MIC4451YN datasheet(10/24 Pages) MICROCHIP

10 / 24 page

MIC4451/52

DS20006616A-page 10



2021 Microchip Technology Inc. and its subsidiaries

4.0

APPLICATIONS INFORMATION

4.1

Supply Bypassing

Charging and discharging large capacitive loads

quickly requires large currents. For example, changing

a 10,000 pF load to 18V in 50 ns requires 3.6A.

The MIC4451 and MIC4452 have double bonding on

the supply pins, the ground pins, and output pins. This

reduces parasitic lead inductance. Low inductance

enables large currents to be switched rapidly. It also

reduces internal ringing that can cause voltage

breakdown when the driver is operated at or near the

maximum rated voltage.

Internal ringing can also cause output oscillation due to

feedback. This feedback is added to the input signal

because it is referenced to the same ground.

To ensure low supply impedance over a wide frequency

range,

parallel

capacitor

combination

recommended for supply bypassing. Low inductance

ceramic disc capacitors with short lead lengths

(<0.5 inch) should be used. A 1 µF low ESR film

capacitor in parallel with two 0.1 µF low ESR ceramic

capacitors, (such as AVX RAM GUARD®), provides

adequate bypassing. Connect one ceramic capacitor

directly between pins 1 and 4. Connect the second

ceramic capacitor directly between pins 8 and 5.

4.2

Grounding

The high current capability of the MIC4451/52

demands careful PC board layout for best

performance. Because the MIC4451 is an inverting

driver, any ground lead impedance will appear as

negative feedback which can degrade switching

speed. Feedback is especially noticeable with slow rise

time inputs. The MIC4451 input structure includes

200 mV of hysteresis to ensure clean transitions and

freedom from oscillation, but attention to layout is still

recommended.

Figure 4-1 shows the feedback effect in detail. As the

MIC4451 input begins to go positive, the output goes

negative and several amperes of current flow in the

ground lead. As little as 0.05Ω of PC trace resistance

can produce hundreds of millivolts at the MIC4451

ground pins. If the driving logic is referenced to power

ground, the effective logic input level is reduced and

oscillation may result.

To ensure optimum performance, separate ground

traces should be provided for the logic and power

connections. Connecting the logic ground directly to

the MIC4451 GND pins will ensure full logic drive to the

input and ensure fast output switching. Both of the

MIC4451 GND pins should, however, still be connected

to power ground.

4.3

Input Stage

The input voltage level of the MIC4451 changes the

quiescent supply current. The N-channel MOSFET

input stage transistor drives a 320 µA current source

load. With a logic “1” input, the maximum quiescent

supply current is 400 µA. Logic “0” input level signals

reduce quiescent current to 80 µA typical.

The MIC4451/52 input is designed to provide 200 mV

of hysteresis. This provides clean transitions, reduces

noise sensitivity, and minimizes output stage current

spiking when changing states. Input voltage threshold

level is approximately 1.5V, making the device

TTL-compatible over the full temperature and operating

supply voltage ranges. Input current is less than

±10 µA.

The MIC4451 can be directly driven by the TL494,

SG1526/1527, SG1524, TSC170, MIC38C42, and

similar switch mode power supply integrated circuits.

By offloading the power-driving duties to the

MIC4451/52, the power supply controller can operate

at lower dissipation. This can improve performance and

reliability.

The input can be greater than the VS supply, however,

current will flow into the input lead. The input currents

can be as high as 30 mA peak-to-peak (6.4 mARMS)

with the input. No damage will occur to MIC4451/52,

however, and it will not latch.

The input appears as a 7 pF capacitance and does not

change even if the input is driven from an AC source.

While the device will operate and no damage will occur

up to 25V below the negative rail, input current will

increase up to 1 mA/V due to the clamping action of the

input, ESD diode, and 1 kΩ resistor.

4.4

Power Dissipation

CMOS circuits usually permit the user to ignore power

dissipation. Logic families, such as 4000 and 74C,

have outputs that can only supply a few milliamperes of

current, and even shorting outputs to ground will not

force enough current to destroy the device. The

MIC4451/52 on the other hand, can source or sink

several amperes and drive large capacitive loads at

high frequency. The package power dissipation limit

can easily be exceeded. Therefore, some attention

should be given to power dissipation when driving low

impedance loads and/or operating at high frequency.

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