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PDF LT1940EFE Data sheet ( Hoja de datos )
| Número de pieza | LT1940EFE | |
| Descripción | Dual Monolithic 1.4A/ 1.1MHz Step-Down Switching Regulator | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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Final Electrical Specifications
LT1940
Dual Monolithic 1.4A,
1.1MHz Step-Down Switching Regulator
FEATURES
DESCRIPTIO
August 2002
s Wide Input Voltage Range: 3.6V to 25V
s Two 1.4A Output Switching Regulators with
Internal Power Switches
s Small 16-Lead TSSOP Surface Mount Package
s Constant 1.1MHz Switching Frequency
s Anti-Phase Switching Reduces Ripple
s Independent Shutdown/Soft-Start Pins
s Independent Power Good Indicators Ease
Supply Sequencing
s Uses Small Inductors and Ceramic Capacitors
U
APPLICATIO S
s Disk Drives
s DSP Power Supplies
s Wall Transformer Regulation
s Distributed Power Regulation
s DSL Modems
s Cable Modems
, LTC and LT are registered trademarks of Linear Technology Corporation.
The LT®1940 is a dual current mode PWM step-down
DC/DC converter with internal 2A power switches. Both
converters are synchronized to a single 1.1MHz oscillator
and run with opposite phases, reducing input ripple cur-
rent. The output voltages are set with external resistor
dividers, and each regulator has independent shutdown
and soft-start circuits. Each regulator generates a power-
good signal when its output is in regulation, easing power
supply sequencing and interfacing with microcontrollers
and DSPs.
The LT1940’s 1.1MHz switching frequency allows the use
of tiny inductors and capacitors, resulting in a very small
dual 1.4A output solution. Constant frequency and ce-
ramic capacitors combine to produce low, predictable
output ripple voltage. With its wide input range of 3.6V to
25V, the LT1940 regulates a wide variety of power sources,
from 4-cell batteries and 5V logic rails to unregulated wall
transformers, lead acid batteries and distributed-power
supplies. A current mode PWM architecture provides fast
transient response with simple compensation compo-
nents and cycle-by-cycle current limiting. Frequency
foldback and thermal shutdown provide additional protec-
tion.
TYPICAL APPLICATIO
VIN
7V TO 25V
4.7µF
CMDSH-3
OUT1
3.3V
1.4A
0.1µF
3.3µH
UPS140
16.5k
VIN
BOOST1 BOOST2
LT1940
SW1 SW2
FB1 FB2
CMDSH-3
0.1µF
4.7µH
UPS140
30.1k
OUT2
5V
1.4A
10.0k
15k
VC1
RUN/SS1
VC2
PG1
15k 10.0k
10µF
10µF
330pF
1nF
RUN/SS2
PG2
GND
330pF
100k
POWER
1940 F01 GOOD
Figure 1. 3.3V and 5V Dual Output Step-Down
Converter with Output Sequencing
Efficiency vs Load Current
100
VIN = 12V
90
VOUT = 5V
80
VOUT = 3.3V
70
60
0
0.5 1.0
LOAD CURRENT (A)
1.5
1940 F01b
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
1940i
1
1 page  
LT1940
PI FU CTIO S
BOOST1, BOOST2 (Pins 1, 8): The BOOST pins are used
to provide drive voltages, higher than the input voltage, to
the internal bipolar NPN power switches.Tie through a
diode from VOUT or from VIN.
SW1, SW2 (Pins 2, 7): The SW pins are the outputs of the
internal power switches. Connect these pins to the induc-
tors, catch diodes and boost capacitors.
VIN (Pins 3, 4, 5, 6): The VIN pins supply current to the
LT1940’s internal regulator and to the internal power
switches. These pins must be tied to the same source, and
must be locally bypassed.
FB1, FB2 (Pins 9, 16): The LT1940 regulates each feed-
back pin to 1.25V. Connect the feedback resistor divider
taps to these pins.
VC1, VC2 (Pins 10, 15): The VC pins are the outputs of the
internal error amps. The voltages on these pins control the
peak switch currents. These pins are normally used to
compensate the control loops, but can also be used to
override the loops. Pull these pins to ground with an open
drain to shut down each switching regulator.
