ADMCF328BN datasheet(19/36 Pages) AD

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ADMCF328

–19–

REV. A

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

14 for a tuned ICONST.

2. Program PWMSYNCWT to proper count as in Figure 13.

3. The ADC Max Count is calculated, as described in a previ-

ous section.

4. The target reference conversion is calculated as TARGET =

(Max Count)

× (2.5 V/3.5 V).

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

reference conversion.

8. If this value is greater than the TARGET, the ICONST_TRIM

value is incremented by one, and Step 7 is repeated.

9. If the calibration channel value is less than the TARGET,

the calibration is completed.

VREF

3.5V

0.3V

MINIMUM

RAMP

TARGET

RAMP

Figure 15. Current Ramp

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.

IWINDING

CURRENT SENSE SIGNAL

CIRCUIT COMMON

–V

IWINDING

IBUS

Figure 16. Typical Power Inverter Switching Topology

for DC Brushless Motor Control

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, IBUS, shown in Figure 17, is sampled at the midpoint of

the lower device-on period using the PWMSYNC pulse. The cap-

tured value represents the current in the motor winding IWINDING.

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.

ADMCF328BN Datasheet(PDF) 19 Page - Analog Devices

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