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LT3990HMSE#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P22
LT3990/LT3990-3.3/LT3990-5
10
3990fa
APPLICATIONS INFORMATION
FB Resistor Network
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the 1% resis-
tors according to:
R1= R2
V
OUT
1.21
–1
Reference designators refer to the Block Diagram. Note
that choosing larger resistors will decrease the quiescent
current of the application circuit.
Setting the Switching Frequency
The LT3990 uses a constant frequency PWM architecture
that can be programmed to switch from 200kHz to 2.2MHz
by using a resistor tied from the RT pin to ground. A table
showing the necessary R
T
value for a desired switching
frequency is in Table 1.
Table 1. Switching Frequency vs R
T
Value
SWITCHING FREQUENCY (MHz) R
T
VALUE (kΩ)
0.2
0.3
0.4
0.5
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
787
511
374
287
232
169
127
102
84.5
69.8
59
51.1
44.2
Operating Frequency Trade-Offs
Selection of the operating frequency is a trade-off between
efficiency, component size, minimum dropout voltage and
maximum input voltage. The advantage of high frequency
operation is that smaller inductor and capacitor values may
be used. The disadvantages are lower efficiency, lower
maximum input voltage, and higher dropout voltage. The
highest acceptable switching frequency (f
SW(MAX)
) for a
given application can be calculated as follows:
f
SW(MAX)
=
V
OUT
+ V
D
t
ON(MIN)
V
IN
– V
SW
+ V
D
( )
where V
IN
is the typical input voltage, V
OUT
is the output
voltage, V
D
is the integrated catch diode drop (~0.7V),
and V
SW
is the internal switch drop (~0.5V at max load).
This equation shows that slower switching frequency is
necessary to accommodate high V
IN
/V
OUT
ratio.
Lower frequency also allows a lower dropout voltage.
The input voltage range depends on the switching fre-
quency because the LT3990 switch has finite minimum
on and off times. The switch can turn off for a minimum
of ~160ns but the minimum on-time is a strong function
of temperature. Use the minimum switch on-time curve
(see Typical Performance Characteristics) to design for
an application’s maximum temperature, while adding
about 30% for part-to-part variation. The minimum and
maximum duty cycles that can be achieved taking these
on and off times into account are:
DC
MIN
= f
SW
• t
ON(MIN)
DC
MAX
= 1 – f
SW
• t
OFF(MIN)
where f
SW
is the switching frequency, the t
ON(MIN)
is the
minimum switch on-time, and the t
OFF(MIN)
is the minimum
switch off-time (~160ns). These equations show that
duty cycle range increases when switching frequency is
decreased.
A good choice of switching frequency should allow ad-
equate input voltage range (see next section) and keep
the inductor and capacitor values small.
Input Voltage Range
The minimum input voltage is determined by either the
LT3990’s minimum operating voltage of 4.2V or by its
maximum duty cycle (as explained in previous section).
The minimum input voltage due to duty cycle is:
V
IN(MIN)
=
V
OUT
+ V
D
1– f
SW
• t
OFF(MIN)
– V
D
+ V
SW
where V
IN(MIN)
is the minimum input voltage, V
OUT
is the
output voltage, V
D
is the catch diode drop (~0.7V), V
SW
is the internal switch drop (~0.5V at max load), f
SW
is
the switching frequency (set by RT), and t
OFF(MIN)
is the
minimum switch off-time (160ns). Note that higher switch-
ing frequency will increase the minimum input voltage.
LT3990/LT3990-3.3/LT3990-5
11
3990fa
APPLICATIONS INFORMATION
If a lower dropout voltage is desired, a lower switching
frequency should be used.
The highest allowed V
IN
during normal operation
(V
IN(OP-MAX)
) is limited by minimum duty cycle and can
be calculated by the following equation:
V
IN(OP-MAX)
=
V
OUT
+ V
D
f
SW
• t
ON(MIN)
– V
D
+ V
SW
where t
ON(MIN)
is the minimum switch on-time.
However, the circuit will tolerate inputs up to the absolute
maximum ratings of the V
IN
and BOOST pins, regardless of
chosen switching frequency. During such transients where
V
IN
is higher than V
IN(OP-MAX)
, the switching frequency will
be reduced below the programmed frequency to prevent
damage to the part. The output voltage ripple and inductor
current ripple may also be higher than in typical operation,
however the output will still be in regulation.
