MIC5013 Datasheet, PDF (8/16 Pages) Micrel Semiconductor – Protected High- or Low-Side MOSFET Driver

MIC5013
Applications Information (Continued)
V
LOAD
Control Input
R _TH
20kΩ
MIC5013
V+ =15V
1 _{Input Fault} 8
2 _{Thresh V+} 7
3 _{Sense Gate} 6
10µF
+
LOAD
4 _{Source Gnd} 5
SENSE
IRCZ44
(S=2590,
R=11mΩ)
R_S
22Ω
KELVIN
SOURCE
SR V
R_S =
TRIP
R I –V
L TRIP
2200
R=
TH _V
TRIP
–1000
For this example:
I _L =20A (trip current)
V =100mV
TRIP
Micrel
Figure 4. Low-Side Driver with
Current-Sensing MOSFET
supply should be limited to 15V in low-side topologies;
otherwise, a large current will be forced through the gate
clamp zener.
Low-side drivers constructed with the MIC501X family are
also fast; the MOSFET gate is driven to near supply
immediately when commanded ON. Typical circuits achieve
10V enhancement in 10µs or less on a 12 to 15V supply.
High-Side Driver with Current Shunt (Figure 3). The
comparator input pins (source and sense) float with the
current sensing resistor (R_S) on top of the load. R1 and R2
add a small, additional potential to V_TRIP to prevent false-
triggering of the over-current shutdown circuit with open or
inductive loads. R1 is sized for a current flow of 1mA, while
R2 contributes a drop of 100mV. The shunt voltage should
be 200 to 500mV at the trip point. The example of Figure 3
gives a 10A trip current when the output is near supply. The
trip point is somewhat reduced when the output is at ground
as the voltage drop across R1 (and therefore R2) is zero.
High-side drivers implemented with MIC5013 drivers are
self-protected against inductive switching transients. Dur-
ing turn-off an inductive load will force the MOSFET source
5V or more below ground, while the driver holds the gate at
ground potential. The MOSFET is forced into conduction,
and it dissipates the energy stored in the load inductance.
The MIC5013 source and sense pins (3 and 4) are designed
to withstand this negative excursion without damage. Exter-
nal clamp diodes are unnecessary.
Current Shunts (R_S). Low-valued resistors are necessary
for use at R_S.Values for R_S range from 5 to 50mΩ, at 2 to
10W. Worthy of special mention are Kelvin-sensed, “four-
terminal” units supplied by a number of manufacturers^†
(see next page). Kelvin-sensed resistors eliminate errors
caused by lead and terminal resistances, and simplify
product assembly. 10% tolerance is normally adequate,
and with shunt potentials of 200mV thermocouple effects
are insignificant. Temperature coefficient is important; a
linear, 500 ppm/°C change will contribute as much as 10%
shift in the over-current trip point. Most power resistors
designed for current shunt service drift less than 100 ppm/
°C.
Low-Side Driver with Current Sensing MOSFET (Figure
4). Several manufacturers now supply power MOSFETs in
which a small sampling of the total load current is diverted
to a “sense” pin. One additional pin, called “Kelvin source,”
is included to eliminate the effects of resistance in the
source bond wires. Current-sensing MOSFETs are speci-
fied with a sensing ratio “S” which describes the relationship
between the on-resistance of the sense connection and the
body resistance “R” of the main source pin. Current sensing
MOSFETs eliminate the current shunt required by standard
MOSFETs.
The design equations for a low-side driver using a current
sensing MOSFET are shown in Figure 4. “S” is specified on
the MOSFET’s datasheet, and “R” must be measured or
estimated. V_TRIP must be less than R × I_L, or else R_S will
become negative. Substituting a MOSFET with higher on-
resistance, or reducing V_TRIP fixes this problem. V_TRIP =
100 to 200mV is suggested. Although the load supply is
limited only by MOSFET ratings, the MIC5013 supply
should be limited to 15V to prevent damage to the gate
clamp zener. Output clamping is necessary for inductive
loads.
“R” is the body resistance of the MOSFET, excluding bond
resistances. R_DS(ON) as specified on MOSFET data sheets
† Suppliers of Kelvin-sensed power resistors:
Dale Electronics, Inc., 2064 12th Ave., Columbus, NE 68601. Tel: [phone]
International Resistive Co., P.O. Box 1860, Boone, NC 28607-1860. Tel: [phone]
Kelvin, 14724 Ventura Blvd., Ste. 1003, Sherman Oaks, CA 91403-3501. Tel: [phone]
RCD Components, Inc., 520 E. Industrial Pk. Dr., Manchester, NH 03103. Tel: [phone]
Ultronix, Inc., P.O. Box 1090, Grand Junction, CO 81502. Tel: [phone]
MIC5013
8
July 2000