Data Sheet

ADL8100

RECOMMENDED POWER MANAGEMENT CIRCUIT

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Rev. 0 | 26 of 27

Figure 107 shows a recommended power management circuit for

the ADL8100. The LT8607 step-down regulator is used to step

down a 12 V rail to a 6.5 V rail, which is then applied to the LT3045

low dropout (LDO) linear regulator to generate a low noise 5 V

output. Even though the circuit shown in Figure 107 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)

The switching frequency (fSW) is set to 2 MHz by the 18.2 kΩ

resistor (R1) on the RT pin of the LT8607. 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 LT3045 is set by the R4 resistor connect-

ed to the SET pin, according to the following equation:

VOUT = 100 μA × R4

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 LT3045 has 1% initial tolerance and another

1% variation over temperature. The PGFB tolerance is roughly 3%

over temperature, and adding resistors results in a bit more (5%).

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 10 provides the

recommended resistor values for operation at 5 V, 3.3 V, and 3 V.

Table 10. Recommended Resistor Values for Operating at 5 V, 3.3 V, and 3 V

LDO VOUT (V)

R4 (kΩ)

R7 (kΩ)

R8 (kΩ)

5

49.9

442

30.1

3.3

33.2

287

30.1

3

30.1

255

30.1

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

LT3045 can source a maximum current of 500 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.

Figure 107. Recommended Power Management Circuit