AMC7834 datasheet(39/97 Pages) TI2

39 / 97 page

PA Controller

V(DRAIN)

RF Out

DAC

RF In

HPA

V(BIAS)

PA_ON

Control

PA_ON

PAVDD

PMOS Switch

AMC7834

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SLAS972B – NOVEMBER 2014 – REVISED MARCH 2016

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7.3.5 Drain Switch Control

The AMC7834 device includes an output-control voltage (PA_ON pin) capable of driving an external PMOS

switch that turns on and off the drain current to a PA FET. The use of this control signal in conjunction with the

DAC clamp option allows control of the sequence in which the PA FET is powered up and powered down.

The OFF and ON states of the PA_ON signal are equal to the PAVDD and AGND pins, respectively. The default

state of the PA_ON signal is off (PMOS switch off).

Figure 53. PA_ON Operation

The maximum output voltage is determined by the PAVDD pin and limited to a maximum of 20 V. For PA FETs

with drain voltages higher than 20 V, tying the PAVDD pin to one of the other supply devices (preferably AVDD)

and scaling the control signal externally is recommended.

The PA_ON signal state can be set through a register write, but it can also be configured to be triggered

automatically by the ALARMOUT pin, any of the SLEEP signals or by the special AVSS and AVDD monitoring

circuits.

For FETs requiring a negative bias voltage, such as GaN, ensuring that the bias voltage remains within an

acceptable range is crucial otherwise significant and irreversible damage to the FET can occur. The AMC7834

bipolar DAC operation and clamping mechanism rely on the AVDD and AVSS voltages for proper operation. For

this reason, when either the AVDD or AVSS voltage falls outside its acceptable range, turning off the drain current

to the FET is desirable.

The AVDD detection circuit is set to trigger the PA_ON signal to the OFF state in response to an out of range

event. Additionally, the AVSS detection alarm can be set to trigger the PA_ON signal to the OFF state by setting

the PAON_AVSS bit to 1 in the AMC configuration 1 register (address 0x11). The AVSS alarm is set by default to

prevent the PA_ON output from entering the ON state (PMOS switch on). In this case writing to the PA_ON

clearing the PAON_AVSS bit.

AMC7834 Datasheet(PDF) 39 Page - Texas Instruments