RISEN(Ω) = KISEN ×

VFS(V)

IMAX(A)

Equation - RISEN Setting

For example, if the ADC operates up to 1V FS and the maximum operating output current is 2A, then RISEN would be

7500 x 1V/2A = 3.75KΩ.

The RISEN value also sets the output impedance of the Current Sense Output circuit (ISEN output impedance). Normally,

the input impedance of the ADC is much higher than RISEN enabling a direct connection to the ISEN pin without

attenuation. In case a low input impedance ADC is used, a pre-amplifier (buffer) could be required.

The Current Sense Output circuit bandwidth and step response performances (see Specifications) ensure that the current

monitor tracks the driver current in PWM motor drive applications.

Current Drive Regulation

The MAX22211 features embedded CDR. The embedded CDR provides an accurate control of the current flowing into

the motor windings. The bridge current is sensed by non-dissipative ICS and it is then compared with the threshold

current (ITRIP). As soon as the bridge current exceeds the threshold, the device enforces slow decay for a fixed OFF-time

(tOFF). Once tOFF has elapsed, the driver is re-enabled for the next PWM cycle. During current regulation, the PWM duty

cycle and frequency depend on the supply voltage, motor inductance, and motor speed and load conditions.

Setting the Current Regulation Threshold – ITRIP

Connect resistors from REFA and REFB to GND to set the current regulation thresholds for Full Bridge A and Full Bridge

B, respectively (ITRIPA, ITRIPB) .

The following equation shows the typical ITRIP current as a function of the RREF_ shunt resistor connected to pin REF_.

ITRIP =

KIFS(kV)

RREF(KΩ)

× HFS( % )

The proportionality constant KIFS is typically 36kV when HFS is logic low and 18.4kV when HFS is logic high. The

external resistor RREF can range between 9.5kΩ and 72kΩ.

The HFS depends on the status of the Logic Input pin HFS. When HFS is set logic low, the scalar coefficient is 100%

and the power FETs RDS(ON) is set to a minimum 0.25Ω (high-side + low-side). When the HFS is set logic high, the

scalar coefficient is 50% and the power FETs have higher RDS(ON) of 0.375Ω (high-side + low-side). This setting is

recommended for applications in which the maximum current does not exceed 1.9A and high accuracy at low current is

desirable.

The Table 2 summarizes the HFS settings.

Table 2. HFS Truth Table

HFS

ITRIP

(%)

MAXIMUM

OUTPUT

CURRENT

TYPICAL

RDS(ON)

(HIGH-SIDE +

LOW-SIDE)

NOTES

0

100%

3.8A

0.25Ω

Optimized efficiency and extended operating range up to 3.8AMAX

1

50%

1.9A

0.375Ω

Reduced operating range up to 1.9AMAX. Improved current accuracy

control in the bottom end of the current range

Thermal constraints and limitations must be carefully considered when setting the ITRIP current threshold. These

constraints depend on the PCB layout, ground plane thickness, number of layers, air ventilation, heat sinks, number of

thermal vias, maximum ambient temperature, etc. A maximum of 2ARMS is recommended per H-bridge RMS current

when using a standard four-layer board with no forced air or heatsinks. With a proper thermal designed board, a higher

peak current can be delivered when the burst duration is significantly shorter than the PCB thermal time constant. For

example, using our evaluation board without forced air and with two H-bridges operating simultaneously the device was

able to deliver up to 3.8A per H-bridge for a 100ms long burst at room temperature.

MAX22211

36V, 3.8A Two H-Bridge for Dual Brushed or

Single Stepper Motor Drive

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