DS75325 datasheet(13/14 Pages) NSC

13 / 14 page

Applications

EXTERNAL RESISTOR CALCULATION

A typical magnetic-memory word drive requirement is shown

Figure 11 A source-output transistor of one DS75325

delivers load current (IL) The sink-output transistor of an-

other DS75325 sinks this current

The value of the external pull-up resistor (Rext) for a particu-

lar memory application may be determined using the follow-

ing equation

Rext e

16 VCC2(Min) b VS b 22

IL b16 VCC2(Min) b VS b 29

(1)

where Rext is in kX

VCC2(Min) is the lowest expected value of VCC2 in volts VS

is the source output voltage in volts with respect to ground

IL is in mA

The power dissipated in resistor Rext during the load current

pulse duration is calculated using Equation 2

PRext

VCC2(Min) b VS b 2

(2)

where PRext is in mW

After solving for Rext the magnitude of the source collector

current (ICS) is determined from Equation 3

(3)

ICS

094 IL

where ICS is in mA

As an example let VCC2(Min) e 20V and VL e 3V while IL

of 500 mA flows Using Equation 1

Rext e

16 (20 b 3 b 22)

500 b 16 (20 b 3 b 29)

05 kX

and from Equation 2

PRext

500

20 b 3 b 2

470 mW

The amount of the memory system current source (ICS)

from Equation 3 is

ICS

094 (500)

470 mA

In this example the regulated source-output transistor base

current through the external pull-up resistor (Rext) and the

source gate is approximately 30 mA This current and ICS

comprise IL

TLF9755 – 15

Note 1

For clarity partial logic diagrams of two DS55325s are shown

Note 2

Source and sink shown are in different packages

FIGURE 11 Typical Application Data

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