MIC4420 datasheet(12/26 Pages) MICROCHIP

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MIC4420/9

DS20006092B-page 12



2018 - 2022 Microchip Technology Inc. and its subsidiaries.

MIC4420/4429, 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 propagation

delay for tD2 will increase to as much as 400 ns at room

temperature. The input currents can be as high as

30 mA peak-to-peak (6.4 mARMS) with the input, 6 V

greater than the supply voltage. No damage will occur

to MIC4420/4429 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.

Care should be taken so that the input does not go

more than 5 volts below the negative rail.

FIGURE 4-2:

Switching Time Degradation

Due to Negative Feedback.

4.4

Power Dissipation

CMOS circuits usually permit the user to ignore power

dissipation. Logic families such as 4000 and 74C have

outputs which can only supply a few milliamperes of

current, and even shorting outputs to ground will not

force enough current to destroy the device. The

MIC4420/4429 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.

The supply current vs. frequency and supply current vs.

capacitive load characteristic curves aid in determining

power dissipation calculations. Table 4-1 lists the

maximum safe operating frequency for several power

supply voltages when driving a 2500 pF load. More

accurate power dissipation figures can be obtained by

summing the three dissipation sources.

Given the power dissipation in the device, and the

thermal resistance of the package, junction operating

temperature for any ambient is easy to calculate. For

example, the thermal resistance of the 8-pin MSOP

package, from the data sheet, is 250°C/W. In a 25°C

ambient, then, using a maximum junction temperature

of 150°C, this package will dissipate 500 mW.

Accurate power dissipation numbers can be obtained

by summing the three sources of power dissipation in

the device:

• Load power dissipation (PL)

• Quiescent power dissipation (PQ)

• Transition power dissipation (PT)

Calculation of load power dissipation differs depending

on whether the load is capacitive, resistive or inductive.

4.4.1

RESISTIVE LOAD POWER

DISSIPATION

Dissipation caused by a resistive load can be

calculated as:

EQUATION 4-1:

4.4.2

CAPACITIVE LOAD DISSIPATION

Dissipation caused by a capacitive load is simply the

energy placed in, or removed from, the load

capacitance by the driver. The energy stored in a

capacitor is described by Equation 4-2:

EQUATION 4-2:

MIC4429

6, 7

+18 V

0.1μF

TEK CURRENT

PROBE 6302

2,500 pF

POLYCARBONATE

5.0V

0 V

18 V

0 V

WIMA

MKS-2

1 μF

LOGIC

GROUND

POWER

GROUND

6 AMPS

300 mV

TABLE 4-1:

MIC4429 MAX. OPERATING

FREQUENCY

Maximum Frequency

18V

500 kHz

15V

700 kHz

10V

1.6 MHz

Note 1: Conditions: DIP package (θJA =

130°C/W), TA = 25°C, CL = 2500 pF.

PL I

2 R



Where:

= The current drawn by the load.

= The output resistance of the driver

when the output is high, at the power

supply voltage used.

= Fraction of the time the load is

conducting (duty cycle).

1/2C V2



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