TC426 datasheet(6/16 Pages) MICROCHIP

6 / 16 page

TC426/TC427/TC428

DS21415C-page 6

3.0

APPLICATIONS INFORMATION

3.1

Supply Bypassing

Charging and discharging large capacitive loads

quickly requires large currents. For example, charging

a 1000 pF load to 18V in 25 nsec requires an 0.72A

current from the device power supply.

To ensure low supply impedance over a wide frequency

range, a parallel capacitor combination is recom-

mended for supply bypassing. Low-inductance ceramic

disk capacitors with short lead lengths (< 0.5 in.) should

be used. A 1

μF film capacitor in parallel with one or two

0.1

μF ceramic disk capacitors normally provides

adequate bypassing.

3.2

Grounding

The TC426 and TC428 contain inverting drivers.

Ground potential drops developed in common ground

impedances from input to output will appear as

negative feedback and degrade switching speed

characteristics.

Individual ground returns for the input and output

circuits or a ground plane should be used.

3.3

Input Stage

The input voltage level changes the no-load or

quiescent supply current. The N-channel MOSFET

input stage transistor drives a 2.5 mA current source

load. With a logic ‘1’ input, the maximum quiescent

supply current is 8 mA. Logic ‘0’ input level signals

reduce quiescent current to 0.4 mA maximum.

Minimum power dissipation occurs for logic ‘0’ inputs

for the TC426/TC427/TC428. Unused driver inputs

must be connected to VDD or GND.

The drivers are designed with 100 mV of hysteresis.

This provides clean transitions and minimizes output

stage current spiking when changing states. Input

voltage thresholds are approximately 1.5V, making the

device TTL compatible over the 4.5V to 18V supply

operating range. Input current is less than 1

μA over

this range.

The TC426/TC427/TC428 may be directly driven by

the TL494, SG1526/1527, SG1524, SE5560, and

similar switch-mode power supply integrated circuits.

3.4

Power Dissipation

The supply current vs frequency and supply current

vs capacitive load characteristic curves will aid in

determining power dissipation calculations.

The TC426/TC427/TC428 CMOS drivers have greatly

reduced quiescent DC power consumption. Maximum

quiescent current is 8 mA compared to the DS0026 40

mA specification. For a 15V supply, power dissipation

is typically 40 mW.

Two other power dissipation components are:

• Output stage AC and DC load power.

• Transition state power.

Output stage power is:

Po = PDC + PAC

= Vo (IDC) + f CL VS

Where:

Vo = DC output voltage

IDC = DC output load current

= Switching frequency

Vs = Supply voltage

In power MOSFET drive applications the PDC term is

negligible. MOSFET power transistors are high-imped-

ance, capacitive input devices. In applications where

resistive loads or relays are driven, the PDC component

will normally dominate.

The magnitude of PAC is readily estimated for several

cases:

1. f

= 200 kHZ

1. f

= 200 kHz

2. CL =1000 pf

3. Vs

= 18V

3. Vs

= 15V

4. PAC = 65 mW

4. PAC = 45 mW

During output level state changes, a current surge will

flow through the series connected N and P channel

output MOSFETS as one device is turning “ON” while

the other is turning “OFF”. The current spike flows only

during output transitions. The input levels should not be

maintained between the logic ‘0’ and logic ‘1’ levels.

Unused driver inputs must be tied to ground and

not be allowed to float. Average power dissipation will

be reduced by minimizing input rise times. As shown in

the characteristic curves, average supply current is

frequency dependent.

TC426 Scheda tecnica(PDF) 6 Page - Microchip Technology

TC426 Scheda tecnica(HTML) 6 Page - Microchip Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Scheda tecnica Scarica

Link URL