MIC5010 Datasheet, PDF (9/16 Pages) Micrel Semiconductor – Full-Featured High- or Low-Side MOSFET Driver

MIC5010
Applications Information (Continued)
Control Input
R
TH
20kΩ
MIC5010
1 _{Inhibit Fault} 14
2 _NC
V+ ¹³
3 _{Input NC} 12
4 _{Thresh C1} 11
5 _{Sense Com} 10
6 _{Source C2} 9
7 _{Gnd Gate} 8
R1
24kΩ
V+ =24V
+
10µF
V+
R1=
1mA
R2=100Ω
IRF541
100Ω
R2
R
S
18mΩ
IRC 4LPW-5*
100mV+V
R=
TRIP
S
I_L
2200
R_TH = _V
–1000
TRIP
For this example:
I_L =10A (trip current)
V =100mV
TRIP
LOAD
*International Resistive Company
Micrel
Figure 3. High-Side Driver
with Current Shunt
self-protected against inductive switching transients. Dur- body resistance “R” of the main source pin. Current sensing
ing turn-off an inductive load will force the MOSFET source MOSFETs eliminate the current shunt required by standard
5V or more below ground, while the driver holds the gate at MOSFETs.
ground potential. The MOSFET is forced into conduction, _{The design equations for a low-side driver using a current}
5
and it dissipates the energy stored in the load inductance. _{sensing MOSFET are shown in Figure 4. “S” is specified on}
The MIC5010 source and sense pins (5 and 6) are designed _{the MOSFET’s datasheet, and “R” must be measured or}
to withstand this negative excursion without damage. Exter-
nal clamp diodes are unnecessary, but may be added to
estimated. V_TRIP must be less than R × I_L, or else R_S will
become negative. Substituting a MOSFET with higher on-
reduce power dissipation in the MOSFET.
resistance, or reducing V_TRIP fixes this problem. V_TRIP =
Current Shunts (R_S). Low-valued resistors are necessary 100 to 200mV is suggested. Although the load supply is
for use at R_S.Values for R_S range from 5 to 50mΩ, at 2 to limited only by MOSFET ratings, the MIC5010 supply
10W. Worthy of special mention are Kelvin-sensed, “four- should be limited to 15V to prevent damage to the gate
terminal” units supplied by a number of manufacturers^†. clamp zener. Output clamping is necessary for inductive
Kelvin-sensed resistors eliminate errors that are caused by loads.
lead and terminal resistances, and simplify product assem-
bly. 10% tolerance is normally adequate, and with shunt
potentials of 200mV thermocouple effects are insignificant.
Temperature coefficient is important; a linear, 500ppm/°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 100ppm/°C.
Low-Side Driver with Current Sensing MOSFET (Figure
4). Several manufacturers now supply power MOSFETs in
“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
(using TAB and SOURCE for forcing, and SENSE and
KELVIN for sensing) is the best method of evaluating “R.”
Alternatively, “R” can be estimated for large MOSFETs
(R_DS(ON) ≤ 100mΩ) by simply halving the stated R_DS(ON), or
by subtracting 20 to 50mΩ from the stated R_DS(ON) for
smaller MOSFETs.
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
High-Side Driver with Current Sensing MOSFET (Figure
1). The design starts by determining the value of “S” and “R”
for the MOSFET (use the guidelines described for the low-
side version). Let V_TRIP = 100 mV, and calculate R_S for a
desired trip current. Next calculate R_TH and R1. The trip
† 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]
April 1998
5-95