AN2475 Freescale Semiconductor / Motorola, AN2475 Datasheet - Page 5

• Manufacturer Part Number: AN2475
• Description: Generating a PWM Signal Modulated by an Analog Input Using the MC68HC908QY4 Microcontroller
• Manufacturer: Freescale Semiconductor / Motorola

Application Circuit
MOTOROLA
NOTE:
active channel to prevent writing a new value to the active channel. Writing
to the active channel registers is the same as generating unbuffered PWM
signals.
Since in this application the PWM duty cycle varies from 0 to 100% in
accordance with the voltage applied on the ADC, input buffered PWM allows
eliminating potentially harmful glitches when the pulse width setting is changed.
This is accomplished without requiring a synchronization method as in
unbuffered PWM signal generation.
Figure 1
MC68HC908QY4 outputs a PWM waveform used to control the PMOS switch
and, therefore the effective DC regulated voltage applied on the load. The MCU
reads the instantaneous regulated output voltage through the ADC and
performs a comparison between the digital word obtained and an internal
digital reference previously defined. The load voltage is acquired by the ADC
through a feedback path cyclically thus, forcing the voltage regulation. If the
load demands more current the regulator output voltage decreases. If the
voltage input to the ADC is below the pre-set internal reference value the duty
cycle is reduced (decremented), thus increasing the driver current of the
converter compensating for the reduced voltage supplied to the load. On the
other hand, if the load demands less current than the nominal value the MCU
grows the duty cycle reducing the output voltage. When regulation is attained
the green light emitting diode (LED) is turned on. Conversely, turning on the red
LED indicates lack of regulation.
modulated by an analog voltage signal applied on the ADC input whereas,
Figure 3
application circuit.
It is possible to achieve a regulated voltage of 2.5 V with less than 30 mV
variation driving a current load from 20 mA to 280 mA.
Regarding the application circuit in
capacitor create an energy storage reservoir to store enough energy to
maintain load voltage during the off-time of the transistor. The transistor serves
to replace lost energy during its off-time. Operation of the power switch can be
broken down into two periods.
Generating a PWM Signal Modulated by an Analog Input
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Freescale Semiconductor, Inc.
For More Information On This Product,
Using the MC68HC908QY4 Microcontroller
When the power switch is on, load current passes through the inductor
to the load and the diode will be reversed biased.
When the power switch turns off, current must still pass through the
inductor. The current path through the off transistor is now open. The
diode will be forward biased, thus maintaining a closed current loop
through the load.
shows a typical application circuit involving the PWM feature. The
shows the regulated output voltage versus the load current for the
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Figure 2
Figure 1
illustrates the PWM waveform
the diode, inductor, and shunt
Application Circuit
AN2475/D
5