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PDF LT3645 Data sheet ( Hoja de datos )
| Número de pieza | LT3645 | |
| Descripción | 36V 500mA Step-Down Regulator and 200mA LDO | |
| Fabricantes | Linear | |
| Logotipo |  |
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LT3645
36V 500mA Step-Down
Regulator and 200mA LDO
FEATURES
n Wide Input Range:
Operation from 3.6V to 36V
Overvoltage Lockout Protects Circuit Through
55V Transients on Input
n 500mA Output Current Switching Regulator
n High Switching Frequency: 750kHz
n 200mA Low Dropout Linear Regulator
1.2V to 16V Input; 0.8V to 8V Output
310mV Dropout Voltage VCC2 to OUT2
n Precision Programmable Undervoltage Lockout
n Short-Circuit Robust
n Internal Soft-Start
n <2μA Shutdown Current
n Small Thermally Enhanced 16-Lead (3mm × 3mm)
QFN and 12-Lead MSE Packages
APPLICATIONS
n Automotive CMOS Image Sensors
n Industrial/Automotive Micro-Controller Supply
DESCRIPTION
The LT®3645 is a dual output regulator combining a 500mA
buck regulator and a 200mA low dropout linear regula-
tor (LDO). The wide input voltage range of 3.6V to 36V
makes the LT3645 suitable for regulating power from a
wide variety of sources, including 24V industrial supplies
and automotive batteries. Its high operating frequency
allows the use of tiny, low cost inductors and capacitors,
resulting in a very small solution.
Cycle-by-cycle current limit and frequency foldback pro-
vide protection against shorted outputs. Soft-start and
frequency foldback eliminate input current surge during
start-up.
The linear regulator operates from the VCC2 pin at voltages
down to 1.2V. It supplies 200mA of output current with a
typical dropout voltage of 310mV.
Other features of the LT3645 include a <2μA shutdown,
short circuit protection, soft-start and thermal shutdown.
The LT3645 is available in the thermally enhanced 16-lead
(3mm × 3mm) QFN package, or a 12-lead MSE package.
L, LT, LTC, LTM, Linear Technology, Burst Mode and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their respective
owners.
TYPICAL APPLICATION
3.3V/5V Step-Down Converter
6.2V TO 36V
ON OFF
BOOST
VIN SW
1μF LT3645
EN/UVLO
DA
FB
0.1μF
15μH
52.3k
10μF
10k
5V
300mA
PGOOD
EN2
NPG VCC2
OUT2
FB2
GND
31.6k
2.2μF
10k
3645 TA01a
3.3V
200mA
Buck Regulator Efficiency
90
VOUT = 5V
80
VOUT = 3.3V
70
60
VIN = 12V
50
0 100 200 300 400 500
LOAD CURRENT (mA)
3645 TA01b
3645f
1
1 page  
TYPICAL PERFORMANCE CHARACTERISTICS
LT3645
Buck Power Switch Voltage Drop
400
350
300
250
200
150
100
50
0
0 50 100 150 200 250 300 350 400 450 500
SWITCH CURRENT (mA)
3645 G07
Buck Power Switch Current Limit
1000
950
900
850
800
750
700
650
600
–50
0 50 100
TEMPERATURE (°C)
150
3645 G08
Overvoltage Lockout
39.5
RISING
FALLING
39.0
38.5
38.0
37.5
37.0
36.5
–50
0 50 100
TEMPERATURE (°C)
150
3645 G10
Switching Frequency
800
790
780
770
760
750
740
730
720
710
700
–50
0 50 100
TEMPERATURE (°C)
150
3645 G11
Undervoltage Lockout
3.8
3.7
3.6
3.5
3.4
3.3
3.2
–50 –30 –10 10 30 50 70 90 110 130 150
TEMPERATURE (°C)
3645 G09
LDO Power Transistor Current
Limit
340
320
300
280
260
240
220
200
–50
0 50 100
TEMPERATURE (°C)
150
3645 G12
LDO Dropout Voltage to VCC2
350
300
250
200
150
100
50 OUT2 = 0.8V
OUT2 = 3.3V
0
0 50 100 150 200
LOAD CURRENT (mA)
3645 G13
LDO Dropout Voltage
350
300
250
200
150
100
50
0
–60 –40 –20 0 20 40 60 80 100 120 140 160
TEMPERATURE (°C)
3645 G14
LDO Load Regulation
0
–0.02
–0.04
–0.06
–0.08
–0.10
–0.12
–0.14
–0.16
0
50 100 150
OUTPUT CURRENT (mA)
200
3645 G15
3645f
5
5 Page 
LT3645
APPLICATIONS INFORMATION
Table 1. Inductor Vendors
