Datasheet

LT3580
14
3580fg
APPLICATIONS INFORMATION
source voltage conditions. UVLO prevents the regulator
from operating at source voltages where these problems
might occur.
The shutdown pin comparator has voltage hysteresis with
typical thresholds of 1.32V (rising) and 1.29V (falling).
Resistor R
UVLO2
is optional. R
UVLO2
can be included
to reduce the overall UVLO voltage variation caused by
variations in SHDN pin current (see the Electrical Character-
istics). A good choice for R
UVLO2
is ≤10k ±1%. After
choosing a value for R
UVLO2
, R
UVLO1
can be determined
from either of the following:
R
UVLO1
=
V
IN
+
−1.32V
1.32V
R
UVLO2
⎛
⎝
⎜
⎞
⎠
⎟
+11.6μA
or
R
UVLO1
=
V
IN
−
−1.29V
1.29V
R
UVLO2
⎛
⎝
⎜
⎞
⎠
⎟
+11.6μA
where V
IN
+
and V
IN
–
are the V
IN
voltages when rising or
falling respectively.
For example, to disable the LT3580 for V
IN
voltages below
3.5V using the single resistor configuration, choose:
R
UVLO1
=
3.5V −1.29V
1.29V
∞
⎛
⎝
⎜
⎞
⎠
⎟
+11.6μA
=190.5k
Figure 7. Configurable UVLO
To activate the LT3580 for V
IN
voltage greater than
4.5V using the double resistor configuration, choose
R
UVLO2
= 10k and:
R
UVLO1
=
4.5V −1.32V
1.32V
10k
⎛
⎝
⎜
⎞
⎠
⎟
+11.6μA
=22.1k
Internal Undervoltage Lockout
The LT3580 monitors the V
IN
supply voltage in case V
IN
drops below a minimum operating level (typically about
2.3V). When V
IN
is detected low, the power switch is
deactivated, and while sufficient V
IN
voltage persists, the
soft-start capacitor is discharged. After V
IN
is detected
high, the power switch will be reactivated and the soft-start
capacitor will begin charging.
Thermal Considerations
For the LT3580 to deliver its full output power, it is imperative
that a good thermal path be provided to dissipate the heat
generated within the package. This is accomplished by
taking advantage of the thermal pad on the underside of
the IC. It is recommended that multiple vias in the printed
circuit board be used to conduct heat away from the IC and
into a copper plane with as much area as possible.
Thermal Lockout
If the die temperature reaches approximately 165°C, the
part will go into thermal lockout, the power switch will be
turned off and the soft-start capacitor will be discharged.
The part will be enabled again when the die temperature
has dropped by ~5°C (nominal).
Thermal Calculations
Power dissipation in the LT3580 chip comes from four
primary sources: switch I
2
R loss, NPN base drive (AC), NPN
base drive (DC), and additional input current. The following
formulas can be used to approximate the power losses.
These formulas assume continuous mode operation,
R
UVLO2
(OPTIONAL)
1.3V
R
UVLO1
3580 F07
V
IN
V
IN
ACTIVE/
LOCKOUT
GND
11.6μA
AT 1.3V
–
+
SHDN