REF2125 datasheet(16/24 Pages) TI1

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P u

PEAK

466 A 13.54 A R

REF2125

SBAS798 – SEPTEMBER 2017

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Feature Description (continued)

8.3.4 Clean Start Feature

In many applications (for example, loop powered applications), the supply at VIN has inductive impedance. This

can cause the supply to dip during start-up because of the large output capacitor connected to the voltage

reference and the inductive supply. The REF2125 family has an internal clean start block to control the peak of

the inrush current during start-up. This feature is illustrated in Functional Block Diagram. The peak of inrush

current can be calculated as Equation 5:

where

•

IPEAK = Peak of inrush current (µA), has a range of [0.5 mA, 19 mA],

•

Rcs = External resistor connected to the CS pin

(5)

During power up, IPEAK is split between the device current and output current. The output current (IOUT) is split

between output capacitor and load current (ILOAD). The device current can be estimated to be IQ+IOUT/183, where

IQ is quiescent current at no load. Hence for a given ILOAD it is important to choose Rcs such that IPEAK is larger

than ILOAD. Above equations capture typical characteristics and hence it is suggested to include ±25% margins

while budgeting for inrush current and also while choosing Rcs for a given ILOAD. This inrush current continues to

stay at the limiting value (IPEAK) till output reaches close to VREF (2.5 V).

When a Ccs is also connected in parallel to Rcs, The inrush current limit shall rise exponentially to the steady

state value (IPEAK) as calculated using above equations, with a time constant of Rcs × Ccs. Hence the initial (and

maximum) rate of rise of inrush current shall be IPEAK /(Rcs × Ccs). Because the inrush current rate is limited, the

loop powered supply dip is controlled.

8.4 Device Functional Modes

8.4.1

EN Pin

When the ENABLE pin of the REF2125 is pulled high, the device is in active mode. The device must be in active

mode for normal operation. The REF2125 can be placed in a low-power mode by pulling the ENABLE pin low.

When in shutdown mode, the output of the device becomes high impedance and the quiescent current of the

device reduces to 2 µA in shutdown mode. The EN pin must not be pulled higher than VIN supply voltage. See

the Thermal Information for logic high and logic low voltage levels.

8.4.2 Negative Reference Voltage

For applications requiring a negative and positive reference voltage, the REF2125 and OPA735 can be used to

provide a dual-supply reference from a 5-V supply. Figure 27 shows the REF2125 used to provide a 2.5-V supply

reference voltage. The low drift performance of the REF2125 complements the low offset voltage and zero drift of

the OPA735 to provide an accurate solution for split-supply applications. Take care to match the temperature

coefficients of R1 and R2.

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