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LTC3444EDD#PBF

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13
LTC3444
3444fb
Table 1. Shows some typical resistor value combinations
for several V
CONTROL
vs V
OUT
voltage ranges. One percent
(1%) resistor tolerances were assumed.
Table 1. Typical Resistor Values for V
OUT
vs V
CONTROL
V
CONTROL
(V) V
OUT
(V) RESISTANCE (kΩ)
MIN MAX MIN MAX R1 R2 R3
0.35 2.4 0.8 4.2 340 271 205
0.35 2.5 0.5 5.0 340 210 162
0.8 2.35 0.8 4.2 340 200 154
0.5 2.5 0.5 4.2 340 249 182
Figure 6. Recommended Component Placement
COMPONENT SELECTION
Recommended Component Placement
Figure 6. Shows a recommended component placement.
Traces carrying high current should be made short and
wide. Trace area at FB and V
C
pins should be minimized.
Lead lengths to the battery should be kept short. V
OUT
and
V
IN
ceramic capacitors should be placed close to the IC
pins. Multiple vias should be used between layers.
OPERATIO
U
3444 F06
V
IN
V
IN
V
OUT
LTC3444
MULTIPLE VIAS
5
6
7
8
V
OUT
V
IN
V
C
FB
4
3
2
1
SW2
GND
SW1
SHDN
V
CONTROL
14
LTC3444
3444fb
Inductor Selection
The high frequency operation of the LTC3444 allows the
use of small surface mount inductors. The internal loop
compensation is designed to work with a 2.2μH inductor
(1.5μH for V
IN
< 3.1V). The 2.2μH inductor was selected to
optimize the transient response to the control input. The
use of a 2.2μH inductor pushes out the right half plane
(RHP) zero frequency and allows the loop crossover to
occur at frequencies higher than the output L-C double
pole.
For external compensation the inductor selection is based
on the desired inductor ripple current. The inductor ripple
current is typically set to 20% to 40% of the average
inductor current. Increased inductance results in lower
ripple current, however, higher inductance pulls in the
RHP zero frequency and limits the maximum crossover
frequency possible. Refer to Closing the Feedback Loop
for more information on the RHP zero. For a given ripple
the inductance terms are given as follows:
L
VVV
fI
BOOST
IN MIN OUT IN MIN
OUT MAX
>
•
••
() ()
()
(– )
ΔΔIV
H
L OUT
•
L
VV V
fI I V
H
BUCK
OUT IN MAX OUT
OUT MAX L IN MAX
>
•
•••
(–)
()
() ()
Δ
where f = operating frequency, Hz
ΔI
L
= inductor ripple current, A
V
IN(MIN)
= minimum input voltage, V
V
IN(MAX)
= maximum input voltage, V
V
OUT
= output voltage, V
I
OUT(MAX)
= maximum output load current
In most cases, the boost configuration will be used to
determine the minimum inductance allowed for a given
ripple current.
For high efficiency, choose a ferrite inductor with a high
frequency core material to reduce core loses. The inductor
should have low ESR (equivalent series resistance) to
reduce the I
2
R losses, and must be able to handle the peak
inductor current without saturating. To minimize radiated
noise, use a shielded inductor. See Table 2 for a suggested
list of inductor suppliers.
Table 2. Inductor Vendor Information
SUPPLIER PHONE FAX WEB SITE
Coilcraft (847) 639-6400 (847) 639-1469 www.coilcraft.com
CoEv Magnetics (800) 227-7040 (650) 361-2508 www.circuitprotection.com/magnetics.asp
COOPER Bussmann (636) 394-2877 1-800-544-2570 www.coooperET.com
Murata (814) 237-1431 (814) 238-0490 www.murata.com
(800) 831-9172
Sumida USA: (847) 956-0666 USA: (847) 956-0702 www.sumida.com
Japan: 81(3) 3607-5111 Japan: 81(3) 3607-5144
TDK (847) 803-6100 (847) 803-6296 www.component.tdk.com
TOKO (847) 297-0070 (847) 699-7864 www.tokoam.com
OPERATIO
U
15
LTC3444
3444fb
Output Capacitor Selection
A 4.7μF, X5R or X7R type ceramic capacitor should be
used when using the internal loop compensation. When
using external compensation, larger values of output
capacitance can be used, however, larger output capaci-
tance will increase the time needed to slew the output
voltage as required in typical WCDMA applications. The
bulk value of the output filter capacitor is set to reduce the
ripple due to charge into the capacitor each cycle. The
steady state ripple due to charge is given by:
%_
(– )
%
()
RIPPLE BOOST
IVV
CV f
OUT OUT IN MIN
OUT OUT
=
••
••
100
2
%_
(–)
%
() ()
()
RIPPLE BUCK
IVV
CV Vf
OUT MAX IN MAX OUT
OUT IN MAX OUT
=
••
•••
100
where C
OUT
= output filter capacitor in farads
f = switching frequency in Hz.
In a typical application the output capacitance may be
many times larger than that calculated above in order to
handle the transient load response requirements of the
converter. For a rule of thumb, the ratio of the operating
frequency to the unity-gain bandwidth of the converter is
the amount the output capacitance will have to increase
from the above calculations in order to maintain the
desired transient response. However, in WCDMA applica-
tions the output capacitance should be kept at a minimum
to maximize the output slew rate. Refer to the Loop
Compensation Networks section of this datasheet.
The other component of ripple is due to the ESR (equiva-
lent series resistance) of the output capacitor. Low ESR
capacitors should be used to minimize output voltage
ripple. For surface mount applications, Taiyo Yuden or
TDK ceramic capacitors, AVX TPS series tantalum capaci-
tors or Sanyo POSCAP are recommended. See Table 3 for
contact information.
Ceramic output capacitors should use case size 1206 or
larger. Smaller case sizes have a larger voltage coefficient
that can greatly reduce the output capacitance value at
higher output voltages.
Input Capacitor Selection
Since the V
IN
pin is the supply voltage for the LTC3444, as
well as the input to the power stage of the converter, it is
recommended to place at least a 4.7μF, X5R or X7R
ceramic bypass capacitor close to the V
IN
and GND pins.
It is also important to minimize any stray resistance from
the converter to the battery or other power source.
OPERATIO
U
Table 3. Capacitor Vendor Information
SUPPLIER PHONE FAX WEB SITE
AVX (803) 448-9411 (803) 448-1943 www.avxcorp.com
Sanyo (619) 661-6322 (619) 661-1055 www.sanyovideo.com
Taio Yuden (408) 573-4150 (408) 573-4159 www.t-yuden.com
TDK (847) 803-6100 (847) 803-6296 www.component.tdk.com
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LTC3444EDD#PBF

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Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators Micropower Sych Buck-Boost DC/DC Converter
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