Datasheet

LT3514
15
3514fa
For more information www.linear.com/LT3514
Figure 5. Circuit to Prevent Switching When V
IN
< 10V, with 700mV of Hysteresis
APPLICATIONS INFORMATION
is required to regulate the output, with little overshoot.
Figure 4 shows the start-up waveforms with and without
a soft-start capacitor (C
SS
) on the RUN/SS pin.
Undervoltage Lockout
The LT3514 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
( )
R2
20.5k
R1
133k
GND
LT3514
EN/UVLO
V
IN
V
IN
3514 F05
NOT
SWITCHING
SWITCHING
V
IN, FALLING
= 10V
V
IN
(V)V
IN
(V)
9 10 11 12
V
IN, RISING
= 11V
Figure 4a. Inductor Current Waveform During
Start-Up without a Soft-Start Capacitor
Figure 4b. Inductor Current Waveform During
Start-Up with a 1nF Soft-Start Capacitor (C
SS
)
100µs/DIV
3514 F04a
I
L
0.5A/DIV
V
OUT
2V/DIV
100µs/DIV
3514 F04b
I
L
0.5A/DIV
V
OUT
2V/DIV