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LT3514IUFD#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P26
LT3514
16
3514fa
For more information www.linear.com/LT3514
APPLICATIONS INFORMATION
For a falling threshold of 10V, the minimum hysteresis
is 0.827V. For a falling threshold of 30V, the minimum
hysteresis is 2.48V.
R2 can be calculated once R1 is known:
R2 = R1•
1.33
V
IN, FALLING
−1.33
The circuit shown in Figure 5 will start when the input
voltage rises above 11V and will shutdown when the input
voltage falls below 10V.
Inductor Selection and Maximum Output Current
A good first choice for the inductor value is:
L = 2 • (V
OUT
+ V
D
)/f
SW
for Channels 1, 4
L = (V
OUT
+ V
D
)/f
SW
for Channel 3
where V
D
is the voltage drop of the catch diode (~0.4V),
L is in µH and f
SW
is in MHz. With this value there will
be no subharmonic oscillation for applications with 50%
or greater duty cycle. The inductor’s RMS current rating
must be greater than your maximum load current and
its saturation current should be about 30% higher. For
robust operation in fault conditions, the saturation cur
-
rent should be above 2A for CH1, CH4 and above 4A for
CH3. To keep efficiency high, the series resistance (DCR)
should be less than 0.1
. Table 2 lists several vendors
and types that are suitable.
Of course, such a simple design guide will not always
result in the optimum inductor for your application. A
larger value provides a higher maximum load current and
reduces output voltage ripple at the expense of slower
transient response. If your load is lower than 1A for CH1,
CH4 or below 2A for CH3, 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. Low
inductance may result in discontinuous mode operation,
which is okay, but further reduces maximum load current.
For details on maximum output current and discontinuous
mode operation, see Linear Technology Application Note 44.
Catch Diode
Use a 1A Schottky diode for channels 1 and 4 and a 2A
Schottky diode for channel 3. The diode must have a re
-
verse voltage rating equal to or greater than the maximum
input voltage.
Input Capacitor
The input of the LT3514 cir
cuit must be bypassed with a
X7R or X5R type ceramic capacitor. Y5V types have poor
performance over temperature and amplified voltage
and should not be used. There are four V
IN
pins. Each
V
IN
pin should be bypassed to the nearest ground pin.
However it is not necessary to use a dedicated capaci-
tor for each V
IN
pin. Pins 9 and 11 may be tied together
on the board layout so that both pins can share a single
bypass capacitor. Since the channels running on Pins 9
and 11 are 180 degrees out-of-phase, it is not necessary
to double the capacitor value either. Similarly, Pins 26
and 28 may be tied together on the board layout to save
a bypass capacitor. For switching frequencies greater than
750kHz, a 1µF capacitor or higher value ceramic capacitor
should be used to bypass each group of two V
IN
pins. For
switching frequencies less than 750kHz, a 2.2µF or higher
value ceramic capacitor should be used to bypass each
Table 2. Inductor Vendors
VENDOR URL PART SERIES INDUCTANCE (µH) SIZE (mm)
Sumida www.sumida.com CDRH4D28
CDRH5D28
CDRH5D28
1.2 TO 4.7
2.5 TO 10
2.5 TO 33
4.5 × 4.5
5.5 × 5.5
8.3 × 8.3
Toko www
.toko.com A916CY
D585LC
2 TO 12
1.1 TO 39
6.3 × 6.2
8.1 × 8
Würth Elektronik www.we-online.com
WE-TPC(M)
WE-PD2(M)
WE-PD(S)
1 TO 10
2.2 TO 22
1 TO 27
4.8 × 4.8
5.2 × 5.8
7.3 × 7.3
LT3514
17
3514fa
For more information www.linear.com/LT3514
APPLICATIONS INFORMATION
group of two V
IN
pins. The ceramic bypass capacitors
should be located as close to the V
IN
pins as possible.
See the sample layout shown in the PCB Layout section.
All four V
IN
pins should be tied together on the board and
bypassing with a low performance electrolytic capacitor
is recommended especially if the input power source has
high impedance, or there is significant inductance due to
long wires or cables.
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 LT3514 and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
To accomplish this task, the input bypass capacitor must
be placed close to the LT3514 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 LT3514. A ceramic input capacitor
combined with trace or cable inductance forms a high
quality (underdamped) tank circuit. If the LT3514 circuit
is plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT3514’s
voltage rating. This situation can be easily avoided by add
-
ing an electrolytic capacitor in parallel with the ceramic
input capacitors. See Application Note 88.
