AN2434 Freescale Semiconductor / Motorola, AN2434 Datasheet - Page 17

• Manufacturer Part Number: AN2434
• Description: Input/Output (I/O) Pin Drivers
• Manufacturer: Freescale Semiconductor / Motorola

Inductive Loading
Inductive Loading
When driving a load such as an inductor or capacitor that might result in voltages beyond V
or below
DD
V
, one should take care to provide external circuitry (such as Schottky diodes), which will protect both
SS
the MCU and the internal ESD diodes. Schottky diodes are suggested because they have a faster
switching time and a lower forward voltage (of approximately 300 mV) than the internal ESD diodes
(which have a forward voltage of approximately 700 mV). Using external Schottky diodes will ensure the
external diodes conduct before the internal diodes thus protecting the MCU from damage as a result of
reverse EMF or back EMF (electro-motive force).
The example waveforms
(Figure 15
and
Figure
16) show the results of driving a 5-V relay with and without
an external Schottky diode. The waveform is a square wave which switches between +5 Vdc and 0 Vdc.
The falling edge of the waveform shows an overshoot, which is a result of the back EMF being generated
when current flowing through the relay coil is interrupted. The coil attempts to preserve the flow of current
and in doing so causes a negative voltage to be generated briefly. (The relay coil impedance is 250 Ω and
740 µH.)
INDUCTIVE SPIKE
INDUCTIVE SPIKE
Figure 15. Inductive Spike
Back EMF Clamped by Internal I/O Diodes
The back EMF is clamped in
Figure 15
by the ESD protection diode internal to the MCU pin. After applying
a reverse-biased Schottky diode from the pin to ground, the effect of reduced back EMF amplitude can
be seen in the
Figure
16.
Input/Output (I/O) Pin Drivers, Rev. 0
Freescale Semiconductor
17