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LTC3601IUD#TRPBF

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LTC3601
10
3601fc
For more information www.linear.com/LTC3601
APPLICATIONS INFORMATION
A general LTC3601 application circuit is shown on the first
page of this data sheet. External component selection is
largely driven by the load requirement and begins with the
selection of the inductor L. Once the inductor is chosen,
the input capacitor, C
IN
, the output capacitor, C
OUT
, the
internal regulator capacitor, C
INTVCC
, and the boost capaci-
tor, C
BOOST
, can be selected. Next, the feedback resistors
are selected to set the desired output voltage. Finally, the
remaining optional external components can be selected
for functions such as external loop compensation, track/
soft-start, externally programmed oscillator frequency
and PGOOD.
Operating Frequency
Selection of the operating frequency is a trade-off between
efficiency and component size. High frequency operation
allows the use of smaller inductor and capacitor values.
Operation at lower frequencies improves efficiency by
reducing internal gate charge losses but requires larger
inductance values and/or capacitance to maintain low
output ripple voltage.
The operating frequency, f
O
, of the LTC3601 is determined
by an external resistor that is connected between the RT
pin and ground. The value of the resistor sets the ramp
current that is used to charge and discharge an internal
timing capacitor within the oscillator and can be calculated
by using the following equation:
R
RT
=
3.2 E11
f
O
where R
RT
is in Ω and f
O
is in Hz.
Connecting the RT pin to INTV
CC
will default the converter
to f
O
= 2MHz; however, this switching frequency will be
more sensitive to process and temperature variations than
when using a resistor on RT (see Typical Performance
Characteristics).
Inductor Selection
For a given input and output voltage, the inductor value and
operating frequency determine the inductor ripple current.
More specifically, the inductor ripple current decreases
with higher inductor value or higher operating frequency
according to the following equation:
ΔI
L
=
V
OUT
f • L
1–
V
OUT
V
IN
where ∆I
L
= inductor ripple current, f = operating frequency
and L = inductor value. A trade-off between component
size, efficiency and operating frequency can be seen from
this equation. Accepting larger values of ∆I
L
allows the use
of lower value inductors but results in greater core loss
in the inductor, greater ESR loss in the output capacitor,
and larger output ripple. Generally, highest efficiency op-
eration is obtained at low operating frequency with small
ripple current.
A reasonable starting point for setting the ripple current is
about 40% of I
OUT(MAX)
. Note that the largest ripple current
occurs at the highest V
IN
. To guarantee the ripple current
does not exceed a specified maximum the inductance
should be chosen according to:
L =
V
OUT
f • ΔI
L(MAX)
1–
V
OUT
V
IN(MAX)
However, the inductor ripple current must not be so large
that its valley current level exceeds the negative current
limit of –1.2A (typical) when the circuit is operating in
forced continuous mode. If the inductor current trough
reaches the negative current limit while the part is in
forced continuous mode operation, V
OUT
may charge up
to above its target regulation voltage. In such instances,
choose a larger inductor value to reduce the ripple current.
The alternative is to reduce the inductor ripple current by
decreasing the R
T
resistor value, which will increase the
switching frequency.
RT (kΩ)
0
0
FREQUENCY (kHz)
1000
2000
3000
4000
6000
100
200 300 400
3601 F01
500 600
5000
Figure 1. Switching Frequency vs RT
LTC3601
11
3601fc
For more information www.linear.com/LTC3601
APPLICATIONS INFORMATION
Once the value for L is known the type of inductor must
be selected. Actual core loss is independent of core size
for a fixed inductor value but is very dependent on the
inductance selected. As the inductance increases, core loss
decreases. Unfortunately, increased inductance requires
more turns of wire leading to increased copper loss.
Ferrite designs exhibit very low core loss and are pre-
ferred at high switching frequencies, so design goals can
concentrate on copper loss and preventing saturation.
Ferrite core materials saturate “hard,” meaning the induc-
tance collapses abruptly when the peak design current is
exceeded. This collapse will result in an abrupt increase
in inductor ripple current, so it is important to ensure the
core will not saturate.
Different core materials and shapes will change the
size/current and price/current relationship of an inductor.
Toroid or shielded pot cores in ferrite or permalloy materi-
als are small and don’t radiate much energy but generally
cost more than powdered iron core inductors with similar
characteristics. The choice of which style inductor to use
mainly depends on the price versus size requirements and
any radiated field/EMI requirements. New designs for sur-
face mount inductors are available from Toko, Vishay, NEC/
Tokin, Cooper, Coilcraft, TDK and Wurth Electronik. Table
1 gives a sampling of available surface mount inductors.
C
IN
and C
OUT
Selection
The input capacitance, C
IN
, is needed to filter the trapezoi-
dal wave current at the drain of the top power MOSFET.
To prevent large voltage transients from occurring a low
ESR input capacitor sized for the maximum RMS current
is recommended. The maximum RMS current is given by:
I
RMS
= I
OUT(MAX)
V
OUT
V
IN
– V
OUT
(
)
V
IN
where I
OUT(MAX)
equals the maximum average output
current. This formula has a maximum at V
IN
= 2V
OUT
,
where I
RMS
= I
OUT
/2. This simple worst-case condition
is commonly used for design because even significant
deviations do not offer much relief. Note that ripple cur-
rent ratings from capacitor manufacturers are often based
on only 2000 hours of life which makes it advisable to
further de-rate the capacitor or choose a capacitor rated
at a higher temperature than required.
