where I
M
is the motor current and R
ON
is the on-resis-
tance 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
x 2 x V
DS
= I
M
x 2 x (1/2 x V
DD
x f
SW
x t
R
)
where I
M
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
x 2 x V
BF
x t
DEAD
x f
SW
Operation Without Current Regulation
To operate the MAX14870/MAX14872 without internal or
external current regulation, connect SNS directly to GND.
No sense resistor is required for this configuration.
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
MAX14870/MAX14872 and use external regulation, con-
nect SNS directly to ground.
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.
Figure 5. Operation with External Current Regulation
V
UVLO
DRIVER
V
DD
SNS
PWM*
DIR*
MAX14870
MAX14872
GND
COM
M1
M2
EN
FAULT
DRIVER
V
DD
V
DD
CURRENT
REGULATION
R
SENSE
µC
IRQ
PWM
GPO
ADC
9V
3.3V
3.3V
M
A
*THESE PIN NAMES ARE FOR THE MAX14870. ON THE MAX14872, THESE ARE THE FWD AND REV INPUTS.
MAX14870/MAX14872 Compact 4.5V to 36V
Full-Bridge DC Motor Drivers
www.maximintegrated.com
Maxim Integrated
│
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