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PDF LT1976EFE Data sheet ( Hoja de datos )
| Número de pieza | LT1976EFE | |
| Descripción | High Voltage 1.5A/ 200kHz Step-Down Switching Regulator with 100mA Quiescent Current | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LT1976
High Voltage 1.5A, 200kHz
Step-Down Switching Regulator
with 100µA Quiescent Current
FEATURES
s Wide Input Range: 3.3V to 60V
s 1.5A Peak Switch Current
s Burst Mode® Operation: 100µA Quiescent Current**
s Low Shutdown Current: IQ < 1µA
s Power Good Flag with Programmable Threshold
s Load Dump Protection to 60V
s 200kHz Switching Frequency
s Saturating Switch Design: 0.2Ω On-Resistance
s Peak Switch Current Maintained Over
Full Duty Cycle Range*
s 1.25V Feedback Reference Voltage
s Easily Synchronizable
s Soft-Start Capability
s Small 16-Pin Thermally Enhanced TSSOP Package
U
APPLICATIO S
s High Voltage Power Conversion
s 14V and 42V Automotive Systems
s Industrial Power Systems
s Distributed Power Systems
s Battery-Powered Systems
DESCRIPTIO
The LT®1976 is a 200kHz 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 LT1976 is available in a 16-pin TSSOP package with
exposed pad leadframe for low thermal resistance.
, 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 **See Burst Mode Operation section for conditions
TYPICAL APPLICATIO
14V to 3.3V Step-Down Converter with
100µA No Load Quiescent Current
VIN
3.3V TO 60V
0.047µF
12.5k
4.7µF
100V
CER
100pF
1µF
VIN BOOST
SHDN
SW
0.33µF 33µH 4148
LT1976 0.1µF
10MQ100N
VC CSS
CT
SYNC
GND
VBIAS
FB
PGFB
PG
27pF
165k
1%
100k
1%
VOUT
3.3V
1A
100µF
6.3V
TANT
1976 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)
1976 F05
Efficiency vs Load Current
100
90
VOUT = 5V
80
TA = 25°C
70 VOUT = 3.3V
60
50
40
30
20
10
0
0.0001 0.001 0.01
0.1
LOAD CURRENT (A)
1
10
1976 TA02
1976f
1
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
LT1976
Switch Peak Current Limit
3.5
3.0
2.5
2.0
1.5
–50 –25
–0 25 50 75
TEMPERATURE (°C)
100 125
1976 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)
1976 G13
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)
1976 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)
1976 F05
Oscillator Frequency
vs FB Voltage
250
TJ = 25°C
200
150
100
50
0
0 0.2 0.4 0.6 0.8 1.0 1.2
FB VOLTAGE (V)
1976 G12
Minimum Input Voltage
7.5
VOUT = 5V
7.0
START-UP
6.5
RUNNING
6.0
5.5 VOUT = 3.3V
5.0 START-UP
4.5 RUNNING
4.0
3.5
3.0
0
0.25 0.50 0.75 1.00 1.25 1.50
LOAD CURRENT (A)
1976 G19
Burst Mode Operation
Burst Mode Operation
VOUT
50mV/DIV
VOUT
50mV/DIV
ISW
100mA/DIV
VIN = 12V
VOUT = 3.3V
IQ = 100µA
TIME (5ms/DIV)
0A
1976 G14
ISW
100mA/DIV
VIN = 12V TIME (10µs/DIV)
VOUT = 3.3V
IQ = 100µA
0A
1976 G15
1976f
5
5 Page 
LT1976
APPLICATIO S I FOR ATIO
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 volt-
age 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 LT1976 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 2 shows some
typical solid tantalum surface mount capacitors.
Table 2. 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
triangular with a typical value of 200mARMS. The formula
to calculate this is:
Output capacitor ripple current (RMS)
IRIPPLE(RMS)
=
0.29(VOUT)(VIN – VOUT)
(L)(f)(VIN)
=
IP-P
12
CERAMIC CAPACITORS
Higher value, lower cost ceramic capacitors are now
becoming available. They are generally chosen for their
good high frequency operation, small size and very low
ESR (effective series resistance). Low ESR reduces output
ripple voltage but also removes a useful zero in the loop
frequency response, common to tantalum capacitors. To
compensate for this a resistor RC can be placed in series
with the VC compensation capacitor CC (Figure 10). Care
must be taken however since this resistor sets the high
frequency gain of the error amplifier including the gain at
the switching frequency. If the gain of the error amplifier
is high enough at the switching frequency output ripple
voltage (although smaller for a ceramic output capacitor)
may still affect the proper operation of the regulator. A
filter capacitor CF in parallel with the RC/CC network, along
with a small feedforward capacitor CFB, is suggested to
control possible ripple at the VC pin. The LT1976 can be
stabilized using a 47µF ceramic output capacitor and VC
component values of CC = 0.047µF, RC = 12.5k, CF = 100pF
and CFB = 27pF.
OUTPUT RIPPLE VOLTAGE
Figure 3 shows a typical output ripple voltage waveform
for the LT1976. Ripple voltage is determined by the
impedance of the output capacitor and ripple current
through the inductor. Peak-to-peak ripple current through
the inductor into the output capacitor is:
IP-P
=
( )VOUT VIN – VOUT
(VIN)(L)(f)
1976f
11
11 Page | ||
| Páginas | Total 24 Páginas | |
| PDF Descargar | [ Datasheet LT1976EFE.PDF ] | |
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