LT3580
APPLICATIONS INFORMATION
V
IN
R
UVLO1
SHDN
R
UVLO2
(OPTIONAL)
V
IN
1.3V
–
+
ACTIVE/
LOCKOUT
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
=
22.1k
1.32V
+
11.6μA
10k
11.6μA
AT 1.3V
GND
3580 F07
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 impera-
tive 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
Figure 7. Configurable UVLO
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 varia-
tions 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 deter-
mined from either of the following:
R
UVLO1
=
V
IN
+
1.32V
1.32V
+
11.6μA
R
UVLO2
V
IN
1.29V
or
R
UVLO1
=
1.29V
+
11.6μA
R
UVLO2
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
=
190.5k
1.29V
+
11.6μA
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 opera-
3580fc
15
LINER [ LINEAR TECHNOLOGY ]
