MC33263 datasheet(6/12 Pages) ONSEMI | Ultra Low Noise 150 mA Low Dropout Voltage Regulator with ON/OFF Control

6 page

MC33263

http://onsemi.com

MC33263 Wake–up Improvement – In portable

applications, an immediate response to an enable signal is

vital. If noise is not of concern, the MC33263 without a

bypass capacitor settles in nearly 20

ms and typically delivers

mVRMS between 100 Hz and 100 kHz.

In ultra low–noise systems, the designer needs a 10 nF

bypass capacitor to decrease the noise down to 25

mVRMS

between 100 Hz and 100 kHz. With the adjunction of the

10 nF capacitor, the wake–up time expands up to 1 ms as

shown on the data–sheet curves. If an immediate response

is wanted, following figure’s circuit gives a solution to

charge the bypass capacitor with the enable signal without

degrading the noise response of the MC33263.

At power–on, C4 is discharged. When the control logic

sends its wake–up signal by going to a high level, the PNP

base is momentarily tight to ground. The PNP switch closes

and immediately charges the bypass capacitor C1 toward its

operating value. After a few

ms, the PNP opens and becomes

totally transparent to the regulator.

This circuit improves the response time of the regulator

which drops from 1 ms down to 30

ms. The value of C4 needs

to be tweaked in order to avoid any bypass capacitor

overload during the wake–up transient.

Figure 4. A PNP Transistor Drives the

Bypass Pin when Enable Goes High

MC33263

1.0

Input

Output

470 pF

10 nF

220 k

MMBT2902LT1

On/Off

Figure 5. MC33263 Wake–up Improvement with Small PNP Transistor

MC33263 Without

Wake–up Improvement

(Typical Response)

MC33263 With

Wake–up Improvement

(Typical Response)

1 ms

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