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HMC981LP3E

v01.0411

ACTIVE BIAS CONTROLLER

The gate control of the HMC981LP3E is designed to both sink and source current in to the gate of the targeted ampli-

fier (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 HMC981LP3E.

HMC981LP3E limits the maximum current to protect itself under such fault conditions, by turnung of VDRAIN and

VGATE.

The HMC981LP3E 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 HMC981LP3E 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

HMC981LP3E produces a trigger out signal (TRIGOUT pin#13) when VDRAIN output is enabled. This trigger signal

can be used to enable additional HMC981LP3E 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 HMC981LP3Es in an amplification chain. Please note that, only one of the HMC981LP3E is used to generate

the negative voltage and the remaining HMC981LP3E is set to receive external negative voltage (which is provided

from the master HMC981LP3E). Generating negative voltage from a single HMC981LP3E 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.

Information furnished by Analog Devices is believed to be accurate and reliable. However, no

responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other

rights of third parties that may result from its use. Specifications subject to change without notice. No

license is granted by implication or otherwise under any patent or patent rights of Analog Devices.

Trademarks and registered trademarks are the property of their respective owners.

For price, delivery, and to place orders: Analog Devices, Inc.,

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106

Phone: 781-329-4700 • Order online at www.analog.com

Application Support: Phone: 1-800-ANALOG-D