IRS2092 and IRS2092S functional description

Application Note AN-1138
IRS2092(S) Functional Description
By Jun Honda, Xiao-chang Cheng, Wenduo Liu
Table of Contents
Page
General Description................................................................................................. 1
Typical Implementation............................................................................................ 1
PWM Modulator....................................................................................................... 3
MOSFET Selection.................................................................................................. 6
Protection Design .................................................................................................... 7
Deadtime Generator.............................................................................................. 12
Power Supply ........................................................................................................ 14
Junction Temperature Estimation.......................................................................... 15
Board Layout Considerations ................................................................................ 15
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AN-1138
IRS2092(S) General Description
Input Section
The IRS2092(S) is a Class D audio amplifier driver
with integrated PWM modulator and over current
protection. Combined with two external MOSFETs
and a few external components, the IRS2092(S)
forms a complete Class D amplifier with dual over
current, and shoot-through protection, as well as
UVLO protection for the three bias supplies. The
versatile structure of the analog input section with
an error amplifier and a PWM comparator has the
flexibility of implementing different types of PWM
modulator schemes.
Loss-less current sensing utilizes RDS(on) of the
MOSFETs. The protection control logic monitors the
status of the power supplies and load current across
each MOSFET.
For the convenience of half bridge configuration, the
analog PWM modulator and protection logic are
constructed on a floating well.
The IRS2092(S) implements start-up click noise
elimination to suppress unwanted audible noise
during PWM start-up and shut-down.
The audio input stage of IRS2092(S) is configured
as an inverting error amplifier.
In Figure 2, the voltage gain of the amplifier GV is
determined by input resistor RIN and feedback
resistor RFB.
GV =
RFB
RIN
Since the feedback resistor RFB is part of an
integrator time constant, which determines switching
frequency, changing overall voltage gain by RIN is
simpler and, therefore, recommended in most
cases.
Note that the input impedance of the amplifier is
equal to the input resistor RIN.
A DC blocking capacitor C3 should be connected in
series with RIN to minimize DC offset in the output. A
ceramic capacitor is not recommended due to
potential distortion. Minimizing DC offset is essential
for audible noise-less Turn-ON and -OFF.
Typical Implementation
The connection of the non-inverting input IN+ is a
reference for the error amplifier, and thus is crucial
for audio performance. Connect IN+ to the signal
reference ground in the system, which has same
potential as the negative terminal of the speaker
output.
The following explanations are based on a typical
application circuit with self-oscillating PWM topology
shown in Figure 1. For further information, refer to
the IRAUDAMP5 reference design.
47 kΩ
2.7 kΩ
+B
BAV19WS
VAA
CSH
16
2
GND
VB
15
3.3 kΩ 2.2 nF
Vin
IRF6645
10 kΩ
IN-
HO
14
4
COMP
VS
13
5
CSD
VCC
12
3
10 µF
33 kΩ
22 µF
150
1 nF 2.2 nF
10 µF
10 µF
6
VSS
7
VREF
LO
11
COM
10
DT
9
22 µH
IRF6645
10 µF
10 Ω
3.3 kΩ
8.2 kΩ
8
2.7 kΩ
1.2 kΩ
OCSET
IRS2092
35 V
MURS120
10 Ω
4.7 Ω
10 µF
1
Speaker
0.1 µF 0.47 µF
4Ω
1Ω
0.1 µF
Vcc
12 V
35 V
8.2 kΩ
-B
Figure 1 IRS2092(S) Typical Application Circuit
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AN-1138
2
C1
C2
Cc
R1
COMP
Gate Driver
Vin
C3
RIN
COMP
INGND
PWM
+
RFB
Protection
Figure 2 IRS2092(S) Typical Control Loop Design
OTA
Self-Oscillating Frequency
The front end error amplifier of the IRS2092(S)
features an operational trans-conductance amplifier
(OTA), which is carefully designed to obtain optimal
audio performance. The OTA outputs a current
output to the COMP pin, unlike a voltage output in
an operational amplifier (OPA). The non-inverting
input is internally tied to the GND pin.
The inverting input has clamping diodes to GND to
improve recovery from clipping as well as ensuring
stable start up. The OTA output COMP is internally
connected to the PWM comparator whose threshold
is (VAA-VSS)/2.
For stable operation of the OTA, a compensation
capacitor Cc minimum of 1nF is required.
The OTA is shut off when VCSD<Vth2.
Self oscillating frequency is determined mainly
by the following items in Figure 2.
·
·
·
·
·
Integration capacitors, C1 and C2
Integration resistor, R1
Propagation delay in the gate driver
Feedback resistor, RFB
Duty cycle
Self oscillating frequency has little influences
from bus voltage and input resistance RIN.