Output Capacitor
The output capacitor has two essential functions. Along
with the inductor
, it filters the square wave generated by
the L
T3514 to produce the DC output. In this role it deter-
mines the output ripple so low impedance at the switching
frequency is important. The second function is to store
energy in order to satisfy transient loads and stabilize the
L
T3514’
s control loop.
Ceramic capacitors have very low equivalent series re
-
sistance (ESR) and provide the best ripple performance.
A good value is:
C
OUT
= 33/(V
OUT
• f
SW
) for Channels 1, 4
C
OUT
= 132/(V
OUT
• f
SW
) for Channel 3
where C
OUT
is in µF and f
SW
is in MHz. Use X5R or X7R
types and keep in mind that a ceramic capacitor biased
with V
OUT
will have less than its nominal capacitance. This
choice will provide low output ripple and good transient
response. Transient performance can be improved with a
high value capacitor, if the compensation network is also
adjusted to maintain the loop bandwidth.
A lower value of output capacitor can be used, but tran
-
sient performance will suffer. Also, a lower value output
capacitor may result in increased sensitivity to noise which
can be alleviated by adding a 100pF phase lead capacitor
from FB to V
OUT
.
High performance electrolytic capacitors can be used for
the output capacitor. Low ESR is important, so choose one
that is intended for use in switching regulators. The ESR
should be specified by the supplier and should be 0.1Ω
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 3 lists
several capacitor vendors.
Table 3. Capacitor Vendors
VENDOR PHONE URL PART SERIES COMMENTS
Panasonic (714) 373-7366 www.panasonic.com Ceramic, Polymer, Tantalum EEF Series
Kemet (864) 963-6300 www.kemet.com Ceramic, Tantalum T494, T495
Sanyo (408) 749-9714 www.sanyovideo.com Ceramic, Polymer, Tantalum POSCAP
Murata (404) 436-1300 www.murata.com Ceramic
AVX www.avxcorp.com Ceramic, Tantalum TPS Series
Taiyo Yuden (864) 963-6300 www.taiyo-yuden.com Ceramic
LT3514
18
3514fa
For more information www.linear.com/LT3514
Figure 6 shows the transient response of the LT3514 with
several output capacitor choices. The output is 3.3V. The
load current is stepped from 500mA to 1A and back to
500mA and the oscilloscope traces show the output volt
-
age. The upper photo shows the recommended value. The
second photo shows the improved response (less voltage
drop) resulting from a larger output capacitor and a larger
phase lead capacitor
. The last photo shows the response
to a high performance electrolytic capacitor. Transient per
-
formance is improved due to the large output capacitance.
Shorted and Reversed Input Protection
If the inductor is chosen so that it won’t saturate exces-
sively, an LT3514 buck regulator will tolerate a shorted
output. There is another situation to consider in systems
where the output will be held high when the input to the
LT3514 is absent. This may occur in battery charging ap
-
plications or in battery backup systems where a battery
or some other supply is diode OR-ed with the LT3514’
s
output. If the V
IN
pin is allowed to float and the EN/UVLO
pin is held high (either by a logic signal or because it is
APPLICATIONS INFORMATION
Figure 6. Transient Load Response of the LT3514 with Different Output Capacitors as the
Load Current Is Stepped from 500mA to 1A. V
IN
= 12V, V
OUT
= 3.3V, L = 10µH, R
T
= 19.1k
10µF
31.6k
10k
I
OUT
1A/DIV
V
OUT
20mV/DIV
I
OUT
1A/DIV
V
OUT
20mV/DIV
20µs/DIV
20µs/DIV
I
OUT
1A/DIV
V
OUT
20mV/DIV
20µs/DIV
V
OUT
3514 F06a
3514 F06b
3514 F06c
V
OUT
31.6k
10k
10µF
×2
100pF
31.6k
10k
V
OUT
+
22µF
FB
LT3514
FB
LT3514
FB
LT3514
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P26

LT3514IUFD#PBF

Mfr. #:
Buy LT3514IUFD#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators Triple Step-Down Switching Regulator with 100% Duty Cycle Operation
Lifecycle:
New from this manufacturer.
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