LT3580
APPLICATIONS INFORMATION
To activate the LT3580 for V voltage greater than
IN
V
IN
V
IN
4.5V using the double resistor configuration, choose
ACTIVE/
LOCKOUT
1.3V
–
+
R
UVLO2
= 10k and:
R
UVLO1
SHDN
4.5V −1.32V
RUVLO1
=
=22.1k
11.6μA
AT 1.3V
⎛
⎞
R
1.32V
UVLO2
(OPTIONAL)
+11.6μA
⎜
⎝
⎟
⎠
10k
GND
3580 F07
Internal Undervoltage Lockout
The LT3580 monitors the V supply voltage in case V
IN
IN
Figure 7. Configurable UVLO
drops below a minimum operating level (typically about
2.3V). When V is detected low, the power switch is
source voltage conditions. UVLO prevents the regulator
from operating at source voltages where these problems
might occur.
IN
deactivated, and while sufficient V voltage persists, the
IN
soft-start capacitor is discharged. After V is detected
IN
high,thepowerswitchwillbereactivatedandthesoft-start
The shutdown pin comparator has voltage hysteresis with
typical thresholds of 1.32V (rising) and 1.29V (falling).
capacitor will begin charging.
Resistor R
is optional. R
can be included
UVLO2
UVLO2
Thermal Considerations
to reduce the overall UVLO voltage variation caused by
FortheLT3580todeliveritsfulloutputpower,itisimperative
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
circuitboardbeusedtoconductheatawayfromtheICand
into a copper plane with as much area as possible.
variationsinSHDNpin current(seetheElectricalCharacter-
istics). A good choice for R
UVLO2 UVLO1
from either of the following:
is ≤10k 1%. After
UVLO2
, R
choosing a value for R
can be determined
V
+ −1.32V
IN
RUVLO1
=
=
⎛
⎜
⎝
⎞
⎟
⎠
1.32V
+11.6μA
R
UVLO2
Thermal Lockout
or
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).
V
− −1.29V
IN
RUVLO1
⎛
⎜
⎝
⎞
⎟
⎠
1.29V
+11.6μA
R
UVLO2
+
–
where V and V are the V voltages when rising or
Thermal Calculations
IN
IN
IN
falling respectively.
Power dissipation in the LT3580 chip comes from four
primarysources:switchI Rloss,NPNbasedrive(AC),NPN
basedrive(DC),andadditionalinputcurrent.Thefollowing
formulas can be used to approximate the power losses.
These formulas assume continuous mode operation,
2
For example, to disable the LT3580 for V voltages below
3.5V using the single resistor configuration, choose:
IN
3.5V −1.29V
1.29V
RUVLO1
=
=190.5k
⎛
⎞
+11.6μA
⎜
⎝
⎟
⎠
∞
3580fg
14
Linear [ Linear ]
