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| 型號： | MSA240 |
| 廠商： | Apex Microtechnology Corporation |
| 英文描述： | PULSE WIDTH MODULATION AMPLIFIER |
| 中文描述： | 脈寬調(diào)制放大器 |
| 文件頁數(shù)： | 4/4頁 |
| 文件大?。?/td> | 300K |
| 代理商： | MSA240 |
APEX MICROTECHNOLOGY CORPORATION 5980 NORTH SHANNON ROAD TUCSON, ARIZONA 85741 USA APPLICATIONS HOTLINE: 1 (800) 546-2739
4
OPERATING
CONSIDERATIONS
MSA240
MSA240U REV C JULY 2004 2004 Apex Microtechnology Corp.
GENERAL
Please read Application Note 30 “PWM Basics”. Refer also
to Application Note 1 “General Operating Considerations” for
helpful information regarding power supplies, heat sinking,
mounting, SOA interpretation, and specification interpreta-
tion. Visit www.apexmicrotech.com for design tools that help
automate tasks such as calculations for stability, internal power
dissipation, current limit, heat sink selection, Apex’s complete
Application Notes library, Technical Seminar Workbook and
Evaluation Kits.
OSCILLATOR
The MSA240 includes a user frequency programmable
oscillator. The oscillator determines the switching frequency
of the amplifier. The switching frequency of the amplifier is 1/2
the oscillator frequency. Two resistor values must be chosen
to properly program the switching frequency of the amplifier.
One resistor, R
OSC
, sets the oscillator frequency. The other
resistor, R
, sets the internal ramp amplitude. In all cases
the ramp voltage will oscillate between 1.5V and 3.5V. See
Figure 1. If an external oscillator is applied use the equations
to calculate R
RAMP
.
To program the oscillator, R
OSC
is given by:
R
OSC
= (1.32X10
8
/ F) - 2680
where F is the desired
switching frequency
and:
R
RAMP
= 2 X R
OSC
Use 1% resistors with 100ppm drift (RN55C type resistors,
for example). Maximum
switching frequency
is 50kHz.
Example:
If the desired switching frequency is 22kHz then R
OSC
=
3.32K and R
RAMP
= 6.64K. Choose the closest standard 1%
values:
R
OSC
= 3.32K and R
RAMP
= 6.65K.
FIGURE 1. EXTERNAL OSCILLATOR CONNECTIONS
SHUTDOWN
The MSA240 output stage can be turned off with a shutdown
command voltage applied to Pin 10 as shown in Figure 2. The
shutdown signal is OR’ed with the current limit signal and
simply overrides it. As long as the shutdown signal remains
high the output will be off.
CURRENT SENSING
The low side drive transistors of the MSA240 are brought
out for sensing the current in each half bridge. A resistor from
each sense line to PWR GND (pin 58) develops the current
sense voltage. Choose R and C such that the time constant
is equal to 10 periods of the selected switching frequency. The
internal current limit comparators trip at 200mV. Therefore,
current limit occurs at I = 0.2/R
SENSE
for each half bridge. See
Figure 2. Accurate milliohm power resistors are required and
there are several sources for these listed in the Accessories
Vendors section of the Databook.
FIGURE 2. CURRENT LIMIT WITH OPTIONAL SHUTDOWN
POWER SUPPLY BYPASSING
Bypass capacitors to power supply terminals +V
S
must
be connected physically close to the pins to prevent local
parasitic oscillation and overshoot. All +V
S
pins must be con-
nected together. Place an electrolytic capacitor of at least
10μF per output amp required midpoint between these sets
of pins. In addition place a ceramic capacitor 1μF or greater
directly at each set of pins for high frequency bypassing. V
CC
is bypassed internally.
GROUNDING AND PCB LAYOUT
Switching amplifiers combine millivolt level analog signals
and large amplitude switching voltages and currents with fast
rise times. As such grounding is crucial. Use a single point
ground at SIG GND (pin 26). Connect signal ground pins 2 and
18 directly to the single point ground on pin 26. Connect the
digital return pin 23 directly to pin 26 as well. Connect PWR
GND pin 58 also to pin 26. Connect AC BACKPLATE pin 28
also to the single point ground at pin 26. Connect the ground
terminal of the V
CC
supply directly to pin 26 as well. Make sure
no current from the load return to PWR GND flows in the analog
signal ground. Make sure that the power portion of the PCB
layout does not pass over low-level analog signal traces on
the opposite side of the PCB. Capacitive coupling through the
PCB may inject switching voltages into the analog signal path.
Further, make sure that the power side of the PCB layout does
not come close to the analog signal side. Fast rising output
signal can couple through the trace-to-trace capacitance on
the same side of the PCB.
DETERMINING THE OUTPUT STATE
The input signal is applied to +IN (Pin 13) and varies from
1.5 to 3.5 volts, zero to full scale. As +IN varies from 1.5 to
2.5 volts the A output "high" duty cycle (relative to ground) is
greater than the B output "high" duty cycle. The reverse occurs
as the input signal varies from 2.5 to 3.5 volts. When +IN =
2.5 volts the duty cycles of both A and B outputs are 50%.
Consequently, when the input voltage is 1.5V the A output
is close to 100% duty cycle and the B output is close to 0%
duty cycle. The reverse occurs with an input voltage of 3.5V.
The output duty cycle extremes vary somewhat with switching
frequency and are internally limited to approximately 5% to
95% at 10kHz and 7% to 93% at 50kHz.
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