MIC5013_05 Datasheet, PDF (8/15 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
+
LO A D
4 _{Source Gnd} 5
IRCZ44
(S=2590,
R=11mΩ)
SENSE
R_S
22Ω
KELVIN
SOURCE
R_S = ^{S R VTRI P}
R I _L – V_{TRI P}
R _TH=
2200
V_{TR I P}
–1000
For this example:
I _L =20A (trip current)
V_{TRI P} =100mV
Micrel, Inc.
Figure 4. Low-Side Driver with
Current-Sensing MOSFET
inductive switching transients. The MIC5013 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 imme-
diately 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. During
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. External
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 specified
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
includes bond resistances. A Kelvin-connected ohmmeter
† 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 2005