April 1998

5-119

MIC5012

Micrel

5

Input

Source

Gate

1/2 MIC5012

V+

Gnd

IRF540

10µF

Figure 3. Bootstrapped

High-Side Driver

Control Input

+

LOAD

100nF

1N4001 (2)

1N5817

7 to 15V

Input

Source

Gate

1/2 MIC5012

V+

Gnd

IRFP044 (2)

10µF

Figure 5. 50-Ampere

Industrial Switch

+

LOAD

24V

330k

Ω

100k

Ω

OFF

ON

CR2943-NA102A

(GE)

pitfalls most often encountered during prototyping: Sup-

plies: many bench power supplies have poor transient

response. Circuits that are being pulse tested, or those that

operate by pulse-width modulation will produce strange

results when used with a supply that has poor ripple

rejection, or a peaked transient response. Always monitor

the power supply voltage that appears at the drain of a high-

side driver (or the supply side of the load in a low-side driver)

with an oscilloscope. It is not uncommon to find bench

power supplies in the 1kW class that overshoot or under-

shoot by as much as 50% when pulse loaded. Not only will

the load current and voltage measurements be affected, but

it is possible to over-stress various components—espe-

cially electrolytic capacitors—with possibly catastrophic

results. A 10

µF supply bypass capacitor at the chip is

recommended.

Residual Resistances: Resistances in circuit connections

may also cause confusing results. For example, a circuit

may employ a 50m

Ω power MOSFET for low drop, but

careless construction techniques could easily add 50 to

100m

Ω 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 MIC5012 is suited for use with standard MOSFETs in

high- or low-side driver applications. In addition, the MIC5012

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 Figure

1) 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 (Figures 2 and 4). 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 10

µs to V

of one topology over another is based on a combination of

considerations including speed, voltage, and desired sys-

tem characteristics.

High-Side Driver (Figure 1). The high-side topology works

to 7V supply, a logic-level MOSFET can be substituted

since the MIC5012 will not reach 10V gate enhancement

(10V is the maximum rating for logic-compatible MOSFETs).

100k

Ω

Figure 4. Improved

Opto-Isolator Performance

1k

Ω

To MIC5012

Input

100k

Ω

4N35

33k

Ω

33pF

MPSA05

15V

10mA

Control Input