NCP623, NCV8623

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NCP623 Wake−up Improvement

− In portable

applications, an immediate response to an enable signal is

vital. If noise is not a concern, the NCP623 without a bypass

capacitor settles in nearly 20

ms and typically delivers

65

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 addition of the 10 nF

capacitor, the wake−up time expands up to 1.0 ms as shown

on the data−sheet curves. If an immediate response is

wanted, Figure 3 provides a solution to charge the bypass

capacitor with the enable signal without degrading the noise

response of the NCP623.

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

sends its wake−up signal by going high, the PNP base is

momentarily tied 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 1.0 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 3. A PNP Transistor Drives the

Bypass Pin when Enable Goes High

13

NCP623

+

+

C2

1.0

mF

C3

1.0

mF

Input

Output

C4

470 pF

C1

10 nF

R2

220 k

MMBT2902LT1

Q1

On/Off

2

64

5