Note that as is the nature of a self-oscillating
PWM, the switching frequency decreases as
PWM modulation deviates from idling.
Determining Self-Oscillating Frequency
Choosing switching frequency entails making a
trade off between many aspects.
PWM Modulator
The IRS2092(S) allows the user to choose from
numerous ways of PWM modulator
implementations. In this section, all the
explanations are based on a typical application
circuit of a self oscillating PWM.
Self-Oscillating PWM Modulator Design
The typical application features self oscillating
PWM scheme. For better audio performance,
2nd order integration in the front end is chosen.
At lower switching frequency, the efficiency at
MOSFET stage improves, but inductor ripple
current increases. The output carrier leakage
increases.
At higher switching frequency, the efficiency
degrades due to switching loss, but wider
bandwidth can be achieved. The inductor ripple
decreases yet iron loss increases. The junction
temperature of gate driver IC might be a stopper
for going higher frequency.
For these reasons, 400kHz is chosen for a
typical design example, which can be seen in
the IRAUDAMP5 reference design.
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AN-1138
3
Choosing External Components Value
EXT. CLK
RCK
CCK
C1
C2
R1
For suggested values of components for a given
target self oscillating frequency, refer to Table 1.
COMP
Vin
RIN
The OTA output has limited voltage and current
compliances. These sets of components values
are to ensure that OTA operates within its linear
region so optimal THD+N performance can be
achieved.
In case target frequency is somewhere in
between the frequencies listed in the Table 1,
adjust the frequency by tweaking R1, if
necessary.
INGND
+
RFB
Figure 3 External Clock Sync
Figure 4 shows how self-oscillating frequency locks
up to an external clock frequency.
600
500
C1=C2
(nF)
2.2
2.2
2.2
2.2
2.2
2.2
4.7
10
10
22
Operating Frequency (kHz)
Target SelfOscillation
Frequency
(kHz)
500
450
400
350
300
250
200
150
100
70
R1
(ohms)
200
165
141
124
115
102
41.2
20.0
14.0
4.42
300
200
100
0
10%
20%
30%
40%
50%
60%
70%
80%
90%
Duty Cycle
Figure 4 Typical Lock Range to External Clock
Click Noise Elimination
The IRS2092(S) has a unique feature that
minimizes Turn-ON and -OFF audible click noise.
When CSD is in between Vth1 and Vth2 during start
up, an internal closed loop around the OTA enables
an oscillation that generates voltages at COMP and
IN-, bringing them to steady state values. It runs at
around 1MHz, independent from the switching
oscillation.
Condition:IRS2092 with IRFB4212, Vbus=+/-35V, DT=25ns,
RFB=47k.
Table 1 External Component Values vs. Self
Oscillation Frequency
Clock Synchronization
In the typical PWM control loop design, the selfoscillating frequency can be set and
synchronized to an external clock. Through a
set of resistor and a capacitor, the external
clock injects periodic pulsating charges into the
integrator, forcing oscillation to lock up to the
external clock frequency. Typical setup with
5Vp-p 50% duty clock signal uses RCK=22k and
CCK=33pF in Figure 3. To maximize audio
performance, the self running frequency without
clock injection should be 20 to 30% higher than
the external clock frequency.
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400
AN-1138
C1
C2
Cc
R1
COMP
C3
RIN
COMP
INGND
PWM
+
Vin
RFB
Start-up
Figure 5 Click Noise Elimination
4
As a result, all capacitive components connected to
COMP and IN- pins, such as C1, C2, C3 and Cc in
Figure 5, are pre-charged to their steady state
values during the star up sequence. This allows
instant settling of PWM operation.
VCSD
VAA
Vth1
To utilize the click noise reduction function, following
conditions must be met.
1. CSD pin has slow enough ramp up from
Vth1 to Vth2 such that the voltages in the
capacitors can settle to their target values.
2. High side bootstrap power supply needs to
be charged up prior to starting oscillation.
3. Audio input has to be zero.
4. For internal local loop to override external
feedback during the startup period, DC
offset at speaker output prior to shutdown
release has to satisfy the following
condition.
DCoffset < 30 mA × RFB
Vth2
OTA Operational Mode
Shutdown
OTA in Active
Gate Driver Stage
Release
Shutdown
Figure 6 VCSD and OTA Mode
Self-oscillation Start-up Condition
The IRS2092(S) requires the following
conditions to be met to start PWM oscillation in
the typical application circuit.
CSD Voltage and OTA Operational Mode
The CSD pin determines the operational mode of
the IRS2092(S). The OTA has three operational
modes; cut off, local oscillation and normal
operation while the gate driver section has two
modes; normal and shutdown with CSD voltage.
