Circuit Note

CN-0523

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Rev. 0 | 5 of 8

Figure 12. CN0523 Thermal Performance at 0 dBm RF Input Power

USB POWER MANAGEMENT

The CN0523 derives its power through a micro-USB adapter, which

typically provides a 5 V, 1 A supply via the micro-USB port. The

whole system requires an overall current of approximately 320 mA

during normal operation.

The power requirements are provided using two supply voltages.

The nominal 4.75 V to 5.25 V input from the USB port is boosted to

5.6 V, then post-regulated to 5.0 V and 3.3 V. Figure 13 shows the

complete CN0523 power supply architecture.

Figure 13. CN0523 Power Supply Architecture

The main +5 V power is regulated by an LT8364, a current-mode

DC/DC converter with a 60 V, 4 A switch operating from a 2.8

V to 60 V input. With a unique single feedback pin architecture,

this device is capable of boost, SEPIC or inverting configurations.

Burst mode operation consumes as low as 9 µA quiescent current,

which maintains high efficiency at very low output currents while

keeping typical output ripple below 15 mV. Figure 14 shows the

basic configuration needed to set the LT8364 for a 5.6 V output.

Figure 14. LT8364 Configuration for a 5 .6V Output

The output voltage is programmed with resistor divider from the

output to the FBX pin of the LT8364. Equation 1 shows how to

calculate a desired output voltage with the user-defined resistor

values:

R1=R2  VOUT1.60−1

(1)

where:

VOUT is the desired output voltage in Volts.

R1, R2 are the user-defined resistor values.

For the required output voltage of +5 V, the equation results with

R1 = 1 MΩ and R2 = 402 kΩ.

The two HMC407 amplifier stages also draw additional supply

current at higher temperatures; but with a maximum output current

of 1.7 A, the LT8364 is more than sufficient to handle the current

requirements of the CN0523.

The ADM7171-5.0 LDO regulator is used to generate the 5 V, 220

mA power required by the two HMC407 and the ADT6401. It has an

input range of 2.3 V to 6.5 V and is capable of driving a load of up

to 1 A.

The ADM7171 LDO regulator is used to generate the 3.8 V, 15 mA

power required by the buffer amplifier and power down pin (VPD)

of the HMC407. This device has an input voltage range of 2.3 V to

6.5 V and an initial fixed output voltage of 3.3 V, but the fixed 3.3 V

output of the ADM7171 can be maximized up to 3.8 V output using

Equation 2:

VOUT=3.3 V 1+R1R2

(2)

where:

VOUT is the desired output voltage in Volts.

R1, R2 are the user-defined resistor values.

For the required output voltage of +3.8 V, the equation results with

R1 = 1 kΩ and R2 = 6.49 kΩ which is shown in Figure 15.