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PDF LT1977 Data sheet ( Hoja de datos )
| Número de pieza | LT1977 | |
| Descripción | Step-Down Switching Regulator | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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Hay una vista previa y un enlace de descarga de LT1977 (archivo pdf) en la parte inferior de esta página. Total 24 Páginas | ||
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LT1977
High Voltage 1.5A, 500kHz
Step-Down Switching Regulator
with 100µA Quiescent Current
FEATURES
DESCRIPTIO
■ Wide Input Range: 3.3V to 60V
■ 1.5A Peak Switch Current
■ Burst Mode® Operation: 100µA Quiescent Current**
■ Low Shutdown Current: IQ < 1µA
■ Power Good Flag with Programmable Threshold
■ Load Dump Protection to 60V
■ 500kHz 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
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a registered trademark of Linear Technology Corporation.
*U.S. Patent 6,498,466 and 6,531,909 **See Burst Mode Operation section for conditions
The LT®1977 is a 500kHz monolithic buck switching
regulator that accepts 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 cir-
cuitry. 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. 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
device also has a power good flag with a programmable
threshold and time-out and thermal shutdown protection.
The LT1977 is available in a 16-pin TSSOP package with
exposed pad leadframe for low thermal resistance. The
LT1976, a 200kHz reduced switch frequency version of the
LT1977, is also available. See the Applications Information
section for selection criteria between the LT1976 and
LT1977.
TYPICAL APPLICATIO
14V to 3.3V Step-Down Converter with
100µA No Load Quiescent Current
VIN
1500pF
26k
2.2µF
100V
CER
330pF
1µF
VIN BOOST
SHDN
SW
0.1µF 10µH 1N4148
LT1977
0.1µF
10MQ100N
VC CSS
CT
SYNC
GND
VBIAS
FB
PGFB
PG
10pF
165k
1% +
100k
1%
VOUT
3.3V
1A
100µF
6.3V
TANT
1977 TA01
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)
1977 F05
Efficiency and Power Loss
vs Load Current
100 VIN = 12V
EFFICIENCY
5V
75
3.3V
10
1
50 0.1
TYPICAL
POWER LOSS
25 0.01
0
0.0001 0.001 0.01
0.1
LOAD CURRENT (A)
0.001
1 10
1977 TA02
1977f
1
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
Switch Peak Current Limit
3.5
3.0
2.5
2.0
1.5
–50 –25
–0 25 50 75
TEMPERATURE (°C)
100 125
1977 G10
Switch On Voltage (VCESAT)
500
450
400
350
300
250 TJ = 125°C
200 TJ = 25°C
150
100
50 TJ = –50°C
0
–0.1 0.1 0.3 0.5 0.7 0.9 1.1 1.3 1.5
LOAD CURRENT (A)
1977 G13
Burst Mode Threshold
vs Input Voltage
500
VOUT = 3.3V
450 L = 10µH
400 COUT = 100µF
350
300
Burst Mode EXIT
(INCREASING LOAD)
250
200
Burst Mode ENTER
150 (DECREASING LOAD)
100
50
0
5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
INPUT VOLTAGE (V)
1977 G20
Soft-Start Current Threshold
vs FB Voltage
50
TJ = 25°C
45
40
35
SOFT-START
30 DEFEATED
25
20
15
10
5
0
0 0.2 0.4 0.6 0.8 1.0 1.2
FB VOLTAGE (V)
1977 G11
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)
1977 F05
Minimum On Time
500
450
400
350 LOAD CURRENT = 0.5A
300
250 LOAD CURRENT = 1A
200
150
100
50
0
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1977 G21
LT1977
Frequency Foldback Percentage
100
90
80
70
60
50
40
30
20
10
0
0 0.25 0.5 0.75 1 1.25
FB PIN VOLTAGE (V)
1977 G12
Minimum Input Voltage
8.0
7.5 VOUT = 5V
7.0 START-UP
RUNNING
6.5
6.0
5.5
5.0
VOUT = 3.3V
START-UP
4.5 RUNNING
4.0
3.5
3.0
0
0.25 0.5 0.75
1
LOAD CURRENT (A)
1.25
1977 G19
Boost Current vs Load Current
45
40
35
30
25
20
15
10
5
0
0 0.25 0.5 0.75 1 1.25 1.5
LOAD CURRENT (A)
1977 G22
1977f
5
5 Page 
LT1977
APPLICATIO S I FOR ATIO
INPUT CAPACITOR
Step-down regulators draw current from the input supply
in pulses. The rise and fall times of these pulses are very
fast. The input capacitor is required to reduce the voltage
ripple this causes at the input of LT1977 and force the
switching current into a tight local loop, thereby minimiz-
ing EMI. The RMS ripple current can be calculated from:
( )IRIPPLE(RMS)
=
IOUT
VIN
VOUT VIN – VOUT
Ceramic capacitors are ideal for input bypassing. At 500kHz
switching frequency input capacitor values in the range of
4.7µF to 20µF are suitable for most applications. If opera-
tion is required close to the minimum input required by the
LT1977 a larger value may be required. This is to prevent
excessive ripple causing dips below the minimum operat-
ing voltage resulting in erratic operation.