-
-
When VCSD < Vth2, the IC is in shutdown mode and
the OTA is cut off.
When Vth2< VCSD < Vth1, the HO and LO outputs
are still in shutdown mode. The OTA is activated
and starts local oscillation, which pre-biases all the
capacitive components in the error amplifier.
When VCSD>Vth1, shutdown is released and PWM
operation starts.
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Pop-less
Startup
AN-1138
All the control power supplies, VAA,
VSS, VCC and VBS are above the
under voltage lockout thresholds.
CSD pin voltage is over Vth1 threshold.
iIN < iFB
Where iIN =
VIN
V
, iFB = + B .
RFB
RIN
Note that this condition also limits the maximum
audio input voltage feeding into R1. If this
condition is exceeded, the amplifier stops its
oscillation during the operation period. This
allows a 100% modulation index; however, a
care should be taken so that the high side
floating supply does not decay due to a lack of
low side pulse ON state.
5
MOSFET Selection
There are a couple of limitations on size of MOSFET
to be combined with the IRS2092(S).
1. Power dissipation
Power dissipation from gate driver stage in the
IRS2092(S) is proportional to switching
frequency and gate charge of MOSFET. Higher
the switching frequency the lower the gate
charge that can be used.
Refer to Junction Temperature Estimation later
in this application note for details.
2. Switching Speed
Internal over current protection has a certain
time window to measure the output current. If
switching transition takes too long, the internal
OCP circuitry starts monitoring voltage across
the MOSFET that induces false triggering of
OCP. Less than 40nC of gate charge per output
is recommended.
The IRS2092(S) accommodates a range of IR
Digital Audio MOSFETs, providing a scalable design
for various output power levels. For further
information on MOSFET section, refer to AN-1070,
Class D Amplifier Performance Relationship to
MOSFET Parameters.
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AN-1138
6
Protection Design
2. The CSD pin starts discharging the external
capacitor Ct.
3. When VCSD, the voltage across Ct, falls
below the lower threshold Vth2, an output
signal from COMP2 resets OCL.
4. The CSD pin starts charging the external
capacitor Ct.
5. When VCSD goes above the upper threshold
Vth1, the logic on COMP1 flips and the IC
resumes operation.
Over Current Protection (OCP)
The IRS2092(S) features over current protection to
protect the power MOSFETs during abnormal load
conditions. The IRS2092(S) starts a sequence of
events when it detects an over current condition
during either high side or low side turn on of a pulse.
As soon as either the high side or low side current
sensing block detects over current:
1. The OC Latch (OCL) flips logic states and
shutdowns the outputs LO and HO.
As long as the over current condition exists, the IC
will repeat the over current protection sequence at a
repetition rate dependent upon capacitance in CSD
pin.
VAA
Vth1
VCSD
Vth2
VSS
tOCL / t OCH
OC detection
Charge
CSD Capacitor
Discharge
Shutdown
SD
Release
t SU
Power on mute
tRESET
Normal operation
Protection
reset interval
Normal operation
Figure 7 Over Current Protection Timing Chart
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AN-1138
7
VAA
`
Vth1
COMP1
CSD
OC
S
Q
UVLO(VB)
`
COMP2
Ct
R
OC DET (H)
Vth2
VSS
HV
LEVEL
SHIFT
HV
LEVEL
SHIFT
FLOATING INPUT
HV
LEVEL
SHIFT
FLOATING HIGH SIDE
LOW SIDE
OC DET (L)
UVLO(VCC)
SD
PWM
HO
DEAD TIME
`
LO
Figure 8 Shutdown Functional Block Diagram
Protection Control
The internal protection control block dictates the
operational mode, normal or shutdown, using the
input of the CSD pin. In shutdown mode, the IC
forces LO and HO to output 0V with respect to COM
and VS respectively to turn off the power MOSFETs.
1
VAA
CSH
16
2
GND
VB
15
3
IN-
HO
14
4
COMP
VS
13
5
CSD
VCC
12
6
VSS
LO
11
7
VREF
COM
10
8
OCSET
DT
9
Ct
The CSD pin provides five functions.
1. Power up delay timer
2. Self-reset timer
3. Shutdown input
4. Latched protection configuration
5. Shutdown status output (host I/F)
Figure 9 Self Reset Protection Configuration
Designing Ct
The CSD pin cannot be paralleled with other
IRS2092(S).
Self Reset Protection
By putting a capacitor between CSD and VSS, the
IRS2092(S) resets itself after entering shutdown
mode.
After the OCP event, CSD pin discharges Ct voltage
VCSD down to the lower threshold Vth2 to reset the
internal shutdown latch. Then, the IRS2092(S)
begins to charge Ct in an attempt to resume
operation. Once the voltage of the CSD pin rises
above the upper threshold, V th1 , the IC resumes
normal operation.
