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PDF LT1976B Data sheet ( Hoja de datos )
| Número de pieza | LT1976B | |
| Descripción | Step-Down Switching Regulator | |
| Fabricantes | Linear | |
| Logotipo |  |
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LT1976/LT1976B
High Voltage 1.5A, 200kHz
Step-Down Switching Regulator
with 100μA Quiescent Current
FEATURES
■ Wide Input Range: 3.3V to 60V
■ 1.5A Peak Switch Current (LT1976)
■ 100μA Quiescent Current (LT1976)**
■ 1.6mA Quiescent Current (LT1976B)
■ Low Shutdown Current: IQ < 1μA
■ Power Good Flag with Programmable Threshold
■ Load Dump Protection to 60V
■ 200kHz Switching Frequency
■ Saturating Switch Design: 0.2Ω On-Resistance
■ Peak Switch Current Maintained Over
Full Duty Cycle Range*
■ 1.25V Feedback Reference Voltage
■ Easily Synchronizable
■ Soft-Start Capability
■ Small 16-Pin Thermally Enhanced TSSOP Package
U
APPLICATIO S
■ High Voltage Power Conversion
■ 14V and 42V Automotive Systems
■ Industrial Power Systems
■ Distributed Power Systems
■ Battery-Powered Systems
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
Burst Mode is a registered trademark of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
*Protected by U.S. Patents, including 6498466
**See Burst Mode Operation section for conditions
DESCRIPTIO
The LT®1976/LT1976B are 200kHz monolithic step-down
switching regulators that accept input voltages up to 60V.
A high efficiency 1.5A, 0.2Ω switch is included on the die
along with all the necessary oscillator, control and logic
circuitry. Current mode topology is used for fast transient
response and good loop stability.
Innovative design techniques along with a new high volt-
age process achieve high efficiency over a wide input
range. Efficiency is maintained over a wide output current
range by employing Burst Mode operation at low currents,
utilizing the output to bias the internal circuitry, and by
using a supply boost capacitor to fully saturate the power
switch. The LT1976B does not shift into Burst Mode
operation at low currents, eliminating low frequency out-
put ripple at the expense of efficiency. Patented circuitry
maintains peak switch current over the full duty cycle
range.* Shutdown reduces input supply current to less
than 1μA. External synchronization can be implemented
by driving the SYNC pin with logic-level inputs. A single
capacitor from the CSS pin to the output provides a
controlled output voltage ramp (soft-start). The devices
also have a power good flag with a programmable thresh-
old and time-out and thermal shutdown protection.
The LT1976/LT1976B are available in a 16-pin TSSOP
package with Exposed Pad leadframe for low thermal
resistance.
TYPICAL APPLICATIO
14V to 3.3V Step-Down Converter with
100μA No Load Quiescent Current
VIN
3.3V TO 60V
1500pF
10k
4.7μF
100V
CER
330pF
1μF
VIN BOOST
SHDN
SW
0.33μF 33μH 4148
LT1976 0.1μF
10MQ60N
VC CSS
CT
SYNC
GND
VBIAS
FB
PGFB
PG
47pF
165k
1%
100k
1%
VOUT
3.3V
1A
100μF
6.3V
TANT
1976 TA01
LT1976 Supply Current vs
Input Voltage
150
VOUT = 3.3V
TA = 25°C
125
100
75
50
25
0
0 10 20 30 40 50 60
INPUT VOLTAGE (V)
1976 F05
LT1976 Efficiency and Power
Loss vs Load Current
100 10
EFFICIENCY
75
5V
3.3V
50
25
1
TYPICAL
POWER LOSS
0.1
0.01
0 0.001
0.1 1 10 100 1000 10000
LOAD CURRENT (mA)
1976 TA02
1976bfg
1
1 page  
LT1976/LT1976B
TYPICAL PERFOR A CE CHARACTERISTICS
LT1976 Efficiency and Power Loss
vs Load Current
100 10
EFFICIENCY
75
5V
3.3V
50
25
1
TYPICAL
POWER LOSS
0.1
0.01
LT1976B Efficiency and Power
Loss vs Load Current
100
EFFICIENCY
75
5V
3.3V
50
TYPICAL
POWER LOSS
25
10
1
0.1
0.01
0
0.1 1
0.001
10 100 1000 10000
LOAD CURRENT (mA)
1976 TA02
0
0.1 1
0.001
10 100 1000 10000
LOAD CURRENT (mA)
1976 G25
FB Voltage
1.30
1.29
1.28
1.27
1.26
1.25
1.24
1.23
1.22
1.21
1.20
–50 –25 0 25 50 75 100 125 150
TEMPERATURE (°C)
1976 G01
Oscillator Frequency
250
240
230
220
210
200
190
180
170
160
150
–50 –25 0 25 50 75 100 125 150
TEMPERATURE (°C)
1976 G02
Shutdown Supply Current
25
20
VIN = 60V
15
10
5
VIN = 42V VIN = 12V
