MIC4420 datasheet(11/26 Pages) MICROCHIP

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DS20006092B-page 11

MIC4420/9

4.0

APPLICATION INFORMATION

4.1

Supply Bypassing

Charging and discharging large capacitive loads

quickly requires large currents. For example, charging

a 2500 pF load to 18V in 25 ns requires a 1.8A current

from the device power supply.

The MIC4420/4429 has 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 guarantee low supply impedance over a wide

frequency range, a parallel capacitor combination is

recommended for supply bypassing. Low inductance

ceramic disk capacitors with short lead lengths (less

than 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 MIC4420/4429

demands careful PC board layout for best performance

Because the MIC4429 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 MIC4429 input structure includes 300 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

MIC4429 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 MIC4429

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 MIC4429 GND pins will ensure full logic drive to the

input and ensure fast output switching. Both of the

MIC4429 GND pins should, however, still be connected

to power ground.

FIGURE 4-1:

Self-Contained Voltage Doubler.

4.3

Input Stage

The input voltage level of the 4429 changes the

quiescent supply current. The N channel MOSFET

input stage transistor drives a 450 μA current source

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

supply current is 450 μA. Logic “0” input level signals

reduce quiescent current to 55 μA maximum.

The MIC4420/4429 input is designed to provide

300 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 4.5V to 18V operating

supply voltage range. Input current is less than 10 μA

over this range.

The MIC4429 can be directly driven by the TL494,

SG1526/1527, SG1524, TSC170, MIC38HC42, and

similar switch mode power supply integrated circuits.

By offloading the power-driving duties to the

MIC4429

1μF

50V

MKS2

UNITED CHEMCON SXE

0.1μF

WIMA

MKS2

6, 7

0.1μF

50V

5.6 k

560

+15

220 μF 50V

BYV 10 (x 2)

35 μF 50V

(x2) 1N4448

100 120 140

LINE

Output Voltage vs.

Load Current

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