MIC4426/4427/4428

Micrel

MIC4426/4427/4428

6

September 1999

Applications Information

Supply Bypassing

Large currents are required to charge and discharge large

capacitive loads quickly. For example, changing a 1000pF

load by 16V in 25ns requires 0.8A from the supply input.

To guarantee low supply impedance over a wide frequency

range, parallel capacitors are recommended for power sup-

ply bypassing. Low-inductance ceramic MLC capacitors with

short lead lengths (< 0.5") should be used. A 1.0

µF film

capacitor in parallel with one or two 0.1

µF ceramic MLC

capacitors normally provides adequate bypassing.

Grounding

When using the inverting drivers in the MIC4426 or MIC4428,

individual ground returns for the input and output circuits or a

ground plane are recommended for optimum switching speed.

The voltage drop that occurs between the driver’s ground and

the input signal ground, during normal high-current switching,

will behave as negative feedback and degrade switching

speed.

Control Input

Unused driver inputs must be connected to logic high (which

can be V

(< 500

µA) , connect unused inputs to ground. A logic-high

signal will cause the driver to draw up to 9mA.

The drivers are designed with 100mV of control input hyster-

esis. This provides clean transitions and minimizes output

stage current spikes when changing states. The control input

voltage threshold is approximately 1.5V. The control input

recognizes 1.5V up to V

1

µA within this range.

The MIC4426/7/8 drives the TL494, SG1526/7, MIC38C42,

TSC170 and similar switch-mode power supply integrated

circuits.

Power Dissipation

Power dissipation should be calculated to make sure that the

driver is not operated beyond its thermal ratings. Quiescent

power dissipation is negligible. A practical value for total

power dissipation is the sum of the dissipation caused by the

load and the transition power dissipation (P

Load Dissipation

Power dissipation caused by continuous load current (when

driving a resistive load) through the driver’s output resistance

is:

P

O

For capacitive loads, the dissipation in the driver is:

P

2

Transition Dissipation

In applications switching at a high frequency, transition power

dissipation can be significant. This occurs during switching

transitions when the P-channel and N-channel output FETs

are both conducting for the brief moment when one is turning

on and the other is turning off.

P

Charge (Q) is read from the following graph:

1

8

4

3

2

6

1

4

6

8

10

1214

1618

SUPPLY VOLTAGE (V)

Crossover Energy Loss per Transition