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The timing capacitor, Ct, is used to program tRESET
and tSU.
· tRESET is the amount of time that elapses
from when the IC enters shutdown mode to
the time when the IC resumes operation.
tRESET should be long enough to avoid over
heating the MOSFETs from the repetitive
sequence of shutting down and resuming
operation during over current conditions. In
most of the applications, the minimum
recommended time for tRESET is 0.1 second.
· tSU is the amount of time between powering
up the IC in shutdown mode to the moment
the IC releases shutdown to begin normal
operation.
AN-1138
8
The Ct determines tRESET and tSU as following
equations:
<10k
t RESET =
t SU
Ct × VDD
1.1 × I CSD
Ct × VDD
=
0.7 × I CSD
[s]
SD
VAA
CSH
16
2
GND
VB
15
3
IN-
HO
14
4
COMP
VS
13
5
CSD
VCC
12
6
VSS
LO
11
7
VREF
COM
10
8
OCSET
DT
9
[s]
Figure 11 Latched Protection with Reset Input
where ICSD = the charge/discharge current at the
CSD pin
VDD = the floating input supply voltage with
respect to VSS.
Shutdown Input
The IRS2092(S) can be shut down by an external
shutdown signal SD. Figure 10 shows how to add
an external discharging path to shutdown the PWM.
SD
Interfacing with System Controller
The IRS2092(S) can communicate with an external
system controller through a simple interfacing circuit
shown in Figure 12. A generic PNP transistor U1
detects the sink current at the CSD pin during an
OCP event and outputs a shutdown signal to an
external system controller. Another generic NPN
transistor U2 can then reset the internal protection
logic by pulling the CSD voltage below the lower
threshold Vth2 for a minimum of 200ns. Note that
the CSD pin is configured to operate in latched
OCP. After the power up sequence, a reset signal to
the CSD pin is required to release the IC from
shutdown mode.
1
VAA
CSH
16
2
GND
VB
15
3
IN-
HO
14
4
COMP
VS
13
5
CSD
VCC
12
1
VAA
CSH
16
LO
11
2
GND
VB
15
COM
10
3
IN-
HO
14
DT
9
4
COMP
VS
13
5
CSD
VCC
12
6
VSS
LO
11
7
VREF
COM
10
8
OCSET
DT
9
Ct
6
VSS
7
VREF
8
OCSET
U1
SD
Figure 10 Shutdown Input
<10k
RESET
U2
Latched Protection
Connecting CSD to VDD through a 10k Ω or less
resistor configures the over current protection latch.
The latch locks the IC in shutdown mode after over
current is detected. An external reset switch can be
used to bring CSD below the lower threshold Vth2
for a minimum of 200ns to properly reset the latch.
After the power up sequence, a reset signal to the
CSD pin is required to release the IC from the
latched shutdown mode.
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1
AN-1138
Figure 12 Interfacing with Host Controller
Programming OCP Trip Level
In a Class D audio amplifier, the direction of the load
current alternates with the audio input signal. An
over-current condition can therefore occur during
either a positive current cycle or a negative current
cycle. The IRS2092(S) uses the RDS(on) of the output
MOSFETs as current sensing resistors. Due to the
structural constraints of high voltage ICs, current
sensing is implemented differently for the high side
and low side. If the measured current exceeds a
predetermined threshold, the OCP block outputs a
signal to the protection block, forcing
HO and LO low and protecting the MOSFETs.
9
D1
R2
CSH
UV
DETECT
+B
VB
R1
UV
HIGH
SIDE
CS
Dbs
Q
R3
HO
Q1
Cbs
OUT
HV
LEVEL
SHIFT
FLOATING HIGH SIDE
VS
HV
LEVEL
SHIFT
Vcc
VCC
5V REG
UV
DETECT
DEAD TIME
Q2
LO
SD
-B
COM
R5
LOW SIDE CS
OCSET
R4
VREF
Figure 13 Bi-directional Over Current Protection
Low Side Over Current Sensing
For negative load currents, low side over current
sensing monitors the load condition and shuts down
switching operation if the load current exceeds the
preset trip level.
Low side current sensing is based on the
measurement of VDS across the low side MOFET
during low side on state. In order to avoid triggering
OCP from overshoot, a blanking interval inserted
after LO turn on disables over current detection for
450ns.
The OCSET pin is to program the threshold for low
side over current sensing. When the VDS measured
of the low side MOSFET exceeds the voltage at the
OCSET pin with respect COM, the IRS2092(S)
begins the OCP sequence described earlier.
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AN-1138
Note that programmable OCSET range is 0.5V to
5.0V. To disable low side OCP, connect OCSET to
VCC directly.
