Datasheet

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1
300
200
100
0
10 k 100 k 1 M 10 M 100 M
DAC Fullscale Output Current – A
400
500
RRNG Resistor Value
DAC FULLSCALE OUTPUT CURRENT
vs
RRNG RESISTOR VALUE
600
µ
1000
900
800
700
1100
TSC2301
SLAS371D – SEPTEMBER 2002 – REVISED AUGUST 2004
Figure 71. DAC Output Current Range vs RRNG Resistor Value
For example, consider an LCD that has a contrast control voltage VBIAS that can range from 2 V to 4 V, that
draws 400 µA when used, and has an available 5-V supply. This is higher than the TSC2301 supply voltage, but
it is within the absolute maximum ratings.
The maximum VBIAS voltage is 4 V, and this occurs when the D/A converter current is 0, so only the 400- µA
load current ILOAD is flowing from 5 V to VBIAS. This means 1 V is dropped across R1, so R1 = 1 V/400 µA =
2.5 k Ω.
The minimum VBIAS is 2 V, which occurs when the D/A converter current is at its full scale value, IMAX. In this
case, 5 V - 2 V = 3 V is dropped across R1, so the current through R1 is 3 V/2.5 k Ω = 1.2 mA. This current is
IMAX + ILOAD = IMAX + 400 uA, so IMAX must be set to 800 µA. Looking at Figure 73 , this means that RRNG
should be around 1 M Ω.
Since the voltage at the AOUT pin must not go below 1.5 V, this limits the voltage at the bottom of R2 to be
1.5-V minimum; this occurs when the D/A converter is providing its maximum current, IMAX. In this case, IMAX
+ILOAD flows through R1, and IMAX flows through R2. Thus,
R2 x IMAX + R1(IMAX + ILOAD) = 5 V - 1.5 V = 3.5 V
W R1 = 2.5 k Ω IMAX = 800 µA, ILOAD = 400 µA, thus allowing R2 to be solved as 625 Ω.
In the previous example, when the DAC current is zero, the voltage on the AOUT pin rises above the TSC2301
supply voltage. This is not a problem, however, since V+ was within the absolute maximum ratings of the
TSC2301, so no special precautions are necessary. Many LCD displays require voltages much higher than the
absolute maximum ratings of the TSC2301. In this case, the addition of an NPN transistor, as shown in
Figure 72 , protects the AOUT pin from damage.
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