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PDF LT3685 Data sheet ( Hoja de datos )
| Número de pieza | LT3685 | |
| Descripción | Step-Down Switching Regulator | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LT3685
38V, 2A, 2.4MHz Step-Down
Switching Regulator
FEATURES
■ Wide Input Range:
Operation From 3.6V to 38V
Overvoltage Lockout Protects Circuit through
60V Transients
■ 2A Maximum Output Current
■ Adjustable Switching Frequency: 200kHz to 2.4MHz
■ Low Shutdown Current: IQ < 1μA
■ Integrated Boost Diode
■ Synchronizable Between 250kHz to 2MHz
■ Power Good Flag
■ Saturating Switch Design: 0.25 On-Resistance
■ 0.790V Feedback Reference Voltage
■ Output Voltage: 0.79V to 20V
■ Soft-Start Capability
■ Small 10-Pin Thermally Enhanced MSOP and
(3mm × 3mm) DFN Packages
APPLICATIONS
■ Automotive Battery Regulation
■ Set Top Box
■ Distributed Supply Regulation
■ Industrial Supplies
■ Wall Transformer Regulation
DESCRIPTION
The LT®3685 is an adjustable frequency (200kHz to 2.4MHz)
monolithic step-down switching regulator that accepts
input voltages up to 38V operating and 60V maximum.
An internal overvoltage protection circuit turns off the
power switch when VIN is above 41.5V typical (38V mini-
mum) which then allows the part to safely withstand 60V
transients. A high efficiency 0.25 switch is included on
the die along with a boost Schottky diode and the neces-
sary oscillator, control, and logic circuitry. Current mode
topology is used for fast transient response and good
loop stability. The LT3685’s high operating frequency
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 86% of the programmed
output voltage. The LT3685 is available in 10-Pin MSOP
and 3mm × 3mm DFN packages with exposed pads for
low thermal resistance.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
Burst Mode is a registered trademark of Linear Technology Corporation. All other
trademarks are the property of their respective owners.
TYPICAL APPLICATION
VIN
4.5V TO 38V
TRANSIENT
TO 60V
3.3V Step-Down Converter
OFF ON
VIN
RUN/SS
BD
BOOST
4.7μF
470pF
14k
40.2k
VC LT3685
RT
PG
SYNC
GND
SW
FB
0.47μF 4.7μH
316k
100k
VOUT
3.3V
2A
22μF
3685 TA01
100
90
80
70
60
50
0
Efficiency
VOUT = 5V
VOUT = 3.3V
VIN = 12V
L = 5.6μH
F = 800 kHz
0.5 1.0 1.5
LOAD CURRENT (A)
2
3685 TA01b
3685f
1
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LT3685
TYPICAL PERFOR A CE CHARACTERISTICS TA = 25°C unless otherwise noted.
Feedback Voltage
840
820
800
780
760
–50 –25
0 25 50 75 100 125 150
TEMPERATURE (°C)
3685 G09
Switching Frequency
1.20
RT = 29.4k
1.15
1.10
1.05
1.00
0.95
0.90
0.85
0.80
–50 –25
0 25 50 75 100 125 150
TEMPERATURE (°C)
3685 G10
Frequency Foldback
1200
RT = 29.4k
1000
800
600
400
200
0
0 100 200 300 400 500 600 700 800 900
FB PIN VOLTAGE (mV)
3685 G11
Minimum Switch On-Time
140
120
100
80
60
40
20
0
–50 –25
0 25 50 75 100 125 150
TEMPERATURE (˚C)
3685 G12
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)
3685 G13
RUN/SS Pin Current
12
10
8
6
4
2
0
05
10 15 20 25 30 35
RUN/SS PIN VOLTAGE (V)
3685 G14
Boost Diode
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)
3685 G15
Error Amp Output Current
50
40
30
20
10
0
–10
–20
–30
–40
–50
–200
–100
0
100
FB PIN ERROR VOLTAGE (V)
200
3685 G16
3685f
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LT3685
APPLICATIONS INFORMATION
Table 1. Inductor Vendors
VENDOR URL
Murata www.murata.com
TDK www.componenttdk.com
Toko www.toko.com
Sumida 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
Of course, such a simple design guide will not always re-
sult in the optimum inductor for your application. A larger
value inductor provides a slightly higher maximum load
current and will reduce the output voltage ripple. If your
load is lower than 2A, then you can decrease 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. There are
several graphs in the Typical Performance Characteristics
section of this data sheet that show the maximum load
current as a function of input voltage and inductor value
for several popular output voltages. Low inductance may
result in discontinuous mode operation, which is okay
but further reduces maximum load current. For details of
maximum output current and discontinuous mode opera-
tion, see Linear Technology Application Note 44. Finally,
for duty cycles greater than 50% (VOUT/VIN > 0.5), there
is a minimum inductance required to avoid subharmonic
oscillations. See AN19.
Input Capacitor
Bypass the input of the LT3685 circuit with a ceramic capaci-
tor of X7R or X5R type. Y5V types have poor performance
over temperature and applied voltage, and should not be
used. A 4.7μF to 10μF ceramic capacitor is adequate to
bypass the LT3685 and will easily handle the ripple current.
Note that larger input capacitance is required when a lower
switching frequency is used. If the input power source has
high impedance, or there is significant inductance due to
long wires or cables, additional bulk capacitance may be
necessary. This can be provided with a lower performance
electrolytic capacitor.
Step-down regulators draw current from the input sup-
ply in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage
ripple at the LT3685 and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
A 4.7μF capacitor is capable of this task, but only if it is
placed close to the LT3685 and the catch diode (see the
PCB Layout section). A second precaution regarding the
ceramic input capacitor concerns the maximum input
voltage rating of the LT3685. A ceramic input capacitor
combined with trace or cable inductance forms a high
quality (under damped) tank circuit. If the LT3685 circuit
is plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT3685’s
voltage rating. This situation is easily avoided (see the Hot
Plugging Safety section).
For space sensitive applications, a 2.2μF ceramic capaci-
tor can be used for local bypassing of the LT3685 input.
However, the lower input capacitance will result in in-
creased input current ripple and input voltage ripple, and
may couple noise into other circuitry. Also, the increased
voltage ripple will raise the minimum operating voltage
of the LT3685 to ~3.7V.
Output Capacitor and Output Ripple
The output capacitor has two essential functions. Along
with the inductor, it filters the square wave generated by the
LT3685 to produce the DC output. In this role it determines
the output ripple, and low impedance at the switching
frequency is important. The second function is to store
energy in order to satisfy transient loads and stabilize the
LT3685’s control loop. Ceramic capacitors have very low
equivalent series resistance (ESR) and provide the best
ripple performance. A good starting value is:
COUT
=
100
VOUT fSW
3685f
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| Páginas | Total 24 Páginas | |
| PDF Descargar | [ Datasheet LT3685.PDF ] | |
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