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Data Sheet

ADL8103

RECOMMENDED POWER MANAGEMENT CIRCUIT

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Rev. 0 | 23 of 25

Figure 92 shows a recommended power management circuit for the

ADL8103. The LT8607 step-down regulator is used to step down a

12 V rail to 6.5 V, which is then applied to the LT3042 low dropout

(LDO) linear regulator to generate a low noise 5 V output. While the

circuit shown in Figure 92 has an input voltage of 12 V, the input

range to the LT8607 can be as high as 42 V.

The 6.54 V regulator output of the LT8607 is set by the R2 and R3

resistors according to the following equation:

R2=R3 VOUT/0.778 V –1

(1)

where VOUT is the output voltage.

The switching frequency is set to 2 MHz by the 18.2 kΩ resistor

on the RT pin. The LT8607 data sheet provides a table of resistor

values that can be used to select other switching frequencies

ranging from 0.2 MHz to 2.2 MHz.

The output voltage of the LT3042 is set by the R4 resistor connect-

ed to the SET pin according to the following equation:

VOUT=100 μA×R4

(2)

The PGFB resistors are chosen to trigger the power-good (PG)

signal when the output is just under 95% of the target voltage

of 5 V. The output of the LT3042 has 1% initial tolerance and

another 1% variation over temperature. The PGFB tolerance is

roughly 3% over temperature. Adding resistors results in a bit more

(5%) tolerance, therefore putting 5% between the output and PGFB

works well. In addition, the PG open-collector is pulled up to the 5

V output to give a convenient 0 V to 5 V voltage range. Table 12

provides the recommended resistor values for operation at 3 V to

5.5 V.

Table 12. Recommended Resistor Values for Operating at 3 V to 5.5 V

LDO Output Voltage (V)

R4 (kΩ)

R7 (kΩ)

R8 (kΩ)

30.1

255

30.1

3.5

34.8

316

30.1

40.2

374

30.1

4.5

45.3

422

30.1

49.9

442

30.1

5.5

54.9

487

30.1

The LT8607 can source a maximum current of 750 mA, and the

LT3042 can source a maximum current of 200 mA. If the 5 V power

supply voltage is being developed as a bus supply to serve another

component, higher current devices can be used. The LT8608 and

LT8609 step-down regulators can source a maximum current to 1.5

A and 3 A, respectively, and these devices are pin compatible with

the LT8607. The LT3045 linear regulator, which is pin compatible

with the LT3042, can source a maximum current to 500 mA.

Figure 92. Recommended Power Management Circuit

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