Datasheet
C
SS
=
t
SS
x I
SS
V
REF
VIN
EN
GND
VOUT
R
enb
R
ent
V
IN
C
in1
Master Power Supply
V
OUT1
V
OUT2
C
O1
LMZ10505EXT
VIN
EN
GND
LMZ10505EXT
R
enb
R
ent
V
IN
Cin1
V
IN(UVLO)
= 1.23V x
R
trkb
+ R
trkt
R
trkt
LMZ10505EXT
SNVS669G –JUNE 2010–REVISED OCTOBER 2013
www.ti.com
Additional Features
ENABLE
The LMZ10505EXT features an enable (EN) pin and associated comparator to allow the user to easily sequence
the LMZ10505EXT from an external voltage rail, or to manually set the input UVLO threshold. The turn-on or
rising threshold and hysteresis for this comparator are typically 1.23V and 0.15V respectively. The precise
reference for the enable comparator allows the user to ensure that the LMZ10505EXT will be disabled when the
system demands it to be.
The EN pin should not be left floating. For always-on operation, connect EN to VIN.
ENABLE AND UVLO
Using a resistor divider from VIN to EN as shown in the schematic diagram below, the input voltage at which the
part begins switching can be increased above the normal input UVLO level according to
(19)
For example, suppose that the required input UVLO level is 3.69V. Choosing R
enb
= 10 kΩ, then we calculate
R
ent
= 20 kΩ.
Alternatively, the EN pin can be driven from another voltage source to cater to system sequencing requirements
commonly found in FPGA and other multi-rail applications. The following schematic shows an LMZ10505EXT
that is sequenced to start based on the voltage level of a master system rail (V
OUT1
).
SOFT-START
The LMZ10505EXT begins to operate when both the VIN and EN, voltages exceed the rising UVLO and enable
thresholds, respectively. A controlled soft-start eliminates inrush currents during startup and allows the user more
control and flexibility when sequencing the LMZ10505EXT with other power supplies.
In the event of either VIN or EN decreasing below the falling UVLO or enable threshold respectively, the voltage
on the soft-start pin is collapsed by discharging the soft-start capacitor by a 14 µA (typ.) current sink to ground.
SOFT-START CAPACITOR
Determine the soft-start capacitance with the following relationship
(20)
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