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The TPA005D02 Audio Power Amplifier Evaluation Module
3-13 Details
3.2.3.6 Control and Indicator Circuits
The shutdown circuit, the mute circuit, and the fault indicator circuit are all
low-current circuits and are not as critical in the layout design as the circuits
mentioned previously.
Shutdown
The shutdown control (IC pin 1) is activated by a logic low. Shutdown limits the
supply current of the TPA005D02 to 400
µ
A to conserve power in low power
applications.
The shutdown circuit (IC pin 1) consists of D1, Q1, R1, R2, R3, R4, C1, and
S2, a relatively large number of components. Since space and power
consumption are to be minimized, these components need to be small,
low-power devices. Switch S2 is actually the component that sends the device
into shutdown mode—there is no shutdown control input pin on the EVM. The
remaining components form a delay circuit that eliminates noise created by the
discharge of energy stored in the output filter when the amplifier IC is initially
placed in shutdown.
The PNP transistor needs to be biased so that it is operational from at least
4.5 V to 5.5 V. Zener diode D1 and resistors R1, R2, and R3 set the bias for
the transistor so that it is kept in its linear region and minimizes the supply
current used by this circuit. The maximum zener voltage Vz is then calculated
to be
Vz = V
DD(min)
–V
BE(min)
= 4.5V–0.6V = 3.9V
Zener diode D1 was selected based on Vz and the current required for
operation. Resistor R1 limits the current through the diode, while R2 also helps
limit the base current and isolate the bootstrap capacitor C1. Resistor R3 limits
the collector current during normal operation, and R4 limits the current flowing
to ground when switch S2 is closed.
Mute
The mute control (IC pin 2) is activated by a logic low, in which case the
amplifier low-side output transistors are turned on, shorting the load to ground.
The main consideration is to minimize the current used. A 100-k
Ω
pullup
resistor is used to limit the current to microamps and minimize the power
dissipated in the resistor, allowing a smaller surface-mount package to be
used.