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HMC981

v00.0611

ACTIVE BIAS CONTROLLER

The gate control of the HMC981 is designed to both sink and source current in to the gate of the targeted amplifier (at

least ±0.8 mA). This unique feature is important to achieve nearly constant quiescent bias through the amplifier under

varying gate current at different input power values.

The bias current passing through the external amplifier can be adjusted with RSENSE, where RSENSE is the R10

connected from ISENSE to GND. Use the relation given in equation (2) to set the desired bias current through the

external amplifier.

IDRAIN=32/Rsense (A)

(2)

Self Protection Feature

Due to the small resistance of the internal switch FET a large amount of current may flow through the HMC981.

HMC981 limits the maximum current to protect itself under such fault conditions, by turnung of VDRAIN and VGATE.

The HMC981 will remain in this protection mode until a full power-cycle or enable/disable cycle is applied.

VNEG Fault Detection Feature

In depletion mode operation VNEG is continuously monitored against short circuit fault to GND. If VNEG rises above

a preset value (typically -1V) the system and the external amplifier are disabled by pulling VDRAIN and VG2 to GND

and VGATE to VNEG. The system will stay in this stand-by mode until short fault at VNEG is fixed.

Power-up and Enable Sequencing

To ensure the protection of the external amplifier, the HMC981 provides a power-up sequence for enabling active bias

control loop. During start-up VDRAIN and VG2 are kept at GND while VGATE is taken to the most negative supply

available (VGATE=VNEG). This ensures that external amplifier is completely pinched-off before VDRAIN is applied.

When EN signal is received, VDRAIN is applied and the active bias loop is enabled. After the VDRAIN is applied, VG2

is generated. The final phase of the power-up sequence is completed by increasing the VGATE linearly until the set

IDRAIN value is achieved.

For power-down and disabling the same sequencing is applied in the reverse order.

Daisy-Chain Operation

HMC981 produces a trigger out signal (TRIGOUT pad#13) when VDRAIN output is enabled. This trigger signal can

be used to enable additional HMC981 chips in a chain of amplifiers. The triggering sequence can be routed in any

way, from input to output, or from output to input depending on the use. Figure-1 presents a sample use of three

HMC981s in an amplification chain. Please note that, only one of the HMC981 is used to generate the negative

voltage and the remaining HMC981 is set to receive external negative voltage (which is provided from the master

HMC981). Generating negative voltage from a single HMC981 reduces the number of the components in the system,

and decreases the over all current consumption..

Please note that, to ensure proper start-up, the system enable signal should be applied to the chip that has the

negative voltage generator.