Diodes ZNBG3116 - Manual (Page 7)

ZNBG3115
ZNBG3116
ISSUE 2 - JUNE 2006
7
FUNCTIONAL DIAGRAM
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 ZNBG3115/16 contains 3 such stages. The negative rail generator is common to both
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 ZNBG3115 this is nominally 2.2 volts whilst the ZNBG3116
provides nominally 2 volts.
The drain current taken by the FET is monitored by the low value resistor ID 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
.
Since the FET is a depletion mode transistor, it is often 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
.