MIC4451_11 datasheet(9/14 Pages) MICREL

9 / 14 page

Micrel Inc.

MIC4451/4452

January 2011

M9999-011811

Input Stage

The input voltage level of the MIC4451 changes the

quiescent supply current. The N channel MOSFET input

stage transistor drives a 320µA current source load. With

a logic “1” input, the maximum quiescent supply current

is 400µA. Logic “0” input level signals reduce quiescent

current to 80µA typical.

The MIC4451/4452 input is designed to provide 200mV

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 ±10µA.

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 MIC4451/4452,

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 30mA p-p (6.4mARMS) with the input.

No damage will occur to MIC4451/4452 however, and it

will not latch.

The input appears as a 7pF 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 1mA/V due to the clamping action of the

input, ESD diode, and 1kΩ resistor.

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

MIC4451/4452 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.

MIC4451

6, 7

+18

0.1µF

TEK CURRENT

PROBE 6302

2,500 pF

POLYCARBONATE

5.0V

0 V

18 V

0 V

300 mV

12 AMPS

PC TRACE RESISTANCE = 0.05Ω

LOGIC

GROUND

POWER

GROUND

WIMA

MKS-2

1µF

Figure 5. Switching Time Degradation Due to Negative

Feedback

The supply current vs. frequency and supply current vs

capacitive load characteristic curves aid in determining

power dissipation calculations. Table 1 lists the

maximum safe operating frequency for several power

supply voltages when driving a 10,000pF 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 plastic DIP

package, from the data sheet, is 130°C/W. In a 25°C

ambient, then, using a maximum junction temperature of

125°C, this package will dissipate 960mW.

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.

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