LM3000 datasheet(13/41 Pages) TI1

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VOUT1

VOUT2

1.8V

VOUT2 = VOUT1 x

RT1

RT1 + RT2

VOUT1

VOUT2

3.3V

1.2V

0.8 x 3.3V

0.75 = VOUT1 x

RT1

RT1 + RT2

LM3000

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SNVS612A – JULY 2009 – REVISED JULY 2009

(3)

A value of 10 k

Ω 1% is recommended for RT1 as a good compromise between high precision and low quiescent

current through the divider. Using an example of VOUT1 = 3.3V and VOUT2 = 1.2V, the value of RT2 is 34.4 kΩ 1%.

A timing diagram for VOUT1 controlling VOUT2 is shown in Figure 3. Note that the TRK pin must finish at least 100

mV higher than the 0.6V reference to achieve the full accuracy of the LM3000 regulation. To meet this

requirement the tracking voltage is offset by 150 mV. The tracking output voltage will reach its final value at 80%

of the controlling output voltage.

Figure 3. Tracking with VOUT1 Controlling VOUT2

Alternatively, the tracking feature can be used to create equal slew rates for the output voltages. In order to track

properly, use the highest output voltage to control the slew rate. In this case, the tracking resistors are found

from:

(4)

Again, a value of 10 k

Ω 1% is recommended for RT1. For the example case of VOUT1 = 5V and VOUT2 = 1.8V, RT2

is 17.8 k

Ω 1%. A timing diagram for the case of equal slew rates is shown in Figure 4.

Either method ensures that the output voltage of the tracking supply always reaches regulation before the output

voltage of the controlling supply.

Figure 4. Tracking with Equal Slew Rates

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