external
sn1950 1950fs
11
11 Page | ||
| Páginas | Total 20 Páginas | |
| PDF Descargar | [ Datasheet LT1950.PDF ] | |
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