DS75325 Datasheet, PDF (13/14 Pages) National Semiconductor (TI) – Memory Drivers

Applications
EXTERNAL RESISTOR CALCULATION
A typical magnetic-memory word drive requirement is shown
in Figure 11 A source-output transistor of one DS75325
delivers load current (I_L) The sink-output transistor of an-
other DS75325 sinks this current
The value of the external pull-up resistor (R_ext) for a particu-
lar memory application may be determined using the follow-
ing equation
R_ext
e
I_L
16
b1
V_CC2(Min) b
6 V_CC2(Min)
V_S b
b V_S
22
b2
9
(1)
where R_ext is in kX
V_CC2(Min) is the lowest expected value of V_CC2 in volts V_S
is the source output voltage in volts with respect to ground
I_L is in mA
The power dissipated in resistor R_ext during the load current
pulse duration is calculated using Equation 2
P_Rext
I_L
16
V_CC2(Min) b V_S b 2
(2)
where P_Rext is in mW
After solving for R_ext the magnitude of the source collector
current (I_CS) is determined from Equation 3
I_CS 0 94 I_L
(3)
where I_CS is in mA
As an example let V_CC2(Min) e 20V and V_L e 3V while I_L
of 500 mA flows Using Equation 1
16 (20 b 3 b 2 2)
R_ext e _{500 b 1 6 (20 b 3 b 2 9)} e 0 5 kX
and from Equation 2
500
P_Rext
20 b 3 b 2
16
470 mW
The amount of the memory system current source (I_CS)
from Equation 3 is
I_CS 0 94 (500) 470 mA
In this example the regulated source-output transistor base
current through the external pull-up resistor (R_ext) and the
source gate is approximately 30 mA This current and I_CS
comprise I_L
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
TL F 9755 – 15
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