LMZ10503 datasheet(13/22 Pages) NSC

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Additional Features

Enable

The LMZ10503 features an enable (EN) pin and associated

comparator to allow the user to easily sequence the

LMZ10503 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 guarantee that the LMZ10503 will be dis-

abled 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

enb = 10 kΩ, then we calculate Rent = 20

Ω.

30111844

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 LMZ10503 that is sequenced to

start based on the voltage level of a master system rail

OUT1).

30111845

Soft-Start

The LMZ10503 begins to operate when both the VIN and EN,

voltages exceed the rising UVLO and enable thresholds, re-

spectively. A controlled soft-start eliminates inrush currents

during startup and allows the user more control and flexibility

when sequencing the LMZ10503 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 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

FB is the internal reference voltage (nominally 0.8V),

SS is the soft-start charging current (nominally 2 µA) and

SS is the external soft-start capacitance.

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

startup time is given by

SS = 2.5 nF / ms

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 LMZ10503 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 LMZ10503 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 connected to the SS pin to al-

leviate output voltage overshoot when recovering from a cur-

rent limit fault.

30111857

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,

OUT1, and the LMZ10503 output voltage, VOUT2, both rise to-

gether and reach their target values at the same time. This is

termed ratiometric startup. For this case, the equation gov-

erning the values of tracking divider resistors R

trkb and Rtrkt is

given by

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 LMZ10503. This offset will cause the

LMZ10503 output voltage to reach regulation slightly before

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