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PDF LT1941 Data sheet ( Hoja de datos )
| Número de pieza | LT1941 | |
| Descripción | Triple Monolithic Switching Regulator | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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FEATURES
s Wide Input Range: 3.5V to 25V
s Three Switching Regulators with Internal Power
Switches: 3A Step-Down, 2A Step-Down,
1.5A Inverting/Boost
s Antiphase Switching Reduces Ripple
s Independent Shutdown/Soft-Start Pins
s Independent Power Good Indicators Ease Supply
Sequencing
s Input Voltage Power Good Indicators Monitor Input
Supply
s Uses Small Inductors and Ceramic Capacitors
s Constant 1.1MHz Switching Frequency
s Thermally Enhanced 28-Lead TSSOP Package
U
APPLICATIO S
s Cable Modems
s DSL Modems
s Distributed Power Regulation
s Wall Transformer Regulation
s Disk Drives
s DSP Power
, LTC and LT are registered trademarks of Linear Technology Corporation.
LT1941www.DataSheet4U.com
Triple Monolithic
Switching Regulator
DESCRIPTIO
The LT®1941 is a triple current mode DC/DC converter
with internal power switches. Two of the regulators are
step-down converters with 3A and 2A power switches.
The third regulator can be configured as a boost, inverter
or SEPIC converter and has a 1.5A power switch. All three
converters are synchronized to a 1.1MHz oscillator. The
two step-down converters run with opposite phases,
reducing input ripple current. The output voltages are set
with external resistor dividers and each regulator has
independent shutdown and soft-start circuits. Each regu-
lator generates a power good signal when its output is in
regulation, easing power supply sequencing and interfac-
ing with microcontrollers and DSPs.
The high switching frequency offers several advantages
by permitting the use of small inductors and ceramic
capacitors. Small inductors and capacitors lead to a very
small triple output solution. The constant switching fre-
quency, combined with low impedance ceramic capaci-
tors, result in low, predictable output ripple. With its wide
input voltage range of 3.5V to 25V, the LT1941 regulates
a broad array of power sources from 4-cell batteries and
5V logic rails to unregulated wall transformers, lead acid
batteries and distributed-power supplies.
TYPICAL APPLICATIO
VIN
4.7V TO 14V
5GOOD
12GOOD
VOUT1
1.8V
2.4A
VOUT3*
–12V
350mA
10µF
VOUT1
VOUT2
130k 100k
PGOOD1
5GOOD PGOOD2
12GOOD PGOOD3
3µH
33µF
0.22µF
13.7k
3300pF
7.32k 3.3k
1.5nF
BOOST1 BOOST2
LT1941
SW1 SW2
FB1 FB2
VC1 VC2
RUNSS1 RUNSS2
22µH 1µF
SW3 BIAS1
NFB BIAS2
22µH
10µF
133k 13.7k
VC3
FB3 RUNSS3
GND
100k 100k 100k
0.22µF
1000pF
10.7k
10k 2.49k
1.5nF
3.3µH
22µF
22nF
1.5nF
1.5k
*240mA AT VIN = 5V, 550mA AT VIN = 12V
1941 F01
PGOOD1
PGOOD2
PGOOD3
VOUT2
3.3V
1.4A
Figure 1. Triple Output Power Supply: 3.3V, 1.8V, –12V
RUN/SS
2V/DIV
VOUT1
2V/DIV
VOUT2
5V/DIV
VOUT3
10V/DIV
IVIN(AVE)
1A/DIV
PGOOD2
5V/DIV
Start-Up Waveforms
with Sequencing
1941 F01b
1941f
1
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
LT1941www.DataSheet4U.com
SW1 VCESAT
500
TA = 25°C
400
300
200
100
0
0 0.5 1 1.5 2 2.5 3
SWITCH CURRENT (A)
1941 G02
SW1 Current Limit vs Duty Cycle
5.0
4.5
TYPICAL
4.0
3.5
3.0 MINIMUM
2.5
2.0
1.5
1.0
0.5
0
0 20 40 60 80 100
DUTY CYCLE (%)
1941 G03
BOOST1 Pin Current
50
TA = 25°C
40
30
20
10
0
0 0.5 1 1.5 2 2.5 3
SW1 PIN CURRENT (A)
1941 G04
SW2 VCESAT
600
TA = 25°C
500
400
300
200
100
0
0 0.5 1 1.5 2
SWITCH CURRENT (A)
1941 G09
SW2 Current Limit vs Duty Cycle
3.0
2.5
TYPICAL
2.0
MINIMUM
1.5
1.0
0.5
0
0 20 40 60 80 100
DUTY CYCLE (%)
1941 G06
VFB3 vs Temperature
1.280
1.265
1.250
1.235
1.220
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1941 G05
SW3 VCESAT
500
TA = 25°C
400
300
200
100
0
0 0.25 0.5 0.75 1
1.25 1.5
SWITCH CURRENT (A)
1941 G10
BOOST2 Pin Current
40
TA = 25°C
30
20
10
0
0 0.5 1 1.5 2
SW2 PIN CURRENT (A)
1941 G11
VFB1, VFB2 vs Temperature
0.645
0.635
0.625
0.615
0.605
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1941 G12
