Datasheet

LT3504
13
3504fa
For more information www.linear.com/LT3504
Figure 5. Circuit to Prevent Switching When V
IN
< 10V, with 700mV of Hysteresis
R2
20.5k
R1
133k
GND
LT3504
EN/UVLO
V
IN
V
IN
3504 F05
NOT
SWITCHING
SWITCHING
V
IN, FALLING
= 10V
V
IN
(V)V
IN
(V)
9 10 11 12
V
IN, RISING
= 11V
applicaTions inForMaTion
Undervoltage Lockout
The LT3504 prevents switching when the input voltage
decreases below 3.2V. Alternatively, the EN/UVLO pin
can be used to program an undervoltage lockout at input
voltages exceeding 3.2V by tapping a resistor divider from
V
IN
to EN/UVLO as shown in Figure 5.
The rising threshold on the EN/UVLO pin is 1.44V. The
falling threshold on the EN/UVLO pin is 1.33V. When EN/
UVLO is rising and less than 1.44V then the EN/UVLO pin
sinks 1.3µA of current. This 1.3µA current can be used to
program additional hysteresis on the EN/UVLO pin. For the
circuit in Figure 5, R1 can be determined from:
R1=
V
IN,HYSTERESIS
−
0.11
1.33
V
IN,FALLING
( )
1.3µA
where V
IN,HYSTERESIS
is the desired amount of hysteresis
on the input voltage and V
IN,FALLING
is the desired input
voltage threshold at which the part will shut down. Notice
that for a given falling threshold (V
IN,FALLING
), the amount
of hysteresis (V
IN,HYSTERESIS
) must be at least:
V
IN,HYSTERESIS
>
0.11
1.33
• V
IN,FALLING
( )
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
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 current
should be above 2A. 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, 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 discontinu
-
ous 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.