DIGITAL CONTROL Without Programming
5 10 15
20 25 30 35 40 45 50
Figure 5. PWM drive waveforms for constant 5 V output with
12 V and 15 V supplies.
Figure 4 illustrates a power amplifier that sources or
sinks current while maintaining constant output voltage at
twice (1+2 K/2 K) the analog setpoint (Vset). This circuit
is a switching converter that behaves like a low frequency
( 100 Hz) high power ( 10 V/5 A) op-amp. It requires few
parts that are inherently robust because critical functions
are integrated, including digital signal processing, power
switching, and thermal shutdown.
The half-bridge driver TDA21201 converts PWM
logic levels to a high power pulse train that is averaged to
a DC level by the output LC filter, as shown in Figure 5.
The LC response is fast and susceptible to ringing, so
high speed sampling and control calculations are
required. For this reason, the 128 µS control setting
is used despite the 214 µS time constant of the system
(τ = LC1/2).
Figure 5 illustrates that more drive (higher duty cycle)
is required at a lower supply voltage ( 12 V versus 15 V) to
maintain constant output voltage.
Figure 6 illustrates a thermal controller that uses a
Thermo-Electric-Cooler (TEC) or Peltier Cell to heat
(PWM>50%) or cool (PWM<50%), depending on current
direction through full-bridge driver LMD18201. This circuit
is useful for applications requiring variable temperatures
that include ambient. The full-bridge has two outputs that
switch out-of-phase; one is low while the other is high. Both
filtered outputs are equal at half the input voltage when
NUTS & VOLTS
Circle #111 on the Reader Service Card.
Circle #70 on the Reader Service Card.