ADMC328YN datasheet(18/32 Pages) AD

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ADMC328

–18–

REV. B

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

µA. 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

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 the ADC

Resolution section.

4. The target reference conversion count is calculated as TAR-

GET = (Max Count)

× (2.5 V/3.5 V).

5. Reset or software sets the ICONST_TRIM register to zero.

6. Select the calibration VREF in software on ADC multiplexer

and wait one PWM cycle for updated ADC value.

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 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, IBUS shown in Figure 17, is sampled at the mid-

point of the lower device on period using the PWMSYNC pulse.

The captured value represents the current in the motor winding

IWINDING.

IWINDING

CURRENT SENSE SIGNAL

CIRCUIT COMMON

–V

IWINDING

IBUS

Figure 16. Typical Power Inverter Switching Topology

for DC Brushless Motor Control

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