MIC5011 データシートの表示（PDF） - Micrel

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MIC5011

Minimum Parts High- or Low-Side MOSFET Driver

Micrel 

MIC5011

Applications Information (Continued)

Micrel, Inc.

Construction Hints

High current pulse circuits demand equipment and assembly

techniques that are more stringent than normal, low current

lab practices. The following are the sources of pitfalls most

often encountered during prototyping. Supplies: 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

1 kW class that overshoot or undershoot 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—especially electrolytic

capacitors—with possibly catastrophic results. A 10µF sup-

ply 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 50mΩ power MOSFET for low drop, but

careless construction techniques could easily add 50 to

100mΩ 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

under load.

Circuit Topologies

The MIC5011 is suited for use with standard MOSFETs in

high- or low-side driver applications. In addition, the MIC5011

works well in applications where, for faster switching times,

the supply is bootstrapped from the MOSFET source

output. 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 10µs to VGS

= 1V). The choice of one topology over another is based on

a combination of considerations including speed, voltage,

and desired system characteristics.

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

well down to V+ = 7V with standard MOSFETs. From 4.75 to

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

the MIC5011 will not reach 10V gate enhancement (10V is

the maximum rating for logic-compatible MOSFETs).

High-side drivers implemented with MIC501X 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 MIC5011 holds the gate at

ground potential. The MOSFET is forced into conduction,

and it dissipates the energy stored in the load inductance.

The MIC5011 source pin (3) is designed to withstand this

negative excursion without damage. External clamp diodes

are unnecessary.

Low-Side Driver (Figure 2). A key advantage of the low-

side topology is that the load supply is limited only by the

MOSFET BVDSS rating. Clamping may be required to

protect the MOSFET drain terminal from inductive switching

transients. The MIC5011 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.

Modifying Switching Times (Figure 3). High-side switch-

ing times can be improved by a factor of 2 or more by

adding external charge pump capacitors of 1nF each. In

cost-sensitive applications, omit C1 (C2 has a dominant

effect on speed).

Do not add external capacitors to the MOSFET gate. Add a

resistor (1kΩ to 51kΩ) in series with the gate to slow down

the switching time.

ON

+

10µF

Control Input

OFF

MIC5011

1 V+

C1 8

2 Input Com 7

3 Source C2 6

4 Gnd Gate 5

14.4V

1nF

1nF

IRF531

LO A D

Figure 3. High Side Driver with

External Charge Pump Capacitors

Bootstrapped High-Side Driver (Figure 4). The speed

of a high-side driver can be increased to better than 10µs

by bootstrapping the supply off of the MOSFET source.

This topology can be used where the load is pulse-width

modulated (100Hz to 20kHz), or where it is energized con-

tinuously. The Schottky barrier diode prevents the MIC5011

supply pin from dropping more than 200mV below the drain

supply, and it also improves turn-on time on supplies of less

than 10V. Since the supply current in the “off” state is only a

small leakage, the 100nF bypass capacitor tends to remain

charged for several seconds after the MIC5011 is turned

off. In a PWM application the chip supply is sustained at

a higher potential than the system supply, which improves

switching time.

July 2005

7

MIC5011

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