To program the trip level for over current, the
voltage at OCSET can be calculated using the
equation below.
VOCSET = VDS(LOW SIDE) = ITRIP+ x RDS(on)
In order to minimize the effect of the input bias
current at the OCSET pin, select resistor values for
R4 and R5 such that the current through the voltage
divider is 0.5mA or more.
* Note: Using VREF to generate an input to OCSET
through a resistive divider provides improved
immunity from fluctuations in VCC.
10
+B
Q1
OCREF
OC
REF
5.1V
R4
OCSET
R5
OUT
VS
0.5mA
-
OC
+
OC Comparator
COM
LO
LO
Q2
IRS2092(S)
-B
Figure 14 Low Side Over Current Sensing
Low Side Over Current Setting
Let the low side MOSFET has RDS(on) of 100mΩ. We
wish to set the current trip level at 30A.
VOCSET is given by:
VOCSET = ITRIP+ x RDS(on) = 30A x 100mW = 3.0V
In contrast to low side current sensing, the threshold
at which the CSH pin engages OC protection is
internally fixed at 1.2V. An external resistive divider
R2 and R3 can be used to program a higher
threshold.
Choose R4+R5=10 kW to properly load the VREF
pin.
V
R5 = OCSET ×10kW
VREF
3.0V
=
×10kW
5.1V
= 5.8kW
where VREF = 5.1V
Based on the E-12 series of resistor values, choose
R5 to be 5.6kW and R4 to be 3.9kW to complete the
design.
In general, RDS(on) has a positive temperature
coefficient that needs to be considered when setting
the threshold level. Also, variations in RDS(on) will
affect the selection of external or internal component
values.
High Side Over-Current Sensing
For positive load currents, high side over current
sensing also monitors the load condition and shuts
down switching operation if the load current exceeds
the preset trip level.
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High side current sensing is based on the
measurement of VDS across the high side MOSFET
during high side turn on through pins CSH and Vs.
In order to avoid triggering OCP from overshoot, a
blanking interval inserted after HO turn on disables
over current detection for 450ns.
AN-1138
An external reverse blocking diode, D1, is required
to block high voltages from feeding into the CSH pin
while the high side is off. Due to a forward voltage
drop of 0.6V across D1, the minimum threshold
required for high side over current protection is
0.6V.
VCSH =
R3
× (V DS ( HIGHSIDE ) + V F ( D1) )
R 2 + R3
where VDS(HIGH SIDE) = the drain to source voltage of
the high side MOSFET during high side turn on
VF(D1) = the forward drop voltage of D1
Since VDS(HIGH SIDE) is determined by the product of
drain current ID and RDS(on) of the high side
MOSFET. VCSH can be rewritten as:
VCSH =
R3
× (R DS ( ON ) × I D + V F ( D1) )
R 2 + R3
The reverse blocking diode D1 is forward biased by
a 10kW resistor R1.
11
OC
D1
R2
CSH
CSH
Com parator
VB
+
-
+B
R1
R3
1.2V
HO
HO
Q1
VS
OUT
Vcc
LO
Q2
IRS2092(S)
-B
Figure 15 Programming High Side Over Current Threshold
High Side Over Current Setting
Figure 15 demonstrates the typical circuitry used for
high side current sensing. In the following example,
the over current protection level is set to trip at 30A
using a MOSFET with an RDS(on) of 100mW. The
component values of R2 and R3 can be calculated
using the following formula:
Let R2 + R3=10 kW.
VthOCH
R3 = 10kW ×
V DS + V F
where VthOCL = 1.2V
VF = the forward voltage of reverse blocking
diode D1 = 0.6V.
[email protected]=30A = the voltage drop across the
high side MOSFET when the MOSFET current is
30A.
Therefore, [email protected]=30A = ID x RDS(on) = 30A x 100mW =
3V
Based on the formulas above, R2 = 6.8kW and R3 =
3.3kW.
Choosing the Correct Right Reverse Blocking
Diode
The selection of the appropriate reverse blocking
diode D1 depends on its voltage rating and speed.
To effectively block bus voltages, the reverse
voltage must be higher than the voltage difference
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between +B and –B and the reverse recovery time
must be as fast as the boot strap charging diode. A
diode such as the Philips BAV21W, a 200V, 50ns
high speed switching diode, is more than sufficient.
Dead-Time Generator
Dead-time is the blanking period inserted between
either high-side Turn-OFF and low-side Turn-ON, or
low-side Turn-OFF and high-side Turn-ON. Its
purpose is to prevent shoot through, or a rush of
current through both MOSFETs. In the IRS2092(S),
an internal dead-time generation block allows the
user to select the optimum dead time from a range
of preset values. Selecting a preset dead-time
through the DT/SD pin voltage can easily be done
through an external voltage divider. This way of
setting dead-time prevents outside noise from
modulating the switching timing, which is critical to
the audio performance.
