ADMCF328BRZ Analog Devices Inc, ADMCF328BRZ Datasheet - Page 19

• Manufacturer Part Number: ADMCF328BRZ
• Manufacturer: Analog Devices Inc

Specifications of ADMCF328BRZ

• Operating Current: 123mA
• Operating Temperature Classification: Industrial
• Package Type: SOIC W
• Operating Supply Voltage (min): -0.3V
• Operating Supply Voltage (max): 7V
• Lead Free Status / Rohs Status: Compliant

ADC Reference Ramp Calibration
The peak of the ADC ramp voltage should be as close as possible
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:
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 a previ-
4. The target reference conversion is calculated as TARGET =
5. Reset or software sets the ICONST_TRIM register to zero.
6. Select calibration channel in software on ADC multiplexer.
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,
Current Sense Amplifier
The ADMCF328 analog circuit block also integrates an inverting
amplifier, a sample-and-hold amplifier, and an over-current-
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.
REV. A
14 for a tuned ICONST.
ous section.
(Max Count) × (2.5 V/3.5 V).
reference conversion.
value is incremented by one, and Step 7 is repeated.
the calibration is completed.
V
REF
MINIMUM
RAMP
Figure 15. Current Ramp
TARGET
RAMP
3.5V
0.3V
–19–
Current Sense Amplifier Application
The ADMCF328 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 circuit configuration, illustrated in Figure 16, is one in
which a current sense resistor is placed between the circuit com-
mon 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
the lower device-on period using the PWMSYNC pulse. The cap-
tured value represents the current in the motor winding I
ADC Registers
The configuration of all registers of the ADC System is shown at
the end of the data sheet.
AUXILIARY PWM TIMERS
Overview
The ADMCF328 provides two variable frequency, variable duty
cycle, 8-bit, auxiliary PWM outputs that are available at the AUX1
and AUX0 pins when enabled. These auxiliary PWM outputs
can be used to provide switching signals to other circuits in a
typical motor control system such as power factor corrected
front-end converters or other switching power converters. Alter-
natively, by addition of a suitable filter network, the auxiliary
PWM output signals can be used as simple single-bit digital-to-
analog converters.
Figure 16. Typical Power Inverter Switching Topology
for DC Brushless Motor Control
+V
–V
CURRENT SENSE SIGNAL
I
BUS
BUS
, shown in Figure 17, is sampled at the midpoint of
CH
CL
BL
BH
I
WINDING
AH
AL
CIRCUIT COMMON
ADMCF328
I
WINDING
WINDING
0V
.