ALD110908SA 데이터 시트보기 (PDF) - Advanced Linear Devices
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ALD110908SA
Advanced Linear Devices
ALD110908SA Datasheet PDF : 12 Pages
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PERFORMANCE CHARACTERISTICS OF EPAD®
PRECISION MATCHED PAIR MOSFET FAMILY (cont.)
SUB-THRESHOLD REGION OF OPERATION
ZERO TEMPERATURE COEFFICIENT (ZTC) 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. Rather, it decreases at a
fixed rate of approximately 116mV per decade of drain current de-
crease. Thus, if the threshold voltage is +0.20V, for example, the
drain current is 1µA at VGS = +0.20V. At VGS = +0.09V, the drain
current would decrease to 0.1µA. Extrapolating from this, the drain
current is 0.01µA (10nA) at VGS = -0.03V, 1nA at VGS = -0.14V,
and so forth. This subthreshold characteristic extends all the way
down to current levels below 1nA 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, with VGS(th) = +0.20V, the drain current
still hovers around 20nA when the gate is at zero volts, or ground.
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 the nano-watt region and still provide
a useful and controlled circuit function 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 these temperature
stable operating points is obtained by a ZTC voltage bias condi-
tion, which is 0.55V above a threshold voltage when VGS = VDS,
resulting in a temperature stable current level of about 68µA. 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 to 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 is 1µA at
VDS = 0.1V and VGS = 0.0V. Thus, just by grounding the gate of
the ALD110800, a resistor with RDS(ON) = ~100KΩ is produced.
When an ALD114804 gate is grounded, the drain current IDS =
18.5µA @ VDS = 0.1V, producing RDS(ON) = 5.4KΩ. Similarly,
ALD114813 and ALD114835 produce drain currents of 77µA and
185µA, respectively, at VGS = 0.0V, and RDS(ON) values of 1.3KΩ
and 540Ω, respectively.
MATCHING CHARACTERISTICS
A key benefit of using a matched pair EPAD MOSFET is to main-
tain temperature tracking. In general, for EPAD MOSFET matched
pair devices, one device of the matched pair has gate leakage cur-
rents, junction temperature effects, and drain current temperature
coefficient as a function of bias voltage that cancel out similar ef-
fects of the other device, resulting in a temperature stable circuit.
As mentioned earlier, this temperature stability can be further en-
hanced by biasing the matched-pairs at Zero Tempco (ZTC) point,
even though that could require special circuit configuration and
power consumption design consideration.
ALD110808A/ALD110808/
ALD110908A/ALD110908, Vers. 2.3
Advanced Linear Devices
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