ald110800 Advanced Linear Devices Inc (ALD), ald110800 Datasheet - Page 2

• Manufacturer Part Number: ald110800
• Description: Performance Characteristics Of Epad Matched Pair Mosfet Arrays
• Manufacturer: Advanced Linear Devices Inc (ALD)

ELECTRICAL CHARACTERISTICS
The turn-on and turn-off electrical characteristics of the EPAD
MOSFET products are shown in the Drain-Source On Current vs
Drain-Source On Voltage and Drain-Source On Current vs Gate-
Source Voltage graphs. Each graph show the Drain-Source On
Current versus Drain-Source On Voltage characteristics as a func-
tion of Gate-Source voltage in a different operating region under
different bias conditions. As the threshold voltage is tightly speci-
fied, the Drain-Source On Current at a given gate input voltage is
better controlled and more predictable when compared to many
other types of MOSFETs.
EPAD MOSFETs behave similarly to a standard MOSFET, there-
fore classic equations for a n-channel MOSFET applies to EPAD
MOSFET as well. The Drain current in the linear region (Vds < Vgs
- Vth ) is given by:
Id = u .Cox . W/L . [Vgs - Vgs(th) - Vds/2] . Vds
where
In this region of operation the Ids value is proportional to Vds value
and the device can be used as gate-voltage controlled resistor.
For higher values of Vds where Vds >= Vgs-Vgs(th), the saturation
current Ids is now given by (approx.):
SUB-THRESHOLD REGION OF OPERATION
Low voltage systems, namely those operating at 5V, 3.3V or less,
typically require MOSFETs that have threshold voltage of 1V or
less. The threshold, or turn-on, voltage of the MOSFET is a voltage
below which the MOSFET conduction channel rapidly turns off. For
analog designs, this threshold voltage directly affects the operating
signal voltage range and the operating bias current levels.
At or below threshold voltage, an EPAD MOSFET exhibits a turn-
off characteristic in an operating region called the subthreshold re-
gion. This is when the EPAD MOSFET conduction channel rapidly
turns off as a function of decreasing applied gate voltage. The con-
duction channel induced by the gate voltage on the gate electrode
decreases exponentially and causes the drain current to decrease
exponentially. However, the conduction channel does not shut off
abruptly with decreasing gate voltage, but decreases at a fixed rate
of approximately 116 mV per decade of drain current decrease.
Thus if the threshold voltage is +0.20V, for example, the drain cur-
rent is at 1 uA at Vgs = +0.20V. At Vgs = +0.09V, the drain current
would decrease to 0.1 uA. Extrapolating from this, the drain current
is at 0.01 uA (10 nA) at Vgs = -0.03V, 1 nA at Vgs -0.14V, and so
forth. This subthreshold characteristics extends all the way down
to current levels below 1 nA and is limited by other currents such as
junction leakage currents.
At a drain current to be declared “zero current” by the user, the Vgs
voltage at that zero current can now be estimated. Note that using
the above example the drain current still hovers around 20 nA when
the gate is at zero volt, or ground.
ALD110800/ALD110900/
ALD1108xx/ALD1109xx/ALD1148xx/ALD1149xx
Cox is capacitance per unit area of Gate electrode
Vgs is the Gate to Source voltage
Vgs(th) is the turn-on threshold voltage
Vds is the Drain to Source voltage
W is the channel width and L is the channel length
Ids = u.Cox.W/L . [Vgs-Vgs(th) ] 2
u is the mobility
Advanced Linear Devices
LOW POWER AND NANOPOWER
When supply voltages decrease, the power consumption of a given
load resistor decreases as the square of the supply voltage. So
one of the benefits in reducing supply voltage is to reduce power
consumption. While decreasing power supply voltages and power
consumption go hand-in-hand with decreasing useful AC bandwidth
and at the same time increases noise effects in the circuit, a circuit
designer can make the necessary tradeoffs and adjustments in any
given circuit design and bias the circuit accordingly.
With EPAD MOSFETs, a circuit that performs a specific function
can be designed so that power consumption can be minimized. In
some cases, these circuits operate in low power mode where the
power consumed is measure in micro-watts. In other cases, power
dissipation can be reduced to nano-watt region and still provide a
useful and controlled circuit function operation.
ZERO TEMPERATURE COEFFICIENT (ZTC) OPERATION
For an EPAD MOSFET in this product family, there exist operating
points where the various factors that cause the current to increase
as a function of temperature balance out those that cause the cur-
rent to decrease, thereby canceling each other, and resulting in net
temperature coefficient of near zero. One of this temperature stable
operating point is obtained by a ZTC voltage bias condition, which
is 0.55V above a threshold voltage when Vgs = Vds, resulting in a
temperature stable current level of about 68 uA. For other ZTC
operating points, see ZTC characteristics.
PERFORMANCE CHARACTERISTICS
Performance characteristics of the EPAD MOSFET product family
are shown in the following graphs. In general, the threshold voltage
shift for each member of the product family causes other affected
electrical characteristics to shift with an equivalent linear shift in
Vgs(th) bias voltage. This linear shift in Vgs causes the subthresh-
old I-V curves to shift linearly as well. Accordingly, the subthreshold
operating current can be determined by calculating the gate volt-
age drop relative from its threshold voltage, Vgs(th).
RDS(ON) AT VGS=GROUND
Several of the EPAD MOSFETs produce a fixed resistance when
their gate is grounded. For ALD110800, the drain current at Vds =
0.1V is at 1uA at Vgs=0.0V. Thus just by grounding the gate of the
ALD110800, a resistor with Rds(on)=~100KOhm is produced. When
an ALD114804 gate is grounded, the drain current Ids=18.5 uA@
Vds=0.1V, producing Rds(on)=5.4KOhm. Similarly, ALD114813 and
ALD114835 produces 77 uA and 185 uA respectively at Vgs=0.0V,
producing Rds(on) values of 1.3 KOhm and 540 Ohm respectively.
MATCHING CHARACTERISTICS
A key benefit of using matched-pair EPAD MOSFET is to maintain
temperature tracking. In general, for EPAD MOSFET matched pair
devices, one device of the matched pair has gate leakage currents,
junction temperature effects, and drain current temperature coeffi-
cient as a function of bias voltage that cancel out similar effects of
the other device, resulting in a temperature stable circuit. As men-
tioned earlier, this temperature stability can be further enhanced by
biasing the matched-pairs at Zero Tempco (ZTC) point, even though
that could require special circuit configuration and power consump-
tion design consideration.
2