How to Determine Optimal Dead-Time
The effective dead-time in an actual application
differs from the dead-time specified in this datasheet
due to the switching fall time, tf. The dead-time
value in this datasheet is defined as the time period
between the beginning of turn-off on one side of the
switching stage and the beginning of turn-on on the
other side as shown in Figure 16. The fall time of the
MOSFET gate voltage must be subtracted from the
dead-time value in the datasheet to determine the
effective dead-time of a Class D audio amplifier.
AN-1138
12
(Effective dead-time) = (Dead-time in datasheet) –
tf
90%
Effective dead - time
HO (or LO)
10%
tf
LO (or HO)
Dead-time
in
datasheet
Programming Dead-Time
The IRS2092(S) selects the dead-time from a range
of preset dead-time values based on the voltage
applied at the DT pin. An internal comparator
translates the DT input to a predetermined deadtime by comparing the input with internal reference
voltages. These internal reference voltages are set
in the IC through a resistive voltage divider using
VCC. The relationship between the operation mode
and the voltage at DT pin is illustrated in the
Figure17 below.
Dead-time
10%
25nS
Figure 16 Effective Dead Time
45nS
A longer dead-time period is required for a MOSFET
with a larger gate charge value because of the
longer tf. Although a shorter effective dead-time
setting is beneficial to achieving better linearity in
Class D amplifiers, the likelihood of shoot-through
current increases with narrower dead-time settings.
Negative values of effective dead-time may cause
excessive heat dissipation in the MOSFETs, leading
to potentially serious damage.
To calculate the optimal dead-time in a given
application, the fall time tf for both HO and LO in the
actual circuit need to be taken into account. In
addition, variations in temperature and device
parameters could also affect the effective dead-time
in the actual circuit. Therefore, a minimum effective
dead-time of 10ns is recommended to avoid shootthrough current over the range of operating
temperatures and supply voltages.
75nS
105nS
0.23xVcc
0.36xVcc
0.57xVcc
VDT
Vcc
Figure 17 Dead Time vs. VDT
Table 3 suggests pairs of resistor values used in the
voltage divider for selecting dead-time. Resistors
with up to 5% tolerance are acceptable when using
these values.
IRS2092(S)
>0.5mA
Vcc
R1
DT
R2
COM
Figure 18 External Voltage Divider
Dead-time Mode R1
R2
DT/SD Voltage
DT1
<10k
Open
Vcc
DT2
0.46 x Vcc
5.6kW
4.7kW
DT3
0.29 x Vcc
8.2kW
3.3kW
DT4
Open
<10k
COM
Table 3 Recommended Resistor Values for Dead Time Selection
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AN-1138
13
Supplying VAA and VSS
There are two ways to implement power supplies
VAA and VSS.
1. Supplying VAA and VSS with External
Regulators
For best audio performance, it is preferred to
produce VAA and VSS with external regulators, such
as the three terminal regulators. To keep internal
clamping zener diodes from conducting, the supply
voltage should be VAA < VCLAMPM+ and VSS > VCLAMPM. Standard 7805 and 7905 regulators are suitable.
7805
1
VAA
CSH
16
2
GND
VB
15
3
IN-
HO
14
4
7905
COMP
VS
13
5
CSD
VCC
12
6
VSS
LO
11
COM
10
DT
9
7
VREF
8
OCSET
IAA and ISS, the supply current for VAA and VSS, is
10mA.
This implementation is suggested when the main
bus voltages, +B and –B, are supplied from a
regulated power supply.
Set RAA and RSS values in Figure 21 such that the
supply currents feeding into VAA and VSS are each
10mA.
RAA
RSS
+B
1
VAA
CSH
16
2
GND
VB
15
3
IN -
HO
14
4
COMP
VS
13
5
C SD
VC C
12
6
VSS
LO
11
7
VREF
COM
10
8
OCSET
DT
9
-B
Figure 21 Regulating VAA and VSS with Internal
Zener Diodes
Figure 19 Supplying VAA and VSS with External
Regulators
When switched mode regulators provide VAA and
VSS, it is required to place a two stage noise filter in
the supply lines as shown in Figure 20 to prevent
noise from influencing the switching ripple voltage
on +/-5V.
