ZBNG4000 Datasheet, PDF (5/10 Pages) Zetex Semiconductors – FET BIAS CONTROLLER

FUNCTIONAL DIAGRAM
ZNBG4000 ZNBG4001
ZNBG6000 ZNBG6001
FUNCTIONAL DESCRIPTION
The ZNBG devices provide all the bias requirements for external FETs, including the generation
of the negative supply required for gate biasing, from the single supply voltage.
The diagram above shows a single stage from the ZNBG series. The ZNBG4000/1 contains 4 such
stages, the ZNBG6000/1 contains 6. The negative rail generator is common to all devices.
The drain voltage of the external FET Q_N is set by the ZNBG device to its normal operating voltage.
This is determined by the on board V_D Set reference, for the ZNBG4000/6000 this is nominally
2.2 volts whilst the ZNBG4001/6001 provides nominally 2 volts.
The drain current taken by the FET is monitored by the low value resistor I_D Sense. The amplifier
driving the gate of the FET adjusts the gate voltage of Q_N so that the drain current taken matches
the current called for by an external resistor R_CAL. Both ZNBG devices have the facility to program
different drain currents into selected FETs. Two R_CAL inputs are provided. For the ZNBG4000,
resistor R_CAL1 sets the drain current of FETs 1 and 2, resistor R_CAL2 sets the drain current of FETs
3 and 4. For the ZNBG6000, resistor R_CAL1 sets the drain current of FETs 1 and 4, resistor R_CAL2
sets the drain current of FETs 2, 3, 5 and 6.
Since the FET is a depletion mode transistor, it is usually necessary to drive its gate negative with
respect to ground to obtain the required drain current. To provide this capability powered from
a single positive supply, the device includes a low current negative supply generator. This
generator uses an internal oscillator and two external capacitors, C_NB and C_SUB.
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