April 1998

5-77

MIC4451/4452

Micrel

5

Table 1: MIC4451 Maximum

Operating Frequency

Max Frequency

18V

220kHz

15V

300kHz

10V

640kHz

5V

2MHz

Conditions: 1.

Input Stage

The input voltage level of the MIC4451 changes the quies-

cent 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 MIC4451/4452 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 approxi-

mately 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.

will flow into the input lead. The input currents can be as high

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 1k

Ω 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.

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:

Calculation of load power dissipation differs depending on

whether the load is capacitive, resistive or inductive.

Resistive Load Power Dissipation

Dissipation caused by a resistive load can be calculated as:

where:

I =

the current drawn by the load

high, at the power supply voltage used. (See data

sheet)

D =

fraction of time the load is conducting (duty cycle)

Capacitive Load Power Dissipation

Dissipation caused by a capacitive load is simply the energy

placed in, or removed from, the load capacitance by the

Figure 5. Switching Time Degradation Due to

Negative Feedback

MIC4451

1

8

6, 7

5

4

+18

0.1

µF

0.1

µF

TEK CURRENT

PROBE 6302

2,500 pF

POLYCARBONATE

5.0V

0 V

18 V

0 V

300 mV

12 AMPS

PC TRACE RESISTANCE = 0.05

Ω

LOGIC

GROUND

POWER

GROUND

WIMA

MKS-2

1

µF