0
–50 –25
0 25 50 75 100 125 150
TEMPERATURE (°C)
1976 G05
SHDN Threshold
1.40
1.35
1.30
1.25
1.20
0.15
1.10
1.05
1.00
–50 –25 0 25 50 75 100 125 150
TEMPERATURE (°C)
LT1976 Sleep Mode Supply
Current
1976 G03
240
220
200
180
160
140
120
100
80
60
40
20
0
–50 –25
VBIAS = 0V
VBIAS = 5V
0 25 50 75 100 125 150
TEMPERATURE (°C)
1976 G06
SHDN Pin Current
5.5
TJ = 25°C
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0
0 10 20 30 40
SHDN VOLTAGE (V)
50 60
1976 G04
LT1976 Bias Sleep Current
200
180
160
140
120
100
80
60
40
20
0
–50 –25 0 25 50 75 100 125 150
TEMPERATURE (°C)
1976 G07
1976bfg
5
5 Page 
LT1976/LT1976B
BLOCK DIAGRA
power from the VIN pin, but if the BIAS pin is connected to
an external voltage higher than 3V bias power will be
drawn from the external source (typically the regulated
output voltage). This improves efficiency.
High switch efficiency is attained by using the BOOST pin
to provide a voltage to the switch driver which is higher
than the input voltage, allowing switch to be saturated.
This boosted voltage is generated with an external capaci-
tor and diode.
To further optimize efficiency, the LT1976 automatically
switches to Burst Mode operation in light load situations.
In Burst Mode operation, all circuitry associated with
controlling the output switch is shut down reducing the
input supply current to 45μA.
The only difference between the LT1976 and the LT1976B
is that the LT1976B does not shift into burst mode in light
load situations, eliminating low frequency output ripple at
the expense of light load efficiency.
The LT1976 contains a power good flag with a program-
mable threshold and delay time. A logic-level low on the
SHDN pin disables the IC and reduces input suppy current
to less than 1μA.
APPLICATIO S I FOR ATIO
CHOOSING THE LT1976 OR LT1977
The LT1976/LT1976B and LT1977 are high voltage 1.5A
step-down switching regulators. The LT1976 and LT1977
contain circuitry which shifts into burst mode at light loads
reducing quiescent current to typically 100μA. The LT1976B
pulse skips in light load situations, eliminating low fre-
quency burst mode output ripple at the expense of light
load efficiency. The difference between the LT1976/
LT1976B and LT1977 is that the fixed switching frequency
of the LT1976/LT1976B is 200kHz versus 500kHz for the
LT1977. The switching frequency affects: inductor size,
input voltage range in continuous mode operation, effi-
ciency, thermal loss and EMI.
OUTPUT RIPPLE AND INDUCTOR SIZE
Output ripple current is determined by the input to output
voltage ratio, inductor value and switch frequency. Since
the switch frequency of the LT1977 is 2.5 times greater
than that of the LT1976/LT1976B, the inductance used in
the LT1977 application can be 2.5 times lower than the
LT1976/LT1976B while maintaining the same output ripple
current. The lower value used in the LT1977 application
allows the use of a physically smaller inductor.
INPUT VOLTAGE RANGE
The minimum on and off times for all versions of the ic are
equivalent. This results in a narrower range of continuous
mode operation for the LT1977. Typical minimum and
maximum duty cycles are 6% to 92% for the LT1976/
LT1976B and 15% to 90% for the LT1977. Both parts will
regulate up to an input voltage of 60V but the LT1977 will
transistion into pulse-skipping/Burst Mode operation when
the input voltage is above 30V for a 5V output. At outputs
above 10V the LT1977’s input range will be similar to the
LT1976/LT1976B. Lowering the input voltage below the
maximum duty cycle limitation will cause a dropout in
regulation.
Table 1. LT1976/LT1976B/LT1977 Comparison
PARAMETER
ADVANTAGE
Minimum Duty Cycle
LT1976/LT1976B
Maximum Duty Cycle
LT1976/LT1976B
Inductor Size
LT1977
Output Capacitor Size
LT1977
Efficiency
LT1976
EMI LT1976B
Input Range
LT1976/LT1976B
Output Ripple
LT1977
Light Load Output Ripple
LT1976B
1976bfg
11
11 Page | ||
| Páginas | Total 28 Páginas | |
| PDF Descargar | [ Datasheet LT1976B.PDF ] | |
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