IEEE Solid-State Circuits Magazine - Summer 2017 - 21
Ref
Ramp
VIN
RIN
Loop
Filter
RAUX
Power
Driver
PWM
Gen
RSWP
PWM
GEN
RSPK
RSWN
PWMOUT
FIGURE 19: Programmable driver strength.
Aux
Driver
VGS
RFDBK
RON~1/
(VGS - VT - VDS)
FIGURE 18: A class D amplifier with an auxiliary loop.
FIGURE 20: Switch-on resistance control.
injected in the loop. The loop filter
output includes a large signal swing
when the input signal is large. This
not only increases the current consumption of the amplifier to support
the large output swing but also limits
the error correction range before the
loop is saturated. For the loop to sufficiently suppress the battery disturbance in the mobile system, the signal
level, namely the output power, has to
be limited to keep low distortion.
Furthermore, the fast switching time
and nearly rail-to-rail signal swing of
class D amplifiers result in both high
efficiency and troublesome electromagnetic interference, or the so-called
EMI. In addition, the large switching
current through the speaker coil coupling with the parasitic inductance
from the wire-bond package tends to
induce large overshoots and undershoots at the amplifier output. To
make the device operate reliably, conventional designs reduce the output
signal swings by an amount equivalent to the overshoot or undershoot.
As a result, existing class D amplifiers
with a wire-bond package have very
limited output power capability.
An effective EMI reduction approach
[6] is shown in Figure 17 with the
detailed implementation of the half
H-bridge and predriver circuit. The
output switching edge rate is controlled
by the power switch gate voltage. And
the switch gate voltage is provided
by the predriver, which consists of a
chain degenerated digital gates. Nonoverlapping voltage was provided
During power up, the strength of
the power driver gradually increases
until it reaches its maximum, while
the strength of the auxiliary driver
gradually reduces to zero. This provides a smooth handoff between the
auxiliary and main feedback loops.
When the loop has settled, the amplifier offset is reflected by the PWM. As a
result, both the PWM pulse amplitude
and amplifier offset are ramped simultaneously. The power-down procedure is
reversed with respect to the ramping process during power up. Figure 19 shows
a pair of PMOS and NMOS switches
connecting the speaker to the battery and the ground. Even though
the speaker is directly connected to
the battery, the voltage across the
speaker is attenuated by a factor of
Rspk/(Rswp+Rspk+Rswn), where Rspk
is the speaker resistance and Rswp
and Rswn are the on-resistance for
the P-switch and the N-switch. A weak
driving strength implies large switch
on-resistance. To ramp the drive
strength, the switch on-resistance can
be ramped. Figure 20 conceptually
shows a PMOS switch and its gate driving voltage. As the formula indicates,
the switching on-resistance is controlled by the gate voltage. The switch
gate driver has added source impedance, which slows down the transition. For a narrow pulse input to the
predriver, the output pulse amplitude
is limited due to the slow transition.
Figure 21 shows the detailed implementation of ramping the driver
strength. The right side of the figure
for P and N switches such that they
will not be turned on simultaneously
at any given time. The nonoverlapping time was generated to track the
slow transition edges. The nonoverlapping time must be larger than the
rising/falling time to avoid crowbar
current. However, efficiency and linearity may degrade with increased
edge rates. Design tradeoffs among
efficiency, linearity, and EMI reduction determine the optimal edge rate.
Another unpleasant user experience is the pop/click noise [6] associated with the class D amplifier. The
pop/click noise is caused by the unpredictable transient during power up/
power down due to abrupt changes of
the PWM and the dc offset. To effectively suppress the pop/click noise,
an auxiliary loop [6], as shown in Figure 18, is introduced. During power up,
the auxiliary loop helps establish the
feedback loop. The power driver has
the weakest driving strength, which
effectively disconnects the speaker
from amplifier. So the speaker does
not sense any uncomfortable transients. The driving strength of both
the auxiliary and power drivers is programmable. Their driving strengths
are ramped oppositely during power
up and power down. At the beginning
of power up, the power driver starts
with zero drive strength. At the same
time, the auxiliary driver is at full
drive strength.
The auxiliary path enables the
loop to settle without affecting the
output voltage across the speaker.
IEEE SOLID-STATE CIRCUITS MAGAZINE
SU M M E R 2 0 17
21
Table of Contents for the Digital Edition of IEEE Solid-State Circuits Magazine - Summer 2017
IEEE Solid-State Circuits Magazine - Summer 2017 - Cover1
IEEE Solid-State Circuits Magazine - Summer 2017 - Cover2
IEEE Solid-State Circuits Magazine - Summer 2017 - 1
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