The power dissipated inside of the driver is calculated as:
P
DRIVER
= I
M_LOAD
2
x R
ON
where I
M_LOAD
is the motor current and R
ON
is the on-
resistance of the high and low-side FETs.
P
SW
is the power generated by the driver during the rise/
fall times in switching, and includes both arms of the bridge.
Calculate P
SW
using the following equation:
P
SW
= I
M_LOAD
x 2 x V
DS
= I
M_LOAD
x 2 x (1/2 x V
DD
x f
SW
x t
R
)
where I
M_LOAD
is the motor current, t
R
is the 200ns
(typ) rise or fall time of the driver output, and f
SW
is the
switching frequency.
The internal diodes dissipate power during switching, as
well. Calculate the power dissipated in the diodes as:
P
D
= I
M_LOAD
x 2 x V
BF
x t
DEAD
x f
SW
Operation Without Internal Current Regulation
To operate the MAX14871 without internal or external
current regulation, connect SNS directly to GND. No
sense resistor is required for this configuration. See
Figure 4.
Operation with External Current Regulation
The motor current can be controlled by external PWM
regulation using sense-resistor feedback in a control loop.
To disable the internal current regulation circuitry of the
MAX14871 and use external regulation, connect SNS
directly to ground. See Figure 5.
Note that, if fast decay control is used, the COM voltage
pulses negatively when the H-bridge direction is inverted.
Use of External Capacitors
Maxim does not recommend using external capacitors
across the motor terminals. Added capacitance between
H-bridge outputs increases the power dissipated in the
H-bridge by:
P
D
= V
DD
2
x C x f
SW
where C is the capacitance across M1/M2 and f
SW
is
the M1/M2 switching frequency. This power is dissipated
without good reason.
Note that conducted EMI on the V
DD
lines is also
worsened due to the high-capacitive current spikes.
Figure 5. Operation with External Current Regulation
DRIVER
VDD
SNS
PWM
DIR
MAX14871
GND
COM
M1
M2
EN
FAULT
DRIVER
VDD
VDD
CURRENT
REGULATION
R
SENSE
MODE
VREF
TCOFF
µC
IRQ
PWM
GPO
ADC
9V
3.3V
3.3V
M
A
MAX14871 4.5V to 36V Full-Bridge DC Motor Driver
www.maximintegrated.com
Maxim Integrated
│
13
