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AND8093 Datasheet

Current Sensing Power MOSFETs

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AND8093/D
Current Sensing Power
MOSFETs
SENSEFET) PRODUCT
Current sensing power MOSFETs provide a highly
effective way of measuring load current in power
conditioning circuits. Conceptually simple in nature, these
devices split load current into power and sense
components, and thereby allow signal level resistors to be
used for sampling. Since this technique results in higher
efficiency and lower costs than competing alternatives,
understanding how to use SENSEFET product is an
important design issue.
Getting accustomed to these devices is relatively, but not
completely, straightforward. They are conceptually simple,
but have their own unique set of characteristics and subtle
properties. The following discussion examines both, and
starts with a description of how SENSEFET devices work.
Principle of Operation
Their operation is based on the matched devices
principle that is so commonly used in integrated circuits.
Like integrated circuit transistors, the on-resistance of
individual source cells in a power MOSFET tends to be
well matched. Therefore, if several out of several thousand
cells are connected to a separate sense pin, a ratio between
sense section on-resistance and power section
on-resistance is developed. Then, when the SENSEFET
device is turned on, current flow splits inversely with
respect to the two resistances, and a ratio between sense
current and source current is established.
The separate source connection is called a mirror.
Typically SENSEFET product is designed such that the
ratio between mirror cells and source cells is on the order
of 1:1000 Schematically, this looks like two parallel FETs
with common gate and drain connections, but separate
source leads. An illustration of this configuration appears
in Figure 1. The relative size of the two devices determines
how current is split between source and mirror terminals.
The ratio of source current to mirror current is specified by
n, the “Current Mirror Ratio”. This ratio is defined for
conditions where both source and mirror terminals are held
at the same potential. Since n is on the order of 1000:1, load
current is approximately equal to source current, and the
current mirror ratio also describes the ratio of load current
to sense current.
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APPLICATION NOTE
ILOAD
Drain
Gate
Mirror
ISENSE
Source
ISOURCE
Figure 1. SENSEFET Equivalent Circuit
When a signal level resistor is connected between mirror
and source terminals, a known fraction of load current is
sampled without the insertion loss that is associated with
power sense resistors. For this reason, the technique of
measuring load current with SENSEFET devices is called
“lossless current sensing”. As long as the sense resistor is
less than 10% of the mirror section’s on-resistance
RDM(on), the current that is sampled is approximately load
current divided by the current mirror ratio or ILOAD/n. In
practice, the amount of sense voltage that is developed with
such low values of sense resistance is usually not sufficient
to drive current limiting circuits. Therefore, larger values
of RSENSE are normally used. These larger values appreciably
affect the total resistance in the mirror leg, and therefore,
alter the current mirror ratio. How to model this behavior
and calculate sensing parameters is discussed as follows.
Calculating Sense Resistance
With the aid of the model that is shown in Figure 2,
calculating sense voltage and sense resistance is very
straightforward. In this model, RDS(on) is separated into
bulk and active components. Bulk drain resistance is
common to the entire device, and is represented by Rb.
This document may contain references to devices which are no longer offered. Please contact your ON Semiconductor representative for
information on possible replacement devices.
© Semiconductor Components Industries, LLC, 2002
March, 2017 − Rev. 6
1
Publication Order Number:
AND8093/D


  ON Semiconductor Electronic Components Datasheet  

AND8093 Datasheet

Current Sensing Power MOSFETs

No Preview Available !

AND8093/D
Active components of RDS(on) are modeled by Ra(on) for the
power section, and RDM(on) for the mirror. RSENSE is the
external sense resistor.
Drain
Drain
Rb
Mirror
RSENSE
Source
RDM(on)
Ra(on)
Kelvin
Mirror
RSENSE
Kelvin
a. Model
Source
Gate
Drive
Mirror
RSENSE
Source
Kelvin
Vref
b. Typical Connection
Figure 2. Model and Typical Connection
If RSENSE is an open circuit, the maximum voltage that
can appear at the mirror terminal is VDS(on) × Ra(on)/(Ra(on)
+ Rb). In other words, the mirror terminal does not sample
the full drain-source on voltage, but sees only the fraction
of drain-source voltage that is represented by Ra(on)/(Ra(on)
+ Rb). This ratio is called the mirror compliance ratio, KMC.
Values for Ra(on) and Rb are determined by measuring the
mirror compliance ratio, and multiplying RDS(on) by this
ratio to get Ra(on). Bulk resistance, Rb, is then determined
by subtracting Ra(on) from RDS(on). Given these values,
RDM(on) is determined by multiplying Ra(on) times the
current mirror ratio, n.
Given values for the model’s internal resistors, sense
voltage, sense resistance, and drain current can be
calculated from simple resistive divider equations. These
equations are summarized as follows:
Sensing Equations
VSENSE
+
ID
@
Ra(on)
@
RSENSE
(RSENSE ) RDM(on))
RSENSE
+
VSENSE
@
[(ID
@
RDM(on)
Ra(on)) * VSENSE]
ID
+
VSENSE
@
(RSENSE ) RDM(on))
(Ra(on) @ RSENSE)
(1)
(2)
(3)
The results obtained from using these equations agree
very well with measured values. Using the MTP10N10M
as an example, calculated and measured values are
compared in Table 1. They are based upon 5 amps of drain
current, Ra(on) = 116 milliohms, Rb = 44 milliohms, and
RDM(on) = 209 ohms.
Table 1. CALCULATED VERSUS MEASURED
SENSE VOLTAGE
RSENSE
(W)
Calculated
VSENSE
(mV)
Measured
VSENSE
(mV)
D
(%)
20 51 50 2
47 106 105 1
100 179 185 −3
200 284 290 −2
1.0 k
480
480 0
Since all of the actual values were measured on an
oscilloscope, the discrepancies which are shown here are
all within measurement accuracy. Given static conditions,
the model in Figure 2 does a good job.
In a typical application such as the one shown in
Figure 2b, a current trip is produced when VSENSE is equal
to the comparator’s reference voltage, Vref. Therefore,
substituting Vref for VSENSE in these equations yields
combinations of ID and RSENSE for which a current limit
signal is produced.
For reasons which will soon be discussed, it is generally
advisable to choose a value of RSENSE that does not exceed
RDM(on). As the values in Table 1 indicate, this constraint
produces sense voltages on the order of 250 mV at the
MTP10N10M’s normal operating current. Although this is
sufficient for most applications, lower operating currents
and device types with lower mirror compliance ratios can
lead to problems with generating usable values of sense
voltage. Where higher values of sense voltage are required,
the technique shown in Figure 3 can be used. In this
circuit,the SENSEFET mirror is held at the same potential
as its source, and op amp OA1 generates a negative output
voltage that equals sense current times the feedback
resistor Rf. Sensing equations for this type of virtual ground
circuit are listed as follows:
Virtual Ground Sensing Equations
VSENSE + *ID @ n @ Rf
(4)
Rf + VSENSEńID @ n
(5)
ID + *(VSENSEńRf)n
(6)
These equations assume that the op amp’s input bias
current and input offset voltage are both zero. Using some
of today’s newer op amps, this assumption is a good one.
With an MC34081 for example, room temperature values
for input bias current and input offset voltage are less than
one nanoamp and less than one millivolt, respectively.
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2


Part Number AND8093
Description Current Sensing Power MOSFETs
Maker ON Semiconductor
Total Page 12 Pages
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