Input voltage transients caused by input voltage steps or
by hot plugging the LT1977 to a pre-powered source such
as a wall adapter can exceed maximum VIN ratings. The
sudden application of input voltage will cause a large
surge of current in the input leads that will store energy in
the parasitic inductance of the leads. This energy will
cause the input voltage to swing above the DC level of input
power source and it may exceed the maximum voltage
rating of the input capacitor and LT1977. All input voltage
transient sequences should be observed at the VIN pin of
the LT1977 to ensure that absolute maximum voltage
ratings are not violated.
The easiest way to suppress input voltage transients is to
add a small aluminum electrolytic capacitor in parallel with
the low ESR input capacitor. The selected capacitor needs
to have the right amount of ESR to critically damp the
resonant circuit formed by the input lead inductance and
the input capacitor. The typical values of ESR will fall in the
range of 0.5Ω to 2Ω and capacitance will fall in the range
of 5µF to 50µF.
If tantalum capacitors are used, values in the 22µF to
470µF range are generally needed to minimize ESR and
meet ripple current and surge ratings. Care should be
taken to ensure the ripple and surge ratings are not
exceeded. The AVX TPS and Kemet T495 series are surge
rated. AVX recommends derating capacitor operating
voltage by 2:1 for high surge applications.
OUTPUT CAPACITOR
The output capacitor is normally chosen by its effective
series resistance (ESR) because this is what determines
output ripple voltage. To get low ESR takes volume, so
physically smaller capacitors have higher ESR. The ESR
range for typical LT1977 applications is 0.05Ω to 0.2Ω. A
typical output capacitor is an AVX type TPS, 100µF at 10V,
with a guaranteed ESR less than 0.1Ω. This is a “D” size
surface mount solid tantalum capacitor. TPS capacitors
are specially constructed and tested for low ESR, so they
give the lowest ESR for a given volume. The value in
microfarads is not particularly critical and values from
22µF to greater than 500µF work well, but you cannot
cheat Mother Nature on ESR. If you find a tiny 22µF solid
tantalum capacitor, it will have high ESR and output ripple
voltage could be unacceptable. Table 3 shows some
typical solid tantalum surface mount capacitors.
Table 3. Surface Mount Solid Tantalum Capacitor ESR
and Ripple Current
E CASE SIZE
ESR MAX (Ω)
RIPPLE CURRENT (A)
AVX TPS
0.1 to 0.3
0.7 to 1.1
D CASE SIZE
AVX TPS
0.1 to 0.3
0.7 to 1.1
C CASE SIZE
AVX TPS
0.2
0.5
Many engineers have heard that solid tantalum capacitors
are prone to failure if they undergo high surge currents.
This is historically true and type TPS capacitors are
specially tested for surge capability but surge ruggedness
is not a critical issue with the output capacitor. Solid
tantalum capacitors fail during very high turn-on surges
which do not occur at the output of regulators. High
discharge surges, such as when the regulator output is
dead shorted, do not harm the capacitors.
Unlike the input capacitor RMS, ripple current in the
output capacitor is normally low enough that ripple cur-
rent rating is not an issue. The current waveform is
1977f
11
11 Page | ||
| Páginas | Total 24 Páginas | |
| PDF Descargar | [ Datasheet LT1977.PDF ] | |
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