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13

HMC980

v00.0312

Active BiAs controller

High current

where VDrain is the supply voltage of the external amplifier and iDrain is the desired constant bias current through

the external amplifier.

note that rDs_on resistance of the internal Fet switch can be adjusted through s0, s1 pads based on the Drain

current requirement as shown in table-1. rDs_on is typically equal to 0.7 ohm when s0 and s1 are pulled up to

VDiG, and is typically equal to 2.8 ohm when s0 and s1 are pulled down to GnD. if s0 and s1 pads are left floating,

it is pulled up to VDiG through an internal weak pull-up. recommended settings for the s0 and s1 positions are given

in table-1. not using the hMC980 in the recommended settings may increase the power dissipation of the part and

the part-to-part variation.

Table 1. Recomended Current Range Configuration

negative voltage Generator (vneGoUt)

the hMC980 has internal regulated charge pump circuitry to generate the negative voltage (VneGoUt) required for

depletion mode devices. the hMC980 generates -2.5V at the VneGoUt output in default configuration. it requires

two diodes and two capacitors connected externally as shown in the sample application schematics. it can be dis-

abled through the VGateFB and VneGFB pads, if an enhancement device is targeted or a negative supply is already

available in the system. in this configuration, simply connect the available negative supply to the VneG pad. see

table-2 for the operation mode selection. the hMC980 is designed to reject the ripple on the VneGoUt pad by iso-

lating VneGoUt from the VGate. thus, switching noise of the charge-pump is effectively isolated from the external

amplifier.

enable/Disable (en)

the active bias control loop is enabled when en is pulled up to VDiG, and it is disabled when it is pulled down to GnD.

if en is left floating hMC980 is enabled through an internal weak pull-up. note that VneG operation is independent

of en condition. en signal controls the operation of only VGate, VG2 and VDrain outputs. When en pulled down

to GnD, the hMC980 discharges VDrain and VG2 down to GnD and it pulls the VGate down to VneG. When en

pulled high to VDiG, hM980 enables, VDrain and VG2, and enables the bias control loop to automatically adjust the

VGate voltage. please see the “active Bias Control loop” section for detailed explanation and refer to the enable and

Disable waveforms for transient characteristics.

Active Bias control loop

the hMC980 regulates the bias current (iDrain) of the amplifier under bias through VGate output connected to

the gate of the external amplifier. in this closed loop operation the current passing through the amplifier under bias is

sampled and is used to automatically adjust VGate to achieve constant quiescent bias through the external amplifier.

the hMC980 continuously adjusts VGate voltage to achieve constant Drain current over any supply, tempera-

ture, process variations and threshold drifts due to aging. the part-to-part, temperature, and supply variation of the

hMC980 is excellent. thus, by using an accurate sense resistor connected to the isense pad, expensive calibration

procedures in high volume production could be avoided.

the gate control of the hMC980 is designed to both sink and source current in to the gate of the targeted amplifier (at

least ±4 ma). this unique feature is important to achieve nearly constant quiescent bias through the amplifier under

varying gate current at different input power values.

Current Range (A)

Condition

RD_ON Value (Ohm)

0.05 to 0.3

s1=s0=GnD

2.8

0.3 to 0.6

s1=GnD, s0=VDiG

1.55

0.6 to 1.2

s1=VDiG, s0=GnD

0.85

1.2 to 1.6

s1=VDiG, s0=VDiG

0.7