Datasheet

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