10
IRS2092(S)
10
+5V
10µF
1
VAA
CSH
16
2
GND
VB
15
3
IN-
HO
14
4
COMP
VS
13
5
CSD
VCC
12
2.2µF
10nF
10µF
10
-5V
2.2µF
10
6
VSS
7
VREF
8
OCSET
LO
11
COM
10
DT
9
Figure 20 Supplying VAA and VSS from Switched
Mode Power Supply
2. Regulating VAA and VSS Using Internal
Zener Diodes
VAA and VSS can be supplied with an internal zener
diode clamp as a shunt regulator. Recommended
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Charging VBS Prior to Start
For proper start-up, the high side bootstrap
capacitor is required to be charged prior to PWM
start-up through a resistor RCHARGE from the positive
supply bus to the VB pin. By utilizing an internal
20.8V zener diode between VB and VS, this scheme
eliminates the need to charge the boot strap
capacitor through low side turn on during start up.
The value of this charging resistor is subject to
several constraints:
- The minimum resistance of RCHARGE is
limited by the maximum PWM modulation
index of the system. When HO is high,
RCHARGE drains bootstrap power supply so it
reduces holding up time, hence maximum
continuous HO on time.
- The maximum resistance of RCHARGE is
limited by the current charge capability of
the resistor during startup:
I CHARGE > I QBS
where ICHARGE = the current through
RCHARGE
IQBS = the high side supply
quiescent current.
AN-1138
14
ICHARGE generates a DC offset at the speaker output
prior to PWM start up. Check that the DC offset
does not exceed a condition for click noise
elimination. See Click Noise Elimination section for
more detail.
Rcharge
+B
1
VAA
CSH
16
2
GND
VB
15
3
IN-
HO
14
4
COMP
VS
13
5
CSD
VCC
12
6
VSS
LO
11
7
VREF
COM
10
8
OCSET
DT
9
VSS Negative Bias Clamping
An excessive negative Vss voltage with respect to
COM could damage the IRS2092(S). VSS can go
below COM when a negative supply is missing in a
dual supply configuration. To protect the IC from this
possibility, a diode is recommended for clamping
potential negative biases to VSS. A standard
recovery diode with a current rating of 1A such as
the 1N4002 is sufficient for this purpose.
Vcc
1
VAA
C SH
16
2
GND
VB
15
3
IN-
HO
14
4
COMP
VS
13
5
CSD
VCC
12
6
VSS
LO
11
7
VREF
COM
10
8
OCSET
DT
9
12V
-B
Figure 22 Boot Strap Supply Pre-charging
Start-up Sequence (UVLO)
The protection control block in the IRS2092(S)
monitors the status of VAA and VCC to ensure that
both voltage supplies are above their respective
UVLO (Under Voltage Lock Out) thresholds before
beginning normal operation. If either VAA or VCC is
below the under voltage threshold, LO and HO are
disabled in shutdown mode until both VAA and VCC
rise above their voltage thresholds.
Power-down Sequence
As soon as VAA or VCC falls below its UVLO
threshold, protection logic in the IRS2092(S) turns
off LO and HO, shutting off the power MOSFETs.
VCC
U VLO( VCC )
-B
Figure 24 Negative VSS Clamping
Junction Temperature Estimation
The power dissipation in the IRS2092(S) is
dominated by the following items:
- PMID: Power dissipation of the input floating
logic and protection circuitry
- PLSM: Power dissipation of the Input Level
Shifter
- PLOW : Power dissipation in low side
- PLSH: Power dissipation of the High-side
Level Shifter
- PHIGH: Power dissipation in high side
HO
LO
The following equations are for your reference only.
Because of the non-linear characteristics in gate
drive stage, these assumptions may not be accurate.
Figure 23 IRS2092(S) UVLO Timing Chart
1. PMID: Power Dissipation of the Input
Floating Logic and Protection Circuitry
Power Supply Decoupling
The power dissipation of the input floating
section is given by:
Careful attention must be given to decoupling the
power supplies for proper operation of the IC.
Ceramic capacitors of 0.1µF or more should be
placed close to the power supply pins of the IC on
the board.
PMID = PZENER + POTA
Where
PZENER = the power dissipation from
the internal zener diodes clamping
VAA and VSS
Please refer to the application note AN-978 for
general design considerations of a high voltage gate
driver IC.
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AN-1138
15
Rg(int) = internal gate resistance of the
low side MOSFET, typically 2Ω
POTA = the power dissipation from the
internal OTA
Rg = external gate resistance of the
low side MOSFET
When VAA and VSS are regulated with internal
zener diode clamping, PMID can be simplified as:
PMID » (VAA - VSS ) ×
(V+ BUS - V AA ) + (VSS - V- BUS )
Qg = total gate charge of the low side
MOSFET
R AA + RSS
Where
4. PLSH: Power Dissipation of the High-side
Level Shifter
V+BUS = positive bus voltage feeding
VAA
V-BUS = negative bus voltage feeding
VSS
RAA = resistor feeding VAA from V+BUS
RSS = resistor feeding VSS from
V-BUS
PLSH = 0.4nC x fsw x VBUS
Where
f SW = PWM switching frequency
VBUS = difference between the positive
bus voltage and negative bus voltage
See Figure 21.
