ADL8124ACPZN-R7 datasheet(28/29 Pages) AD

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Data Sheet

ADL8124

RECOMMENDED POWER MANAGEMENT CIRCUIT

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Rev. 0 | 28 of 29

Figure 101 shows a recommended power management circuit for

the ADL8124. The LT8607 step-down regulator is used to step

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

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

Even though the circuit shown in Figure 101 has an input voltage

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

The 4.5V regulator output of the LT8607 is set by the R2 and R3

resistors, according to the following equation:

R2 = R3((VOUT/0.778V) – 1), where VOUT is the output voltage.

The switching frequency (fSW) is set to 2MHz by the 18.2kΩ 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 0.2MHz to 2.200MHz.

The VOUT of the LT3042 is set by the R4 resistor connected to the

SET pin, according to the following equation:

VOUT = 100μA × 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 3.3V. 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 (5%). Therefore, putting 5% between

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

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

voltage range. Table 16 provides the recommended resistor values

for operation at 3.6V to 3V.

Table 16. Recommended Resistor Values for Operating at 3.6V to 3V

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 750mA, and the

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

1.5A and 3A, respectively, and these devices are pin compatible

with the LT8607. The LT3045 linear regulator, which is pin compati-

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

Figure 101. Recommended Power Management Circuit

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