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SC1452
(Rev.:2000)

DUAL 150mA LDO REGULATOR WITH PROGRAMMABLE RESET

Semtech Corporation 

DUAL 150mA LDO REGULATOR WITH

PROGRAMMABLE RESET

SC1452

PRELIMINARY - August 7, 2000

APPLICATIONS INFORMATION

Adjusting RSTB Delay Time

RSTB, the power on reset for regulator B, can be

reduced externally by connecting a capacitor to the

delay time set pin DLYB. If DLYB is connected to

ground, the internally controlled delay time of 150ms

(typ.) will apply.

Referring to the block diagram, as the output of regula-

tor B (VOUTB) rises and reaches the reset threshold

voltage (92% VOUTB(NOM)), two things happen:

1) the internal 150ms timer starts;

2) the 3µA current source turns on, charging CDLYB (if

connected).

If DLYB is connected to ground, RSTB goes high

150ms after VOUTB crosses the threshold voltage. If a

capacitor is connected between DLYB and ground, the

voltage at DLYB can be described by the following

equation:

VDLYB

=

3 • 10 −6 • t

C DLYB

An internal comparator compares this voltage to a

1.25V reference, and triggers the reset high once this

voltage is reached. The delay time can be calculated by

rearranging the above equation, solving for t:

t RSTB

=

C DLYB • 1.25

3 • 10 −6

= 416 ,667 • C DLYB

Note that the maximum delay time is 150ms, as RSTB

goes high when either the internal timer or externally

set timer times out, so if tRSTB is set externally for

200ms, the reset delay will still be 150ms. Thus for a

150ms delay, DLYB should be grounded, and for a

delay time less than 150ms, CDLYB can be calculated

using the equation above, or read from the chart below.

1000

100

tRSTB = 150ms max.

10

1

0.1

0.01

0.1

1

10

100

CDLYB (nF)

1000

© 2000 SEMTECH CORP.

6

652 MITCHELL ROAD NEWBURY PARK CA 91320

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