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PDF LT1938 Data sheet ( Hoja de datos )
| Número de pieza | LT1938 | |
| Descripción | Step-Down Switching Regulator | |
| Fabricantes | Linear | |
| Logotipo |  |
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FEATURES
■ Wide Input Voltage Range: 3.6V to 25V
■ 2.2A Maximum Output Current
■ Adjustable Switching Frequency: 300kHz to 2.8MHz
■ Low Shutdown Current: IQ < 1µA
■ Integrated Boost Diode
■ Power Good Flag
■ Saturating Switch Design: 0.18Ω On-Resistance
■ 1.265V Feedback Reference Voltage
■ Output Voltage: 1.265V to 20V
■ Soft-Start Capability
■ Small 10-Pin Thermally Enhanced (3mm × 3mm)
DFN Package
APPLICATIONS
■ Automotive Battery Regulation
■ Power for Portable Products
■ Distributed Supply Regulation
■ Industrial Supplies
■ Wall Transformer Regulation
LT1938
25V, 2.2A, 2.8MHz
Step-Down
Switching Regulator
DESCRIPTION
The LT®1938 is an adjustable frequency (300kHz to
2.8MHz) monolithic buck switching regulator that accepts
input voltages up to 25V. A high efficiency 0.18Ω switch
is included on the die along with a boost Schottky diode
and the necessary oscillator, control and logic circuitry.
Current mode topology is used for fast transient response
and good loop stability. The LT1938’s high operating fre-
quency allows the use of small, low cost inductors and
ceramic capacitors resulting in low output ripple while
keeping total solution size to a minimum. The low current
shutdown mode reduces input supply current to less than
1µA while a resistor and capacitor on the RUN/SS pin
provide a controlled output voltage ramp (soft-start). A
power good flag signals when VOUT reaches 90% of the
programmed output voltage. The LT1938 is available in
a 3mm × 3mm DFN package with Exposed Pad for low
thermal resistance.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
TYPICAL APPLICATION
VIN
4.5V TO
25V
3.3V Step-Down Converter
4.7µF
OFF ON
16.2k
680pF
VIN
RUN/SS
BD
BOOST
VC SW
LT1938
RT
PG BIAS
60.4k
GND FB
0.47µF 4.7µH
324k
200k
VOUT
3.3V
2.2A
22µF
1938 TA01
Efficiency (VOUT = 3.3V)
90
VIN = 7V
85
80
VIN = 24V
VIN = 12V
75
70
65
60
55 L: NEC PLC-0745-4R7
f = 800kHz
50
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
LOAD CURRENT (A)
1938 G02
1938fa
1
1 page  
LT1938
typical performance characteristics (TA = 25°C unless otherwise noted)
Feedback Voltage
1.290
1.285
1.280
1.275
1.270
1.265
1.260
1.255
1.250
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1938 G10
Minimum Switch On-Time
140
120
100
80
60
40
20
0
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1938 G13
Boost Diode
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
0 0.5 1.0 1.5 2.0
BOOST DIODE CURRENT (A)
1938 G16
Switching Frequency
1.20
RT = 45.3k
1.15
1.10
1.05
1.00
0.95
0.90
0.85
0.80
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1938 G11
Soft-Start
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0 0.5 1 1.5 2 2.5 3 3.5
RUN/SS PIN VOLTAGE (V)
1938 G14
Error Amp Output Current
100
80
60
40
20
0
–20
–40
–60
–80
1.065
1.165 1.265 1.365
FB PIN VOLTAGE (V)
1.465
1938 G17
Frequency Foldback
1200
RT = 45.3k
1000
800
600
400
200
0
0 200 400 600 800 1000 1200 1400
FB PIN VOLTAGE (mV)
1938 G12
RUN/SS Pin Current
12
10
8
6
4
2
0
0 5 10 15 20 25
RUN/SS PIN VOLTAGE (V)
1938 G15
Minimum Input Voltage
4.5
4.0
3.5
3.0
2.5
2.0
0.001
VOUT = 3.3V
TA = 25°C
L = 4.7µH
f = 800kHz
0.01 0.1
1
LOAD CURRENT (A)
10
1938 G18
1938fa
5
5 Page 
LT1938
Applications Information
frequency will be necessary to achieve safe operation at
high input voltages.
