ADMC328TN AD [Analog Devices], ADMC328TN Datasheet - Page 18

• Manufacturer Part Number: ADMC328TN
• Description: 28-Lead ROM-Based DSP Motor Controller with Current Sense
• Manufacturer: AD [Analog Devices]

ADMC328
Programmable Current Source
The ADMC328 has an internal current source that is used to
charge an external capacitor, generating the voltage ramp used
for conversion. The magnitude of the output of the current source
circuit is subject to manufacturing variations and can vary from
one device to the next. Therefore, the ADMC328 incudes a pro-
grammable current source whose output can always be tuned to
within 5% of the target 100 A. A 3-bit register, ICONST_TRIM,
allows the user to make this adjustment. The output current is
proportional to the value written to the register: 0x0 produces
the minimum output, and 0x7 produces the maximum output.
The default value of ICONST_TRIM after reset is 0x0.
ADC Reference Ramp Calibration
The peak of the ADC ramp voltage should be as close as pos-
sible to 3.5 V to achieve the optimum ADC resolution and
signal range. When the current source is in the Default State,
the peak of the ADC ramp slope will be lower than this “3.5 V”
target ramp. When the current source value is increased, the
ADC ramp slope will become closer to the target value. The
“tuned” ramp slope is the one closest to the target ramp.
A simple calibration procedure using the internal 2.5 V reference
voltage allows the selection of the ICONST_TRIM register
value to reach this “tuned” ramp slope:
1. A high quality linear ADC capacitor is selected using Figure
2. Program PWMSYNCWT to proper count as in Figure 13.
3. The ADC Max Count is calculated, as described in the ADC
4. The target reference conversion count is calculated as TAR-
5. Reset or software sets the ICONST_TRIM register to zero.
6. Select the calibration V
7. The calibration channel value is compared with the target
8. If this value is greater than the TARGET, the ICONST_TRIM
9. If the calibration channel value is less than the TARGET, the
14 for a tuned ICONST.
Resolution section.
GET = (Max Count)
and wait one PWM cycle for updated ADC value.
reference conversion.
value is incremented by one, and Step 7 is repeated.
calibration is completed.
V
REF
MINIMUM
RAMP
Figure 15. Current Ramp
TARGET
RAMP
REF
(2.5 V/3.5 V).
in software on ADC multiplexer
3.5V
0.3V
–18–
Current Sense Amplifier
The ADMC328 analog circuit block also integrates an inverting
amplifier, a sample-and-hold amplifier, and an overcurrent-
trip comparator. The current sense amplifier input signal range
is matched to the requirements of medium to low power motor
control applications. There is an output offset that matches the
amplifier output signal range to the input signal range of the A/D
converter. This amplifier is followed by a sample-and-hold
amplifier that samples the current sense signal on the falling
edge of the PWMSYNC pulse. This sampling amplifier system
can be used to capture the winding current signal in a brushless
dc motor.
Current Sense Amplifier Application
The ADMC328 current sense amplifier system has been provided
to simplify the measurement of the motor winding currents in
brushless dc motor control systems. The assumed power cir-
cuit configuration, illustrated in Figure 16 is one in which a
current sense resistor is placed between the circuit common and
the return path to the negative power bus. The normal PWM
modulation scheme keeps one upper device fully conducting
while the duty cycle of one of the lower power switches is varied.
In this case, there is a negative going voltage across the resistive
shunt when the complementary upper diode conducts. The
shunt signal, I
point of the lower device on period using the PWMSYNC pulse.
The captured value represents the current in the motor winding
I
WINDING
+V
–V
Figure 16. Typical Power Inverter Switching Topology
for DC Brushless Motor Control
CURRENT SENSE SIGNAL
I
BUS
.
CH
CL
BUS
shown in Figure 17, is sampled at the mid-
BL
BH
I
WINDING
AH
AL
CIRCUIT COMMON
I
WINDING
REV. B
0V