Electrical Characteristics

USB2SER Data Sheet, Rev. 0

Freescale Semiconductor

15

3. Power supply must maintain regulation within operating VDD range during instantaneous and operating maximum current

conditions. If positive injection current (VIn > VDD) is greater than IDD, the injection current may flow out of VDD and can result

in the external power supply going out of regulation. Ensure external VDD load will shunt current greater than maximum

injection current. This will be the greatest risk when the MCU is not consuming power. Examples are: if no system clock is

present, or if the clock rate is very low which reduces overall power consumption.

A.3

Thermal Characteristics

This section provides information about operating temperature range, power dissipation, and package thermal resistance. Power

dissipation on I/O pins is usually small compared to the power dissipation in on-chip logic. It is user-determined rather than

being controlled by the MCU design. To take PI/O into account in power calculations, determine the difference between actual

1. Junction temperature is a function of die size, on-chip power dissipation, package thermal resistance, mounting site (board)

temperature, ambient temperature, air flow, power dissipation of other components on the board, and board thermal resistance

2. Junction to Ambient Natural Convection

3. 1s — Single layer board, one signal layer

4. 2s2p — Four layer board, 2 signal and 2 power layers

The average chip-junction temperature (TJ) in × ° C can be obtained from:

Eqn. 1

Where:

TA — Ambient temperature,° C

JA — Package thermal resistance, junction-to-ambient,° C/W

is:

Eqn. 2

Table A-3. Absolute maximum ratings

Rating

Symbol

Value

Unit

Operating temperature range (packaged)

T

A

T

-40 to 85

24-pin QFN

1s

2s2p



JA

92

33

T

j

T

A

P

D

Iq

JA

P

D





JA

+

=

PD

K

T

J

273

C

+





=