If the output is in regulation and no short-circuit or start-up
events are expected, then input voltage transients of up to
25V are acceptable regardless of the switching frequency.
In this mode, the LT1938 may enter pulse skipping opera-
tion where some switching pulses are skipped to maintain
output regulation. In this mode the output voltage ripple
and inductor current ripple will be higher than in normal
operation.
The minimum input voltage is determined by either the
LT1938’s minimum operating voltage of ~3.6V or by its
maximum duty cycle (see equation in previous section).
The minimum input voltage due to duty cycle is:
VIN(MIN)
=
1–
VOUT + VD
fSW tOFF(MIN)
–
VD
+
VSW
where VIN(MIN) is the minimum input voltage, and tOFF(MIN)
is the minimum switch off time (150ns). Note that higher
switching frequency will increase the minimum input
voltage. If a lower dropout voltage is desired, a lower
switching frequency should be used.
Inductor Selection
For a given input and output voltage, the inductor value
and switching frequency will determine the ripple current.
The ripple current ∆IL increases with higher VIN or VOUT
and decreases with higher inductance and faster switch-
ing frequency. A reasonable starting point for selecting
the ripple current is:
∆IL = 0.4(IOUT(MAX))
where IOUT(MAX) is the maximum output load current. To
guarantee sufficient output current, peak inductor current
must be lower than the LT1938’s switch current limit (ILIM).
The peak inductor current is:
IL(PEAK) = IOUT(MAX) + ∆IL/2
where IL(PEAK) is the peak inductor current, IOUT(MAX) is
the maximum output load current, and ∆IL is the inductor
ripple current. The LT1938’s switch current limit (ILIM) is
at least 3.5A at low duty cycles and decreases linearly to
2.5A at DC = 0.8. The maximum output current is a func-
tion of the inductor ripple current:
IOUT(MAX) = ILIM – ∆IL/2
Be sure to pick an inductor ripple current that provides
sufficient maximum output current (IOUT(MAX)).
The largest inductor ripple current occurs at the highest
VIN. To guarantee that the ripple current stays below the
specified maximum, the inductor value should be chosen
according to the following equation:
L
=
⎛
⎝⎜
VOUT +
fΔIL
VD
⎞
⎠⎟
⎛
⎝⎜⎜ 1–
VOUT + VD
VIN(MAX)
⎞
⎠⎟⎟
where VD is the voltage drop of the catch diode (~0.4V),
VIN(MAX) is the maximum input voltage, VOUT is the output
voltage, fSW is the switching frequency (set by RT), and L
is in the inductor value.
The inductor’s RMS current rating must be greater than the
maximum load current and its saturation current should be
about 30% higher. For robust operation in fault conditions
(start-up or short circuit) and high input voltage (>30V),
the saturation current should be above 3A. To keep the
efficiency high, the series resistance (DCR) should be less
than 0.1Ω, and the core material should be intended for
high frequency applications. Table 1 lists several vendors
and suitable types.
Table 1. Inductor Vendors
VENDOR
Murata
TDK
Toko
Sumida
URL
www.murata.com
www.componenttdk.com
www.toko.com
www.sumida.com
PART SERIES
LQH55D
SLF7045
SLF10145
D62CB
D63CB
D75C
D75F
CR54
CDRH74
CDRH6D38
CR75
TYPE
Open
Shielded
Shielded
Shielded
Shielded
Shielded
Open
Open
Shielded
Shielded
Open
1938fa
11
11 Page | ||
| Páginas | Total 24 Páginas | |
| PDF Descargar | [ Datasheet LT1938.PDF ] | |
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