Several capacitors may be paralleled to meet the require-
ments of the design. For low input voltage applications
sufficient bulk input capacitance is needed to minimize
transient effects during output load changes. Even though
the LTC3601 design includes an overvoltage protection
circuit, care must always be taken to ensure input voltage
transients do not pose an overvoltage hazard to the part.
The selection of C
OUT
is primarily determined by the effec-
tive series resistance (ESR) that is required to minimize
Table 1. Inductor Selection Table
INDUCTANCE
(µH)
DCR
(mΩ)
MAX
CURRENT (A)
DIMENSIONS
(mm)
HEIGHT
(mm)
Würth Electronik WE-PD2 Typ MS Series
0.56
0.82
1.2
1.7
2.2
9.5
14
21
27
36
6.5
5.4
4.8
4
3.6
5.2 × 5.8 2
Vishay IHLP-2020BZ-01 Series
0.47
0.68
1
2.2
8.8
12.4
20
50.1
11.5
10
7
4.2
5.2 × 5.5 2
Toko DE3518C Series
0.56
1.2
1.7
24
30
35
3.3
2.4
2.1
3.5 × 3.7 1.8
Sumida CDRH2D18/HP Series
0.56
0.82
1.1
33
39
43
3.7
2.9
2.5
3.2 × 3.2 2
Cooper SD18 Series
0.47
0.82
1.2
1.5
2.2
20.1
24.7
29.4
34.5
39.8
3.58
3.24
2.97
2.73
2.55
5.5 × 5.5 1.8
Coilcraft LPS4018 Series
0.56
1
2.2
30
40
70
4.8
2.8
2.7
4 × 4 1.7
TDK VLS252012 Series
0.47
1
1.5
2.2
56
88
126
155
3.3
2.4
2
1.8
2.5 × 2 1.2
LTC3601
12
3601fc
For more information www.linear.com/LTC3601
APPLICATIONS INFORMATION
voltage ripple and load step transients. The output ripple,
∆V
OUT
, is determined by:
ΔV
OUT
< ΔI
L
ESR +
1
8 • f • C
OUT
The output ripple is highest at maximum input voltage
since ∆I
L
increases with input voltage. Multiple capaci-
tors placed in parallel may be needed to meet the ESR
and RMS current handling requirements. Dry tantalum,
special polymer, aluminum electrolytic, and ceramic
capacitors are all available in surface mount packages.
Special polymer capacitors offer low ESR but have lower
capacitance density than other types. Tantalum capacitors
have the highest capacitance density, but it is important
to only use types that have been surge tested for use in
switching power supplies. Aluminum electrolytic capacitors
have significantly higher ESR but can be used in cost-
sensitive applications provided that consideration is given
to ripple current ratings and long-term reliability. Ceramic
capacitors have excellent low ESR characteristics and small
footprints. Their relatively low value of bulk capacitance
may require multiple capacitors in parallel.
Using Ceramic Input and Output Capacitors
Higher value, lower cost ceramic capacitors are now
available in small case sizes. Their high voltage rating
and low ESR make them ideal for switching regulator
applications. However, due to the self-resonant and high-Q
characteristics of some types of ceramic capacitors, care
must be taken when these capacitors are used at the input
and output. When a ceramic capacitor is used at the input,
and the power is supplied by a wall adapter through long
wires, a load step at the output can induce ringing at the
V
IN
input. At best, this ringing can couple to the output and
be mistaken as loop instability. At worst, a sudden inrush
of current through the long wires can potentially cause a
voltage spike at V
IN
large enough to damage the part. For
a more detailed discussion, refer to Application Note 88.
When choosing the input and output ceramic capacitors
choose the X5R or X7R dielectric formulations. These
dielectrics provide the best temperature and voltage
characteristics for a given value and size.
INTV
CC
Regulator Bypass Capacitor
An internal low dropout (LDO) regulator produces a
3.3V supply voltage used to power much of the internal
LTC3601 circuitry including the power MOSFET gate
drivers. The INTV
CC
pin connects to the output of this
regulator and must have a minimum of 1µF of decoupling
capacitance to ground. The decoupling capacitor should
have low impedance electrical connections to the INTV
CC
and PGND pins to provide the transient currents required
by the
LTC3601. The user may connect a maximum load
current of 5mA to this pin but must take into account the
increased power dissipation and die temperature that
results. Furthermore, this supply is intended only to supply
additional DC load currents as desired and not intended
to regulate large transient or AC behavior this may impact
LTC3601 operation.
Boost Capacitor
The boost capacitor, C
BOOST
, is used to create a voltage rail
above the applied input voltage V
IN
. Specifically, the boost
capacitor is charged to a voltage equal to approximately
INTV
CC
each time the bottom power MOSFET is turned
on. The charge on this capacitor is then used to supply
the required transient current during the remainder of the
switching cycle. When the top MOSFET is turned on, the
BOOST pin voltage will be equal to approximately V
IN
+
3.3V. For most applications a 0.1µF ceramic capacitor will
provide adequate performance.
Output Voltage Programming
The LTC3601 will adjust the output voltage such that V
FB
equals the reference voltage of 0.6V according to:
V
OUT
= 0.6V 1+
R1
R2
The desired output voltage is set by appropriate selection of
resistors R1 and R2 as shown in Figure 2. Choosing large
values for R1 and R2 will result in improved efficiency but
may lead to undesirable noise coupling or phase margin
reduction due to stray capacitances at the FB node. Care
should be taken to route the FB line away from any noise
source, such as the SW line.
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LTC3601IUD#TRPBF

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Manufacturer:
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Description:
Switching Voltage Regulators 1.5A, 15V, 4MHz Monolithic Synchronous Step-Down Regulator in 3x3 QFN
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