Inductor Selection
For a given input and output voltage, the inductor value
and switching frequency will determine the ripple current.
The ripple current increases with higher V
IN
or V
OUT
and
decreases with higher inductance and faster switching
frequency. A good starting point for selecting the induc-
tor value is:
L = 3
V
OUT
+ V
D
f
SW
where V
D
is the voltage drop of the catch diode (~0.7V),
L is in µH and f
SW
is in MHz. The inductor’s RMS current
rating must be greater than the maximum load current
and its saturation current should be about 30% higher.
For robust operation in fault conditions (start-up or short
circuit) and high input voltage (>30V), the saturation
current should be above 800mA. 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 2 lists several vendors and
suitable types.
This simple design guide will not always result in the
optimum inductor selection for a given application. As a
general rule, lower output voltages and higher switching
frequency will require smaller inductor values. If the ap-
plication requires less than 350mA load current, then a
lesser inductor value may be acceptable. This allows use
of 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 for several popular output voltages. Low
inductance may result in discontinuous mode operation,
which is acceptable but reduces maximum load current.
For details of maximum output current and discontinu-
ous mode operation, 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 Application Note 19.
Input Capacitor
Bypass the input of the LT3990 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 1µF to 4.7µF ceramic capacitor
is adequate to bypass the LT3990 and will easily handle
the ripple current. Note that larger input capacitance
is required when a lower switching frequency is used
Table 2. Inductor Vendors
VENDOR URL
Coilcraft www.coilcraft.com
Sumida www.sumida.com
Toko www.tokoam.com
Würth Elektronik www.we-online.com
Coiltronics www.cooperet.com
Murata www.murata.com
LT3990/LT3990-3.3/LT3990-5
12
3990fa
APPLICATIONS INFORMATION
(due to longer on-times). 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 low 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 ripple
at the LT3990 and to force this very high frequency switch-
ing 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 LT3990 (see the PCB Layout section). A second
precaution regarding the ceramic input capacitor concerns
the maximum input voltage rating of the LT3990. A ceramic
input capacitor combined with trace or cable inductance
forms a high quality (under damped) tank circuit. If the
LT3990 circuit is plugged into a live supply, the input volt-
age can ring to twice its nominal value, possibly exceeding
the LT3990’s voltage rating. This situation is easily avoided
(see the Hot Plugging Safely section).
Output Capacitor and Output Ripple
The output capacitor has two essential functions. It stores
energy in order to satisfy transient loads and stabilize the
LT3990’s control loop. Ceramic capacitors have very low
equivalent series resistance (ESR) and provide the best
ripple performance. A good starting value is:
C
OUT
=
50
V
OUT
• f
SW
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 combined with a phase lead capacitor (typically
22pF) between the output and the feedback pin. A lower
value of output capacitor can be used to save space and
cost but transient performance will suffer.
The second function is that the output capacitor, along
with the inductor, filters the square wave generated by the
LT3990 to produce the DC output. In this role it determines
the output ripple, so low impedance (at the switching
frequency) is important. The output ripple decreases with
increasing output capacitance, down to approximately
1mV. See Figure 1. Note that a larger phase lead capacitor
should be used with a large output capacitor.
C
OUT
(µF)
0
0
WORST-CASE OUTPUT RIPPLE (mV)
2
6
8
10
40
80
100
18
3990 F01
4
20 60
12
14
16
FRONT PAGE APPLICATION
f = 600kHz
C
LEAD
= 47pF FOR C
OUT
≥ 47µF
V
IN
= 24V
V
IN
= 12V
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. Table 3 lists several capacitor
vendors.
Table 3. Recommended Ceramic Capacitor Vendors
MANUFACTURER WEBSITE
AVX www.avxcorp.com
Murata www.murata.com
Taiyo Yuden www.t-yuden.com
Vishay Siliconix www.vishay.com
TDK www.tdk.com
Ceramic Capacitors
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT3990 due to their piezoelectric nature.
When in Burst Mode operation, the LT3990’s switching
frequency depends on the load current, and at very light
loads the LT3990 can excite the ceramic capacitor at audio
frequencies, generating audible noise. Since the LT3990
Figure 1. Worst-Case Output Ripple Across Full Load Range
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LT3990HMSE#PBF

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Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 60V, 350mA Step-Down Regulator with 2uA Quiescent Current and Integrated Diodes in DFN
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