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LT1618EDD#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16
LT1618
4
sn1618 1618fas
PIN FUNCTIONS
UUU
FB (Pin 1/Pin 1): Feedback Pin. Set the output voltage by
selecting values for R1 and R2 (see Figure 1):
RR
V
V
OUT
12
1 263
1=
⎛
⎝
⎜
⎞
⎠
⎟
.
–
ISN (Pin 2/Pin 2): Current Sense (–) Pin. The inverting
input to the current sense amplifier.
ISP (Pin 3/Pin 3): Current Sense (+) Pin. The noninverting
input to the current sense amplifier.
I
ADJ
(Pin 4/Pin 4): Current Sense Adjust Pin. A DC voltage
applied to this pin will reduce the current sense voltage. If
this adjustment is not needed, tie this pin to ground.
GND (Pin 5/Pin 5): Ground Pin. Tie this pin directly to local
ground plane.
NC (Pin 6/NA): No Connection for MS Package.
SW (NA/Pin 6): Switch Pin for DD Package. Connect this
pin to Pin 7.
SW (Pin 7/Pin 7): Switch Pin. This is the collector of the
internal NPN power switch. Minimize the metal trace area
connected to this pin to minimize EMI.
V
IN
(Pin 8/Pin 8): Input Supply Pin. Bypass this pin with
a capacitor to ground as close to the device as possible.
SHDN (Pin 9/Pin 9): Shutdown Pin. Tie this pin higher
than 1V to turn on the LT1618; tie below 0.3V to turn it off.
V
C
(Pin 10/Pin 10): Compensation Pin for Error Amplifier.
Connect a series RC from this pin to ground. Typical values
are 2kΩ and 10nF.
Exposed Pad (NA/Pin 11): The Exposed Pad on the DD
package is GND and must be soldered to the PCB GND for
optimum thermal performance.
TYPICAL PERFOR A CE CHARACTERISTICS
UW
TEMPERATURE (°C)
–50
SWITCHING FREQUENCY (MHz)
125
1618 G07
0
75
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1.0
–25 25 50
100
V
IN
= 18V
V
IN
= 1.6V
FEEDBACK PIN VOLTAGE (V)
SWITCHING FREQUENCY (MHz)
1.2
1618 G08
0 0.2
0.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
0.4 0.6
1.0
T
J
= 25°C
SHUTDOWN PIN VOLTAGE (V)
0
SHDN PIN CURRENT (µA)
20
1618 G09
5
10
15
50
45
40
35
30
25
20
15
10
5
0
T
J
= –50°C
T
J
= 25°C
T
J
= 125°C
Switching Frequency Frequency Foldback SHDN Pin Current
(MS/DD)
LT1618
5
sn1618 1618fas
1.4MHz
OSCILLATOR
DRIVER
–
+
–
+
+
+
–
–
+
×5
Σ
Q
Q1
S
R
–
+
A1
×25
5 10
3
2
8
79
1.263V
4
1
SHDN
I
ADJ
GND
V
C
R
C
C
C
V
IN
SW
ISN
ISP
FB
A2
A3
0.02Ω
C1 C2
R
SENSE
V
OUT
V
IN
R1
R2
L1
D1
Figure 1. LT1618 Block Diagram
BLOCK DIAGRA
W
OPERATIO
U
The LT1618 uses a constant frequency, current mode
control scheme to provide excellent line and load regula-
tion. Operation can be best understood by referring to the
Block Diagram in Figure 1. At the start of each oscillator
cycle, the SR latch is set, turning on power switch Q1. The
signal at the noninverting input of PWM comparator A3 is
a scaled-down version of the switch current (summed
together with a portion of the oscillator ramp). When this
signal reaches the level set by the output of error amplifier
A2, comparator A3 resets the latch and turns off the power
switch. In this manner, A2 sets the correct peak current
level to keep the output in regulation. If the error amplifier’s
output increases, more current is delivered to the output;
if it decreases, less current is delivered. A2 has two
inverting inputs, one from the voltage feedback loop, and
one from the current feedback loop. Whichever inverting
input is higher takes precedence, forcing the converter
into either a constant-current or a constant-voltage mode.
