AS1376 datasheet(16/20 Pages) AMSCO

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AS1376

Datasheet

This shows why it is convenient to increase the output capacitor value for a better support for fast load changes. Of course the formula holds for

t < “propagation time”, so that a faster LDO needs a smaller cap at the load to achieve a similar transient response. For instance 50mA load

current step produces 50mV output drop if the LDO response is 1usec and the load cap is 1µF.

There is also a steady state error caused by the finite output impedance of the regulator. This is derived from the load regulation specification

discussed above.

9.7.6 Exit from Shutdown Delay

This specification defines the time taken for the LDO to awake from shutdown. The time is measured from the release of the enable pin to the

time that the output voltage is within 5% of the final value. It assumes that the voltage at VIN is stable and within the regulator min and max limits.

Shutdown reduces the quiescent current to very low, mostly leakage values (<1µA).

9.7.7 Thermal Protection

To prevent operation under extreme fault conditions, such as a permanent short circuit at the output, thermal protection is built into the device.

Die temperature is measured, and when a 150ºC threshold is reached, the device enters shutdown. When the die cools sufficiently, the device

will restart (assuming input voltage exists and the device is enabled). Hysteresis of 25ºC prevents low frequency oscillation between start-up and

shutdown around the temperature threshold.

9.7.8 Power Supply Sequencing

The AS1376 requires two different supply voltages active at the same time for correct operation. They are as given below.

1. VIN, the power input voltage, that is regulated to provide the fixed output voltage.

2. VBIAS, the bias input voltage, supplies internal circuitry.

It's important that VIN does not exceed VBIAS at any time. If the device is used in the typical post regulation application as shown in Figure 1, the

sequencing of the two power supplies is not an issue as VBIAS supplies both, the DC-DC regulator and the AS1376. The output voltage of the

DC-DC regulator will take some time to rise up and supply VIN of AS1376. In this application VIN will always ramp up more slowly than VBIAS. In

case VIN is shorted to VBIAS, the voltages at the two supply pins will ramp up simultaneously causing no problem. Only in applications with two

independent supplies connected to the AS1376 special care must be taken to guarantee that VIN is always = VBIAS.

9.7.9 Auto-Discharge

When the AS1376 is placed in shutdown, a 100

 path to ground is connected at the output. This path speeds up the discharge of the

capacitor(s) connected to the regulator output. Assuming that VIN remains constant and always >VOUT, output discharge time is calculated from

the following relationship:

(EQ 17)

Where:



t = specified time after regulator shutdown (sec)



VREG = Regulated output voltage (initial condition)



R = 100

 (typ) discharge resistance

C = Output capacitance (Farad)

In other words, the output discharge will reach 90% below the regulated output voltage in 2.2RC seconds; R and C defined as above.



REG e

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