2. PLSM: Power Dissipation of the Input
Level Shifter
5. PHIGH: Power Dissipation of High Side
-9
PLSM = 1.5 x 10 x f SW x VSS BIAS
The power dissipation of the high side comes
from the losses of the logic circuitry and the
losses of driving HO.
Where
f SW = the PWM switching frequency
VSS BIAS = the bias voltage of VSS
with respect to COM
PHIGH = PLDD + PHO
3. PLOW: Power Dissipation of Low Side
æ
ö
RO
÷
= (I QBS × VBS ) + çVBS × Qg × f SW ×
ç
RO + Rg + Rg (int) ÷ø
è
The power dissipation of the low side comes
from the losses of the logic circuitry and the
losses of driving LO.
Where
PLDD = power dissipation of the
internal logic circuitry
PLOW = PLDD + PLO
PHO = power dissipation of the gate
drive stage for HO
æ
ö
RO
÷
= (I QCC × VCC ) + ç Vcc × Qg × f SW ×
ç
÷
+
+
R
R
R
O
g
g (int) ø
è
RO = equivalent output impedance of
HO, typically 10 Ω for the IRS2092(S)
Where
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PLDD = power dissipation of the
internal logic circuitry
Rg(int) = the internal gate resistance of
the high side MOSFET, typically 2Ω
PLO = power dissipation from of gate
drive stage for LO
Rg = external gate resistance of the
high side MOSFET
RO = output impedance of LO,
typically 10 Ω for the IRS2092(S)
Qg = total gate charge of the high
side MOSFET
AN-1138
16
6. PD: Total Power Dissipation
plane design because the integration of circuitries
within the IC is referenced to different potentials.
Proper application of the IRS2092(S) employs three
reference potentials.
Total power dissipation, PD, is given by
PD = PMID + PLSM + PLOW + PHSM + PHIGH .
1. Analog Ground
7. Tj: Junction Temperature
Given junction to ambient thermal resistance RthJA,
the junction temperature Tj can be calculated from
the formula provided below and must not exceed
150°C.
TJ = RthJA × Pd + TA < 150°C
Board Layout Considerations
The floating input section of the IRS2092(S)
consists of a low noise OTA error amplifier and a
PWM comparator along with CMOS logic circuitry.
High frequency bypass capacitor CVAA-VSS should be
placed closest to the IRS2092(S) to supply the logic
circuitry. CVAA and CVSS are for stable operation of
the OTA and should be placed close to the IC.
Gate driver supply capacitors CVCC and CVBS provide
gate charging current and should also be placed
close to the IRS2092(S).
IRS2092(S)
VAA
CSH
16
2
GND
VB
15
3
IN-
HO
14
4
COMP
VS
13
5
CSD
VCC
12
1
CVAA
CVAA-VSS
CVSS
6
VSS
7
VREF
8
OCSET
2. Gate Driver Reference
The gate driver stage of the IRS2092(S) is located
between pins 10 and 15 and is referenced to the
negative bus voltage, COM. This is the substrate of
the IC and acts as ground. Although the negative
bus is a noisy node in the system, both of the gate
drivers refer to this node. Therefore, it is important
to shield the gate drive stages with the negative bus
voltage so that all the noise currents due to stray
capacitances flow back to the power supply without
degrading signal ground.
CVBS
LO
11
COM
10
DT
9
3. Power Ground
Power ground is the ground connection that closes
the loops of the bus capacitors and inductor ripple
current circuits. Separate the power ground and
input signal grounds from each other as much as
possible to avoid common stray impedances.
CVCC
Figure 25 Placement Sensitive Bypass
Capacitors
Figure 26 illustrates how to paint out reference
planes. Power GND plane should include negative
bus cap. Power reference plane should include Vcc.
Also, Use distinctly different symbols for the different
grounds.
Ground Plane
In addition to the key component locations
mentioned above, it is important to properly pour
ground planes to obtain good audio performance.
The IRS2092(S) does not accept single ground
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The Input analog section around the OTA is
referenced to the signal ground, or GND, which
should be a quiet reference node for the audio input
signal. The peripheral circuits in the floating input
section such as CSD and COM pins refer to this
ground. These nodes should all be separate from
the switching stages of the system. In order to
prevent potential capacitive coupling to the
switching nodes, use a ground plane only in this part
of the circuit. Do not share the ground plane with
gate driver or power stages.
AN-1138
For further board layout information, refer to AN1135, PCB Layout with IR Class D Audio Gate
Drivers
17
Figure 26 Applying Ground Planes
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AN-1138
18