Additional Features

Enable

The LMZ10503EXT features an enable (EN) pin and associ-

ated comparator to allow the user to easily sequence the

LMZ10503EXT 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 com-

parator allows the user to guarantee that the LMZ10503EXT

will be disabled when the system demands it to be.

The EN pin should not be left floating. For always-on opera-

tion, 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

For example, suppose that the required input UVLO level is

3.69V. Choosing R

k

Ω.

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Alternatively, the EN pin can be driven from another voltage

source to cater to system sequencing requirements common-

ly found in FPGA and other multi-rail applications. The fol-

lowing schematic shows an LMZ10503EXT that is sequenced

to start based on the voltage level of a master system rail

(V

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Soft-Start

The LMZ10503EXT 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 LMZ10503EXT with other power sup-

plies.

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 capac-

itor by a 14 µA (typ.) current sink to ground.

Soft-Start Capacitor

Determine the soft-start capacitance with the following rela-

tionship

where V

I

C

Thus, the required soft-start capacitor per unit output voltage

startup time is given by

C

For example, a 4 ms soft-start time will yield a 10 nF capaci-

tance. The minimum soft-start capacitance is 680 pF.

Tracking

The LMZ10503EXT can track the output of a master power

supply during soft-start by connecting a resistor divider to the

SS pin. In this way, the output voltage slew rate of the

LMZ10503EXT will be controlled by a master supply for loads

that require precise sequencing. When the tracking function

is used, a small value soft-start capacitor should be connect-

ed to the SS pin to alleviate output voltage overshoot when

recovering from a current limit fault.

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Tracking - Equal Soft-Start Time

One way to use the tracking feature is to design the tracking

resistor divider so that the master supply output voltage,

V

rise together and reach their target values at the same time.

This is termed ratiometric startup. For this case, the equation

governing the values of tracking divider resistors R

R

The above equation includes an offset voltage, of 200 mV, to

ensure that the final value of the SS pin voltage exceeds the

reference voltage of the LMZ10503EXT. This offset will cause

the LMZ10503EXT output voltage to reach regulation slightly

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