Current Regulation

The MAX14870/MAX14872 features internal current

regulation to limit the startup current. Regulation is set

when the motor current (IM) exceeds the current limit

defined by the sense resistor (RSENSE):

IM_MAX = 0.1V/RSENSE

When the motor current exceeds the maximum threshold,

the H-bridge drive is reversed for fast decay for a fixed

time (tOFF = 15μs). The H-bridge is turned back to normal

operation after tOFF.

Fast Decay Mode

When the motor current exceeds the set threshold, the

H-bridge polarity is reversed during the 15μs (typ) tOFF

period. This results in faster motor current decay since

–VDD is applied across the motor’s terminals. The motor

current decrease is first order with an L/R time constant

and proportional to (VDD + VEMF).

Note that if tOFF is larger than the motor’s L/R electrical

time constant, the motor current can reverse direction,

causing the motor to not start.

During fast decay, the motor’s inductive current recircu-

lates through the external VDD supply, which charges up

the VDD bypass capacitor. Therefore, the voltage seen

across RSENSE is negative during the tOFF delay.

Applications Information

Layout Considerations

Connect duplicate pins (COM pins and VDD pins) togeth-

er with low-resistance traces. See the Current Sensing

section for further layout recommendations.

Power Considerations

The MAX14870/MAX14872 driver can generate more

power than the package for the device can safely dissi-

pate. Total power dissipation for the device is calculated

using the following equation:

PTOTAL = PDRIVER + PSW + PD

The power dissipated inside of the driver is calculated as:

Figure 4. Layout Example for Accurate Current Sensing

EP

M1

M2

To SNS

COM

COM

GND

GND

GND

GND

GND

GND

MAX14870/MAX14872

Compact 4.5V to 36V

Full-Bridge DC Motor Drivers

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