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LT3980EMSE#TRPBF

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LT3980
10
3980fa
For more information www.linear.com/LT3980
drop (~0.5V), V
SW
is the internal switch drop (~0.5V at
max load), f
SW
is the switching frequency (set by R
T
),
and t
ON(MIN)
is the minimum switch on time (~200ns).
Note that a higher switching frequency will depress the
maximum operating input voltage. Conversely, a lower
switching frequency will be necessary to achieve safe
operation at high input voltages.
Input voltages up to 58V are acceptable regardless of the
switching frequency. In this mode, the LT3980 may enter
pulse-skipping operation where some switching pulses
are skipped to maintain safe inductor current.
The minimum input voltage is determined by either the
LT3980’s minimum operating voltage of ~3.6V or by its
maximum duty cycle (see equation in previous section).
The minimum input voltage due to duty cycle is:
V
VV
ft
VV
IN MIN
OUT D
SW
OFFMIN
DS
W
()
()
=
+
+
1–
–
where V
IN(MIN)
is the minimum input voltage, and t
OFF(MIN)
is the minimum switch off time (200ns). Note that higher
switching frequency will increase the minimum input
voltage. If a lower dropout voltage is desired, a lower
switching frequency should be used.
Inductor Selection
For a given input and output voltage, the inductor value
and switching frequency will determine the ripple current.
The
ripple current ΔI
L
increases with higher V
IN
or V
OUT
and decreases with higher inductance and faster switching
frequency. A reasonable starting point for selecting the
ripple current is:
ΔI
L
= 0.4(I
OUT(MAX)
)
where I
OUT(MAX)
is the maximum output load current. To
guarantee sufficient output current, peak inductor current
must be lower than the LT3980’s switch current limit (I
LIM
).
The peak inductor current is:
I
L(PEAK)
= I
OUT(MAX)
+ ΔI
L
/2
where I
L(PEAK)
is the peak inductor current, I
OUT(MAX)
is
the maximum output load current, and ΔI
L
is the inductor
ripple current. The LT3980’s switch current limit (I
LIM
) is
4A at low duty cycles and decreases linearly to 3A at DC
= 0.8. The maximum output current is a function of the
inductor ripple current:
I
OUT(MAX)
= I
LIM
– ΔI
L
/2
Be sure to pick an inductor ripple current that provides
sufficient maximum output current (I
OUT(MAX)
).
The largest inductor ripple current occurs at the highest
V
IN
. To guarantee that the ripple current stays below the
specified maximum, the inductor value should be chosen
according to the following equation:
L
VV
fI
VV
V
OUT D
SW L
OUT D
IN MAX
=
+
+
∆
1–
()
where V
D
is the voltage drop of the catch diode (~0.4V),
V
IN(MAX)
is the maximum input voltage, V
OUT
is the output
voltage, f
SW
is the switching frequency (set by RT), and
L is in the inductor value.
The inductor’s RMS and saturation current rating must
be greater than the maximum load current. For robust
operation in fault conditions (start-up or short circuit) and
high input voltage (>40V), the saturation current should
be above 3.5A. To keep the efficiency high, the series
resistance (DCR) should be less than 0.1Ω, and the core
material should be intended for high frequency applications.
Table 1 lists several vendors and suitable types.
Table 1. Inductor Vendors
VENDOR URL PART SERIES TYPE
Murata www.murata.com LQH55D Open
TDK www.component.tdk.com SLF10145 Shielded
Toko www.toko.com D75C
D75F
Shielded
Open
Sumida www.sumida.com CDRH74
CR75
CDRH8D43
Shielded
Open
Shielded
NEC www
.nec-tokin.com MPLC073
MPBI0755
Shielded
Shielded
Vishay www
.vishay.com IHLP2525CE01
Shielded
applicaTions inFormaTion
LT3980
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3980fa
For more information www.linear.com/LT3980
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 oper
-
ation, see Linear Technology Application Note 44. Finally,
for duty cycles greater than 50% (V
OUT
/V
IN
> 0.5), there
is a minimum inductance required to avoid subharmonic
oscillations. See AN19.
Input Capacitor
Bypass the input of the LT3980 circuit with a ceramic
capacitor of X7R or X5R type. Y5V types have poor
performance over temperature and applied voltage, and
should not be used. A 10µF to 22µF ceramic capacitor is
adequate to bypass the LT3980 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 rip-
ple at the LT3980 and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
A 10µF capacitor is capable of this task, but only if it is
placed close to the L
T3980 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 LT3980. A ceramic input capacitor
combined with trace or cable inductance forms a high
quality (under damped) tank circuit. If the LT3980 circuit
is plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT3980’s
voltage rating. This situation is easily avoided (see the Hot
Plugging Safety section).