The LT1618 is designed to transition cleanly between the
two modes of operation. Current sense amplifier A1 senses
the voltage between the ISP and ISN pins and provides a
25× level-shifted version to error amplifier A2. When the
voltage between ISP and ISN reaches 50mV, the output of
A1 provides 1.263V to one of the noninverting inputs of A2
and the converter is in constant-current mode. If the
current sense voltage exceeds 50mV, the output of A1 will
increase causing the output of A2 to decrease, thus
reducing the amount of current delivered to the output. In
this manner the current sense voltage is regulated to
50mV. Similarly, if the FB pin increases above 1.263V, the
output of A2 will decrease to reduce the peak current level
and regulate the output (constant-voltage mode).
LT1618
6
sn1618 1618fas
Capacitor Selection
Low ESR (equivalent series resistance) capacitors should
be used at the output to minimize the output ripple voltage.
Multilayer ceramic capacitors are an excellent choice.
They have an extremely low ESR and are available in very
small packages. X5R and X7R dielectrics are preferred, as
these materials retain their capacitance over wider voltage
and temperature ranges than other dielectrics. A 4.7µF to
10µF output capacitor is sufficient for high output current
designs. Converters with lower output currents may need
only a 1µF or 2.2µF output capacitor. Solid tantalum or
OSCON capacitors can be used, but they will occupy more
board area than a ceramic and will have a higher ESR for
APPLICATIONS INFORMATION
WUU
U
Inductor Selection
Several inductors that work well with the LT1618 are listed
in Table 1, although there are many other manufacturers
and devices that can be used. Consult each manufacturer
for more detailed information and for their entire selection
of related parts. Many different sizes and shapes are
available. Ferrite core inductors should be used to obtain
the best efficiency, as core losses at 1.4MHz are much
lower for ferrite cores than for the cheaper powdered-iron
ones. Choose an inductor that can handle the necessary
peak current without saturating, and ensure that the
inductor has a low DCR (copper-wire resistance) to mini-
mize I
2
R power losses. A 4.7µH or 10µH inductor will be
a good choice for many LT1618 designs.
Table 1. Recommended Inductors
L MAX HEIGHT
PART (µH) (mΩ) (mm) VENDOR
CDRH5D18-4R1 4.1 57 2.0 Sumida
CDRH5D18-100 10 124 2.0 (847) 956-0666
CR43-2R2 2.2 71 3.5 www.sumida.com
CR43-4R7 4.7 109 3.5
CR43-100 10 182 3.5
CR54-100 10 100 4.8
LQH3C1R0M24 1.0 78 2.0 Murata
LQH3C2R2M24 2.2 126 2.0 (814) 237-1431
LQH3C4R7M24 4.7 260 2.0 www.murata.com
the same footprint device. Always use a capacitor with a
sufficient voltage rating.
Ceramic capacitors also make a good choice for the input
decoupling capacitor, which should be placed as close as
possible to the V
IN
pin of the LT1618. A 1µF to 4.7µF input
capacitor is sufficient for most applications. Table 2 shows
a list of several ceramic capacitor manufacturers. Consult
the manufacturers for detailed information on their entire
selection of ceramic parts.
Table 2. Recommended Ceramic Capacitor Manufacturers
VENDOR PHONE URL
Taiyo Yuden (408) 573-4150 www.t-yuden.com
Murata (814) 237-1431 www.murata.com
Kemet (408) 986-0424 www.kemet.com
Diode Selection
Schottky diodes, with their low forward voltage drop and
fast switching speed, are the ideal choice for LT1618
applications. Table 3 shows several Schottky diodes that
work well with the LT1618. Many different manufacturers
make equivalent parts, but make sure that the component
chosen has a sufficient current rating and a voltage rating
greater than the output voltage. The diode conducts cur-
rent only when the power switch is turned off (typically
less than half the time), so a 0.5A or 1A diode will be
sufficient for most designs. The companies below also
offer Schottky diodes with higher voltage and current
ratings.
Table 3. Recommended Schottky Diodes
1A PART 0.5A PART VENDOR PHONE/URL
UPS120 Microsemi (510) 353-0822
UPS130 www.microsemi.com
UPS140
MBRM120 MBR0520 ON Semiconductor (800) 282-9855
MBRM130 MBR0530 www.onsemi.com
MBRM140 MBR0540
B120 B0520 Diodes, Inc (805) 446-4800
B130 B0530 www.diodes.com
B140 B0540
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LT1618EDD#PBF

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Buy LT1618EDD#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators Const-C/Const-V 1.4MHz Boost DC/DC Conv
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