LT8580
16
8580fa
For more information www.linear.com/LT8580
applicaTions inForMaTion
Configurable Undervoltage Lockout
Figure 8 shows how to configure an undervoltage lock-
out (UVLO) for the LT8580. Typically, UVLO is used in
situations where the input supply is current-limited, has
a relatively high sour
ce resistance, or ramps up/down
slowly
. A switching regulator draws constant power from
the source, so source current increases as source voltage
drops. This looks like a negative resistance load to the
source and can cause the source to current-limit or latch
low under low 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 1.31V (rising) and 1.27V (falling). Re
-
sistor 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 Characteristics).
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
− 1.31V
1.31V
R
UVLO2
⎛
⎝
⎜
⎞
⎠
⎟
+12µA
or
R
UVLO1
=
V
IN
−
− 1.27V
1.27V
R
UVLO2
⎛
⎝
⎜
⎞
⎠
⎟
+12µA
where V
IN
+
and V
IN
–
are the V
IN
voltages when rising or
falling, respectively.
Figure 8. Configurable UVLO
For example, to disable the LT8580 for V
IN
voltages below
3.5V using the single resistor configuration, choose:
R
UVLO1
=
−
1.27V
∞
⎛
⎝
⎜
⎞
⎠
⎟
+ 12µA
= 187k
To activate the LT8580 for V
IN
voltages greater than
4.5V using the double resistor configuration, choose
R
UVLO2
= 10k and:
R
UVLO1
=
−
1.31V
10k
⎛
⎝
⎜
⎞
⎠
⎟
+ 12µA
= 22.1k
Internal Undervoltage Lockout
The LT8580 monitors the V
IN
supply voltage in case V
IN
drops below a minimum operating level (typically about
2.35V). 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 LT8580 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
cir
cuit board be used to conduct heat away from the IC
and into a copper plane with as much area as possible.
R
UVLO2
(OPTIONAL)
1.3V
R
UVLO1
V
IN
ACTIVE/
LOCKOUT
GND
12µA
AT 1.3V
–
+
SHDN