For space sensitive applications, a 4.7µF ceramic ca
-
pacitor can be used for local bypassing of the LT3980
input. However
, the lower input capacitance will result in
increased 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 LT3980 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
LT3980 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
LT3980’s control loop. Ceramic capacitors have very low
equivalent series resistance (ESR) and provide the best
ripple performance. A good starting value is:
C
Vf
OUT
OUT SW
=
100
where f
SW
is in MHz, and C
OUT
is the recommended output
capacitance in µF. Use X5R or X7R types. This choice will
provide low output ripple and good transient response.
Transient performance can be improved with a higher value
capacitor if the compensation network is also adjusted
to maintain the loop bandwidth. A lower value of output
capacitor can be used to save space and cost but transient
performance will suffer. See the Frequency Compensation
section to choose an appropriate compensation network.
When choosing a capacitor, look carefully through the
data sheet to find out what the actual capacitance is under
operating conditions (applied voltage and temperature).
A physically larger capacitor, or one with a higher voltage
rating, may be required. High performance tantalum or
electrolytic capacitors can be used for the output capacitor.
Low ESR is important, so choose one that is intended for
applicaTions inFormaTion
LT3980
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3980fa
For more information www.linear.com/LT3980
use in switching regulators. The ESR should be specified
by the supplier, and should be 0.05Ω or less. Such a
capacitor will be larger than a ceramic capacitor and will
have a larger capacitance, because the capacitor must be
large to achieve low ESR. Table 2 lists several capacitor
vendors.
Table 2. Capacitor Vendors
VENDOR URL PART SERIES COMMANDS
Panasonic www.panasonic.com Ceramic, Polymer,
Tantalum
EEF Series
Kemet www.kemet.com Ceramic, Tantalum T494, T495
Sanyo www.sanyovideo.com Ceramic, Polymer,
Tantalum
POSCAP
Murata www.murata.com Ceramic SOT-23
AVX www.avxcorp.com Ceramic, Tantalum SOT-23
Taiyo Yuden www.taiyo-yuden.com Ceramic TPS Series
Catch Diode
The catch diode conducts current only during switch off
time. Average forward current in normal operation can
be calculated from:
I
D(AVG)
= I
OUT
(V
IN
– V
OUT
)/V
IN
where I
OUT
is the output load current. The only reason to
consider a diode with a larger current rating than necessary
for nominal operation is for the worst-case condition of
shorted output. The diode current will then increase to the
typical peak switch current. Peak reverse voltage is equal
to the regulator input voltage. Use a Schottky diode with a
reverse voltage rating greater than the input voltage. The
overvoltage protection feature in the LT3980 will keep the
switch off when V
IN
> 64V which allows the use of 64V
rated Schottky even when V
IN
ranges up to 80V.
Ceramic Capacitors
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT3980 due to their piezoelectric nature.
When in Burst Mode operation, the LT3980’s switching
frequency depends on the load current, and at very light
loads the LT3980 can excite the ceramic capacitor at
audio frequencies, generating audible noise. Since the
LT3980 operates at a lower current limit during Burst
Mode operation, the noise is nearly silent to a casual ear.
If this is unacceptable, use a high performance tantalum
or electrolytic capacitor at the output.
Frequency Compensation
The LT3980 uses current mode control to regulate the
output. This simplifies loop compensation. In particular,
the LT3980 does not require the ESR of the output capacitor
for stability, so you are free to use ceramic capacitors to
achieve low output ripple and small circuit size. Frequen
-
cy compensation is provided by the components tied to
the V
C
pin, as shown in Figure 2. Generally a capacitor
(C
C
) and a resistor (R
C
) in series to ground are used. In
addition, there may be lower value capacitor in parallel.
This capacitor (C
F
) is not part of the loop compensation
but is used to filter noise at the switching frequency, and
is required only if a phase-lead capacitor is used or if the
output capacitor has high ESR.
–
+
0.79V
SW
V
C
g
m
=
500µmho
GND
3M
LT3980
3980 F02
R1
OUTPUT
ESR
C
F
C
C
R
C
ERROR
AMPLIFIER
FB
R2
C1
C1
CURRENT MODE
POWER STAGE
g
m
= 5.3mho
+
POLYMER
OR
TANTALUM
CERAMIC
C
PL
Figure 2. Model for Loop Response
applicaTions inFormaTion
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LT3980EMSE#TRPBF

Mfr. #:
Buy LT3980EMSE#TRPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 58V, 2A, 2.4MHz Step-Down Switching Regulator with 75uA Quiescent Current
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