ADP151 datasheet(13/23 Pages) AD

13 / 23 page

Data Sheet

ADP151

APPLICATIONS INFORMATION

analog.com

Rev. J | 13 of 23

Figure 30. ADP151 Typical EN Pin Operation

As shown in Figure 30, the EN pin has hysteresis built in to prevent

on and off oscillations that can occur due to noise on the EN pin as

this pin passes through the threshold points.

The EN pin active and inactive thresholds are derived from the

VIN voltage. Therefore, these thresholds vary with changing input

voltage. Figure 31 shows typical EN active and inactive thresholds

when the input voltage varies from 2.2 V to 5.5 V.

Figure 31. Typical EN Pin Thresholds vs. Input Voltage

The ADP151 uses an internal soft start to limit the inrush current

when the output is enabled. The start-up time for the 3.3 V option

is approximately 160 µs from the time the EN active threshold is

crossed to when the output reaches 90% of its final value. As

shown in Figure 32, the start-up time is dependent on the output

voltage setting.

Figure 32. Typical Start-Up Behavior

ADJUSTABLE OUTPUT VOLTAGE OPERATION

The unique architecture of the ADP151 makes an adjustable ver-

sion difficult to implement in silicon. However, it is possible to create

an adjustable regulator at the expense of increasing the quiescent

current of the regulator circuit.

The ADP151, and similar LDOs, are designed to regulate the output

voltage (VOUT) appearing at the VOUT pin with respect to the GND

pin. If the GND pin is at a potential other than 0 V (for example,

at the offset voltage (VOFFSET)), the ADP151 output voltage is VOUT

+ VOFFSET. By taking advantage of this behavior, it is possible

to create an adjustable ADP151 circuit that retains most of the

desirable characteristics of the ADP151.

Figure 33. Adjustable LDO Using the ADP151

The circuit shown in Figure 33 is an example of an adjustable LDO

using the ADP151. A stable VOFFSET voltage is created by passing

a known current through R2. The current through R2 is determined

by the voltage across R1. Because the voltage across R1 is set by

the voltage between VOUT and GND, the current passing through

R2 is fixed, and VOFFSET is stable.

To minimize the effect variation of the ADP151 ground current

(IGND) with load, it is best to keep R1 as small as possible. It is also

best to size the current passing through R2 to at least 20× greater

than the maximum expected ground current.

To create a 4 V LDO circuit, start with the 3.3 V version of the

ADP151 to minimize the value of R2. Because VOUT is 4 V, VOFFSET

must be 0.7 V, and the current through R2 must be 7 mA. R1 is,

therefore, 3.3 V/7 mA or 471 Ω. A 470 Ω standard value introduces

ADP151 Folha de dados(PDF) 13 Page - Analog Devices

ADP151 Folha de dados(HTML) 13 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Folha de dados Download

Ligação URL