MIC5010 Datasheet, PDF (8/16 Pages) Micrel Semiconductor – Full-Featured High- or Low-Side MOSFET Driver

MIC5010
Applications Information (Continued)
Residual Resistances: Resistances in circuit connections
may also cause confusing results. For example, a circuit
may employ a 50mΩ power MOSFET for low drop, but
careless construction techniques could easily add 50 to 100
mΩ resistance. Do not use a socket for the MOSFET. If the
MOSFET is a TO-220 type package, make high-current
drain connections to the tab. Wiring losses have a profound
effect on high-current circuits. A floating millivoltmeter can
identify connections that are contributing excess drop un-
der load.
Circuit Topologies
The MIC5010 is suited for use in high- or low-side driver
applications with over-current protection for both current-
sensing and standard MOSFETs. In addition, the MIC5010
works well in applications where, for faster switching times,
the supply is bootstrapped from the MOSFET source out-
put. Low voltage, high-side drivers (such as shown in the
Test Circuit) are the slowest; their speed is reflected in the
gate turn-on time specifications. The fastest drivers are the
low-side and bootstrapped high-side types. Load current
switching times are often much faster than the time to full
gate enhancement, depending on the circuit type, the
MOSFET, and the load. Turn-off times are essentially the
same for all circuits (less than 10µs to V_GS = 1V). The choice
of one topology over another is based on a combination of
considerations including speed, voltage, and desired sys-
tem characteristics. Each topology is described in this
section. Note that I_L, as used in the design equations, is the
load current that just trips the over-current comparator.
Low-Side Driver with Current Shunt (Figure 2). The over-
current comparator monitors R_S and trips if I_L × R_S exceeds
V_TRIP. R_TH is selected to produce the desired trip voltage.
As a guideline, keep V_TRIP within the limits of 100mV and
Micrel
500mV (R_TH = 3.3kΩ to 20kΩ). Thresholds at the high end
offer the best noise immunity, but also compromise switch
drop (especially in low voltage applications) and power
dissipation.
The trip current is set higher than the maximum expected
load current--typically twice that value. Trip point accuracy
is a function of resistor tolerances, comparator offset (only
a few millivolts), and threshold bias voltage (V₄). The values
shown in Figure 2 are designed for a trip current of 20
amperes. It is important to ground pin 6 at the current shunt
R_S, to eliminate the effects of ground resistance.
A key advantage of the low-side topology is that the load
supply is limited only by the MOSFET BV_DSS rating. Clamp-
ing may be required to protect the MOSFET drain terminal
from inductive switching transients. The MIC5010 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 MIC501X drivers are
Control Input
R_TH
10kΩ
V+=7 to 15V
MIC5010
1 _{Inhibit Fault} 14
2 _NC
3 _Input
4 _Thresh
V+ ¹³
NC ¹²
C1 ¹¹
+
10µF
5 _{Sense Com} 10
6 _{Source C2} 9
7 _{Gnd Gate} 8
V_LOAD
LOAD
IRF540
V
R_S=
TRIP
I
L
2200
R=
–1000
TH _V
TRIP
For this example:
I =20A (trip current)
L
V = 200mV
TRIP
R
S
10mΩ
IRC 4LPW-5
(International Resistive Company)
Figure 2. Low-Side Driver with
Current Shunt
5-94
April 1998