IS31AP4991A Datasheet, PDF (8/11 Pages) Integrated Silicon Solution, Inc – 1.1W AUDIO POWER AMPLIFIER WITH ACTIVE-LOW SHUTDOWN MODE

IS31AP4991A
APPLICATION INFORMATION
BTL CONFIGURATION PRINCIPLE
The IS31AP4991A is a monolithic power amplifier with
a BTL output type. BTL (bridge tied load) means that
each end of the load is connected to two single-ended
output amplifiers. Thus, we have:
Single-ended output 1 = V_OUT+ = V_OUT (V)
Single ended output 2 = V_OUT- = -V_OUT (V)
and
V_OUT+ - V_OUT- = 2V_OUT (V)
The output power is:
P_OUT

(2V_{OUT RMS} )²
R_L
For the same power supply voltage, the output power
in BTL configuration is four times higher than the
output power in single ended configuration.
GAIN IN A TYPICAL APPLICATION SCHEMATIC
The typical application schematic is shown in Figure 1
on page 1.
In the flat region (no C_IN effect), the output voltage of
the first stage is (in Volts):
V_{OUT }

(V_IN )
R_F
R_IN
For the second stage: V_OUT+ = -V_OUT- (V)
The differential output voltage is (in Volts):
V_{OUT }
 V_{OUT }
 2V_IN
R_F
R_IN
The differential gain, G_V, is given by:
G_v
 ^{VOUT   VOUT }
V_IN
 2 ^RF
R_IN
V_OUT- is in phase with V_IN and V_OUT+ is phased 180°
with V_IN. This means that the positive terminal of the
loudspeaker should be connected to V_OUT+ and the
negative to V_OUT-.
LOW AND HIGH FREQUENCY RESPONSE
In the low frequency region, C_IN starts to have an effect.
C_IN forms with R_IN a high-pass filter with a -3dB cut-off
frequency. f_CL is in Hz.
f _CL

1
2R_IN C_IN
In the high frequency region, you can limit the
bandwidth by adding a capacitor (C_F) in parallel with R_F.
It forms a low-pass filter with a -3dB cut-off frequency.
f_CH is in Hz.
Integrated Silicon Solution, Inc. – www.issi.com
Rev. C, 05/13/2013
f _CH

1
2R_F C_F
DECOUPLING OF THE CIRCUIT
Two capacitors are needed to correctly bypass the
IS31AP4991A: a power supply bypass capacitor C_S
and a bias voltage bypass capacitor C_BYPASS.
C_S has particular influence on the THD+N in the high
frequency region (above 7kHz) and an indirect
influence on power supply disturbances. With a value
for C_S of 1μF, you can expect THD+N levels similar to
those shown in the datasheet.
In the high frequency region, if C_S is lower than 1μF, it
increases THD+N and disturbances on the power
supply rail are less filtered.
On the other hand, if C_S is higher than 1μF, those
disturbances on the power supply rail are more filtered.
C_BYPASS has an influence on THD+N at lower
frequencies, but its function is critical to the final result
of PSRR (with input grounded and in the lower
frequency region).
If C_BYPASS is lower than 0.47μF, THD+N increases at
lower frequencies and PSRR worsens.
If C_BYPASS is higher than 0.47μF, the benefit on THD+N
at lower frequencies is small, but the benefit to PSRR
is substantial.
Note that C_IN has a non-negligible effect on PSRR at
lower frequencies. The lower the value of C_IN, the
higher the PSRR is.
WAKE-UP TIME (t_WU)
When the standby is released to put the device on, the
bypass capacitor C_BYPASS will not be charged
immediately. As C_BYPASS is directly linked to the bias of
the amplifier, the bias will not work properly until the
C_BYPASS voltage is correct. The time to reach this
voltage is called wake-up time or t_WU and specified in
the electrical characteristics table with C_BYPASS =
0.47μF.
POP PERFORMANCE
Pop performance is intimately linked with the size of
the input capacitor C_IN and the bias voltage bypass
capacitor C_BYPASS.
The size of C_IN is dependent on the lower cut-off
frequency and PSRR values requested. The size of
C_BYPASS is dependent on THD+N and PSRR values
requested at lower frequencies.
Moreover, C_BYPASS determines the speed with which
the amplifier turns on.
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