MP6550 – 1.8V TO 22V, 2A, H-BRIDGE MOTOR DRIVER
Current Limit / Regulation
OPERATION
Bridge Control
The MP6550 is controlled using a PWM input
interface, which is compatible with industry-
standard devices. Each output is controlled by a
corresponding input pin.
The current in the outputs is limited using
constant-off-time PWM (pulse-width modulation)
control circuitry. Figure 3 shows this operation.
The following table shows the logic for the
MP6550:
Table 1: Input Logic Truth Table
Function
IN1
IN2
OUT1
OUT2
(DC Motor)
Figure 3: Current Limit Operation
L
L
L
H
L
Z
L
Z
H
L
Coast
Initially, a diagonal pair of MOSFETs turns on
and drives current through the load. The current
increases in the load, which is sensed by the
internal current-sense circuit.
Reverse
Forward
Brake
H
H
H
L
H
L
If the load current reaches the current trip
threshold, the H-bridge switches to slow decay
mode, and the two low-side MOSFETs turn on.
Table 2 shows the recommended PWM input
control for a DC motor driver.
After a fixed off time (tITRIP), if the load current
has fallen at least 20% below the current limit
threshold, the FETs are re-enabled and the
cycle repeats. If the current remains above this
level, the off time is extended until the current
falls to 20% below the current limit threshold.
Table 2: PWM Input Control
Function
IN1
IN2
OUT1 OUT2
(DC Motor)
L
H
L
L
L
L
L
Forward
H
PWM
H
Slow decay
Reverse
The current limit threshold is reached when the
VISEN pin reaches 0.5V. For example, with a
5kꢀ resistor from ISET to ground, the VISEN
voltage is about 0.5V/A of output current. So
when the current reaches 1A, the VISET pin
voltage reaches 0.5V, and a current trip occurs.
L
H
L
PWM
H
H
Slow decay
Current Sensing
The current flowing in the two low-side
MOSFETs is sensed with an internal current-
sensing circuit. A voltage proportional to the
output current is sourced on the VISEN pin.
During current regulation, the nFAULT pin is not
activated.
Blanking Time
The VISEN output voltage scaling is set by a
resistor connected between the ISET pin and
ground. For 1A of output current, 100µA of
current is sourced into the resistor connected to
VISEN. For example, if a 5kꢀ resistor is
connected between ISET and ground, the
output voltage on the VISEN pin will be 0.5V/A
of output current.
There is often a current spike during start-up
due to the body diode’s reverse-recovery
current or the shunt capacitance of the load.
This current spike requires filtering to prevent it
from erroneously shutting down the high-side
MOSFET. An internal fixed blanking time (tOCPD
)
blanks the output of the current sense
comparator when the outputs are switched.
This blanking time also sets the minimum on
time for high-side MOSFET.
Current is sensed any time one of the low-side
MOSFETs is turned on, including during slow
decay (brake) mode.
The load current applied to the VISEN pin
should be kept below 2mA, with no more than
500pF of capacitance.
MP6550 Rev. 1.0
7/16/2019
www.MonolithicPower.com
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2019 MPS. All Rights Reserved.
11
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