ADL8101ACPZN-R7 datasheet(34/35 Pages) AD

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Data Sheet

ADL8101

APPLICATIONS INFORMATION

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Rev. 0 | 34 of 35

RECOMMENDED POWER MANAGEMENT CIRCUIT

Figure 127 shows a recommended power management circuit for

the ADL8101. The LT8607 step-down regulator is used to step

down a 12 V rail to 4.5 V that is then applied to the LT3042 low

dropout (LDO) linear regulator to generate a low noise 3.3 V output.

Even though the circuit shown in Figure 127 has an input voltage

(VIN) of 12 V, the input range to the LT8607 can be as high as 42 V.

The 4.5 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)

where VOUT is the output voltage.

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 2.200 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

The resistors on the PGFB pins of the LT3042 are chosen to trigger

the power-good (PG) signal when the output is just under 95% of

the target voltage of 3.3 V. The output of the LT3042 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 tolerance (5%). Therefore, putting 5% between

the output and PGFB works well. In addition, the PG open-collector

is pulled up to the 3.3 V output to give a convenient 0 V to 3.6 V

voltage range. Table 10 provides the recommended resistor values

for operation at 3.6 V to 3 V.

Table 10. Recommended Resistor Values for Operating at 3.6 V to 3 V

LDO VOUT (V)

R4 (kΩ)

R7 (kΩ)

R8 (kΩ)

3.6

36.5

332

30.1

3.3

33.1

301

30.1

267

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 LT3042, can source a maximum current to 500 mA.

Figure 127. Recommended Power Management Circuit

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