1941f
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5 Page 
LT1941www.DataSheet4U.com
APPLICATIO S I FOR ATIO
The optimum inductor for a given application may differ
from the one indicated by this simple design guide. A
larger value inductor provides a slightly higher maximum
load current and will reduce the output voltage ripple. If
your load is lower than the maximum load current, then
you can relax the value of the inductor and operate with
higher ripple current. This allows you to use a physically
smaller inductor or one with a lower DCR resulting in
higher efficiency. Be aware that if the inductance differs
from the simple rule above, then the maximum load
current will depend on input voltage. In addition, low
inductance may result in discontinuous mode operation,
which further reduces maximum load current. For details
of maximum output current and discontinuous mode
operation, see Linear Technology’s Application Note AN44.
Finally, for duty cycles greater than 50% (VOUT/VIN > 0.5),
a minimum inductance is required to avoid subharmonic
oscillations. See AN19.
The current in the inductor is a triangle wave with an
average value equal to the load current. The peak switch
current is equal to the output current plus half the peak-to-
peak inductor ripple current. The LT1941 limits its switch
current in order to protect itself and the system from
overload faults. Therefore, the maximum output current
that the LT1941 will deliver depends on the switch current
limit, the inductor value and the input and output voltages.
When the switch is off, the potential across the inductor is
the output voltage plus the catch diode drop. This gives the
peak-to-peak ripple current in the inductor:
∆IL = (1 – DC)(VOUT + VF)/(L • f)
where f is the switching frequency of the LT1941 and L is
the value of the inductor. The peak inductor and switch
current is:
ISWPK = ILPK = IOUT + ∆IL/2
To maintain output regulation, this peak current must be
less than the LT1941’s switch current limit ILIM. For SW1,
ILIM is at least 3A at low duty cycles and decreases linearly
to 2.4A at DC = 0.8. For SW2, ILIM is at least 2A for at low
duty cycles and decreases linearly to 1.6A at DC = 0.8. The
maximum output current is a function of the chosen
inductor value:
IOUT(MAX) = ILIM – ∆IL/2
= 3 • (1 – 0.25 • DC) – ∆IL/2 for SW1
= 2 • (1 – 0.25 • DC) – ∆IL/2 for SW2
Choosing an inductor value so that the ripple current is
small will allow a maximum output current near the switch
current limit.
One approach to choosing the inductor is to start with the
simple rule given above, look at the available inductors
and choose one to meet cost or space goals. Then use
these equations to check that the LT1941 will be able to
deliver the required output current. Note again that these
equations assume that the inductor current is continuous.
Discontinuous operation occurs when IOUT is less than
∆IL/2.
Output Capacitor Selection
For 5V and 3.3V outputs, a 10µF, 6.3V ceramic capacitor
(X5R or X7R) at the output results in very low output
voltage ripple and good transient response. For lower
voltages, 10µF is adequate for ripple requirements but
increasing COUT will improve transient performance. Other
types and values will also work; the following discusses
tradeoffs in output ripple and transient performance.
The output capacitor filters the inductor current to gener-
ate an output with low voltage ripple. It also stores energy
in order to satisfy transient loads and stabilize the LT1941’s
control loop. Because the LT1941 operates at a high
frequency, minimal output capacitance is necessary. In
addition, the control loop operates well with or without the
presence of output capacitor series resistance (ESR).
Ceramic capacitors, which achieve very low output ripple
and small circuit size, are therefore an option.
1941f
11
11 Page | ||
| Páginas | Total 24 Páginas | |
| PDF Descargar | [ Datasheet LT1941.PDF ] | |
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