Circuit Note

CN-0518

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Rev. 0 | 5 of 8

Automatic Reset Feature

The CN0518 also includes an automatic reset circuitry that acti-

vates when the power level returns within the acceptable range.

This feature is done through the LTC6991, a programmable low

frequency timer.

As shown in Figure 12, the Q output of the ADL5904 holds the

LTC6991 in reset mode during normal operation. When an over-

powering event occurs, the LTC6991 is enabled and a 4 ms delay

begins. The ADL5904 resets after 4 ms, effectively resampling the

power level. If the overpowering condition persists, the ADL5904

trips again and the control signal of the RF switch enters the low

state. This shift in signal isolates the RF input and output of the

ADG901 switch. Once the overpower event subsides, the ADL5904

starts to resample the power level and then returns to its normal

operation.

Figure 12. CN0518 Automatic Reset Circuitry

Overpower Protection Test

Figure 13 presents the setup used to test the overpower protection

functionality of the CN0518. In this test, the RF signal generator

is set to 915 MHz center frequency and the power to the input is

ramped up from -25 dBm to -15 dBm. The CN0518 output power

is monitored using a high speed oscilloscope, which shows the

response time from the overpowering event to the output power

being attenuated.

Figure 13. RF Overpower Response Test Setup

Figure 14 shows the overpowering protection response time. Based

on the figure, the output of the CN0518 has a response time of

approximately 10 ns from its normal operation to the RF output

power being attenuated.

Figure 14. Typical Overpower Protection Response Time

Figure 15 shows the recovery time from when the overpowering

condition subsides, and the power level returns back to its accepta-

ble range. The data demonstrate a 5 ns delay from the attenuated

RF output to its normal operation.

Figure 15. Typical Recovery Time after an Overpower Event

USB POWER MANAGEMENT

The CN0518 sources its power through a micro-USB adapter,

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

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, 3.3 V, and 2.5 V. The circuit

requires an overall current of approximately 260 mA from the USB

supply during normal operation.

Figure 16 shows the complete CN0518 power tree.