Vendor
URL
Sumida
www.sumida.com
Toko
Würth Elektronik
www.toko.com
www.we-online.com
Part Series
CDRH4D28
CDRH5D28
CDRH8D28
A916CY
D585LC
WE-TPC(M)
WE-PD2(M)
WE-PD(S)
Inductance Range (μH)
1.2 to 4.7
2.5 to 10
2.5 to 33
2 to 12
1.1 to 39
1 to 10
2.2 to 22
1 to 27
Size (mm)
4.5 × 4.5
5.5 × 5.5
8.3 × 8.3
6.3 × 6.2
8.1 × 8.0
4.8 × 4.8
5.2 × 5.8
7.3 × 7.3
Now the required on time has decreased below the mini-
mum on time of 100ns. Instead of the switch pulse width
becoming narrower to accommodate the lower duty cycle
requirement, the part skips a few pulses so that the aver-
age inductor current meets and does not exceed the load
current requirement.
The LT3645 is robust enough to survive prolonged opera-
tion under these conditions as long as the peak inductor
current does not exceed 1.2A. Inductor saturation due
to high current may further limit performance in this
operating region.
Inductor Selection and Maximum Output Current
Choose the inductor value according to:
L = 2.2 •(VOUT + VD)/ƒ
where VD is the forward voltage drop of the catch diode
(~0.4V), f is the switching frequency in MHz and L is in
μH. With this value, there will be no subharmonic oscilla-
tion for applications with 50% or greater duty cycle. For
robust operation in fault conditions, the saturation current
should be above 1.5A. To keep efficiency high, the series
resistance (DCR) should be less than 0.1Ω. Table 1 lists
several inductor vendors. If the buck load current is less
than 500mA, then a lower valued inductor can be used.
Catch Diode
Depending on load current, a 500mA to 1A Schottky diode
is recommended for the catch diode, D1. The diode must
have a reverse voltage rating equal to or greater than the
overvoltage lockout voltage (38.5V). The ON Semiconduc-
tor MBRA140T3 and Central Semiconductor CMMSH1-40
are good choices, as they are rated for 1A continuous
forward current and a maximum reverse voltage of 40V.
Input Filter Network
Bypass VIN with a 1μF or higher ceramic capacitor of X7R
or X5R type. Y5V types have poor performance over tem-
perature and applied voltage and should not be used. A 1μF
ceramic capacitor is adequate to bypass the LT3645 and
will easily handle the ripple current. However, if the input
power source has high impedance, or there is significant
inductance due to long wires or cables, additional bulk
capacitance might be necessary. This can be provided
with a low performance (high ESR) electrolytic capacitor
in parallel with the ceramic device. Step-down regulators
draw current from the input supply in pulses with very
fast rise and fall times. The input capacitor is required to
reduce the resulting voltage ripple at the LT3645 input
and to force this very high frequency switching current
into a tight local loop, minimizing EMI. A 1μF capacitor
is capable of this task, but only if it is placed close to the
LT3645 and catch diode (see the PCB layout section). A
second precaution regarding the ceramic input capacitor
concerns the maximum input voltage rating of the LT3645.
A ceramic input capacitor combined with trace or cable
inductance forms a high quality (underdamped) tank cir-
cuit. If the LT3645 circuit is plugged into a live supply, the
input voltage can ring to twice its nominal value, possibly
exceeding the LT3645’s voltage rating. This situation can
easily be avoided. For more details, see Linear Technology
Application Note 88.
3645f
11
11 Page | ||
| Páginas | Total 24 Páginas | |
| PDF Descargar | [ Datasheet LT3645.PDF ] | |
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