PG1, PG2 (Pins 11, 14): The Power Good pins are the
open collector outputs of an internal comparator. PG
remains low until the FB pin is within 10% of the final
regulation voltage. As well as indicating output regulation,
the PG pins can be used to sequence the two switching
regulators. These pins can be left unconnected. The PG
outputs are valid when VIN is greater than 2.4V and either
of the RUN/SS pins is high. The PG comparators are
disabled in shutdown.
RUN/SS1, RUN/SS2 (Pins 12, 13): The RUN/SS pins are
use to shut down the individual switching regulators and
the internal bias circuits. They also provide a soft-start
function. To shut down either regulator, pull the RUN/SS
pin to ground with an open drain or collector. Tie a
capacitor from these pins to ground to limit switch current
during start-up. If neither feature is used, leave these pins
unconnected.
GND (Underside Metal): The underside exposed pad
metal of the package provides both electrical contact to
ground and good thermal contact to the printed circuit
board. The underside must be soldered to the circuit
board for proper operation.
1940i
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5 Page 
LT1940
APPLICATIO S I FOR ATIO
Table 2. Low-ESR Surface Mount Capacitors
Vendor
Type
Series
Taiyo Yuden
Ceramic X5R, X7R
AVX Ceramic X5R, X7R
Tantalum
TPS
Kemet
Tantalum
Ta Organic
Al Organic
T491,T494,T495
T520
A700
Sanyo
Ta or Al Organic
POSCAP
Panasonic
Al Organic
SP CAP
TDK Ceramic X5R, X7R
Catch Diode
Use a 1A Schottky diode for the catch diode (D1 in
Figure 2). The diode must have a reverse voltage rating
greater than the maximum input voltage. The ON Semi-
conductor MBRM120LT3 (20V) and MBRM130LT3 (30V)
are good choices; they have a tiny package with good
thermal properties. Many vendors have surface mount
versions of the 1N5817 (20V) and 1N5818 (30V) 1A
Schottky diodes such as the Microsemi UPS120 that are
suitable.
Boost Pin Considerations
The capacitor and diode tied to the BOOST pin generate a
voltage that is higher than the input voltage. In most cases
a 0.1µF capacitor and fast switching diode (such as the
CMDSH-3 or FMMD914) will work well. Figure 3 shows
three ways to arrange the boost circuit. The BOOST pin
must be more than 2.5V above the SW pin for full effi-
ciency. For outputs of 3.3V and higher the standard circuit
(Figure 3a) is best. For outputs between 2.8V and 3.3V,
use a small Schottky diode (such as the BAT-54). For lower
output voltages the boost diode can be tied to the input
(Figure␣ 3b). The circuit in Figure 3a is more efficient
because the BOOST pin current comes from a lower
voltage source. Finally, as shown in Figure 3c, the anode
of the boost diode can be tied to another source that is at
least 3V. For example, if you are generating 3.3V and 1.8V
and the 3.3V is on whenever the 1.8V is on, the 1.8V boost
diode can be connected to the 3.3V output. In any case,
you must also be sure that the maximum voltage at the
BOOST pin is less than the maximum specified in the
Absolute Maximum Ratings section.
D2 D2
BOOST
C3
LT1940
VIN VIN SW
GND
VOUT
BOOST
C3
LT1940
VIN VIN SW
GND
VOUT
VIN2 > 3V
VIN
VBOOST – VSW ≅ VOUT
MAX VBOOST ≅ VIN + VOUT
(3a)
D2
BOOST
LT1940
VIN SW
GND
C3
VBOOST – VSW ≅ VIN2
MAX VBOOST ≅ VIN2 + VIN
MINIMUM VALUE FOR VIN2 = 3V
(3c)
VOUT
1940 F03
VIN2
>VIN + 3V
VIN
VBOOST – VSW ≅ VIN
MAX VBOOST ≅ 2VIN
D2
(3b)
BOOST
LT1940
VIN SW
GND
MAX VBOOST – VSW ≅ VIN2
MAX VBOOST ≅ VIN2
MINIMUM VALUE FOR VIN2 = VIN + 3V
(3d)
VOUT
1940 F03
Figure 3. Generating the Boost Voltage
1940i
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| PDF Descargar | [ Datasheet LT1940EFE.PDF ] | |
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