MAX14871_14 Datasheet, PDF (12/15 Pages) Maxim Integrated Products – 4.5V to 36V Full-Bridge DC Motor Driver

MAX14871
4.5V to 36V Full-Bridge DC Motor Driver
or by reversing the H-bridge direction (fast decay). The
H-bridge is turned back to normal polarity after a defined
delay time (TOFF mode) or after the motor current has
reduced by 25% (25% ripple mode). See Table 1. The
MAX14871 Function Tables show how to set the regula-
tion mode.
Regulation Modes
The MAX14871 offers two internal current regulation
modes: Constant off-time (TOFF) and 25% current ripple
regulation. For both modes, regulation is set when the
motor current (IM) exceeds the current limit defined by the
VREF voltage and the sense resistor:
IM_MAX = V_VREF/(AV x R_SENSE)
VREF
Either the internal V_VREF voltage or an external voltage
on VREF can be used for current regulation. Select inter-
nal VREF-based regulation by setting V_VREF < V_{REF_TH}.
The internal V_VREF is 1V (typ).
When an external voltage is used, the range of VREF is
defined by V_VREF.
Fixed Off-Time (TOFF-time) Regulation
Fixed off-time regulation turns the H-bridge driver off for
a fixed time (t_OFF time), as defined by the value of the
C_OFF capacitor connected between TCOFF and GND:
t_OFF (µs)= C_OFF(pF)/10.
If V_VREF < V_{VREF_TH} and TCOFF is left unconnected,
then t_OFF is 15µs (typ).
During the fixed TOFF-time regulation, the H-bridge can
operate in either slow or fast decay mode. See Table 1.
Slow Decay Mode
Slow decay, also called brake mode, is selected by set-
ting V_MODE > 1.5V. In slow decay, both H-bridge low-side
drivers are turned on so that the inductive motor
current recirculates through the low-side transistors and
the motor’s terminals see a differential voltage near
zero (V_DIFF = 2 x IM x R_{ON_LS}). During the slow-decay
TOFF period (t_OFF) motor current does not flow through
the external VDD/GND supply and the voltage across
RSENSE is zero. The current decay during tOFF is a
first-order exponential decay with a time constant equal
to the motor’s electrical time constant (L/R). The rate of
current decay during t_OFF is proportional to the motor’s
back EMF/rotational speed.
Fast Decay Mode
Fast decay mode can be used as an alternative to slow
decay during fixed off-time regulation. Fast decay is
enabled by setting VMODE < 0.2V. In fast decay, the
H-bridge polarity is reversed during the tOFF period,
which 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 inductive current can reverse direction,
causing the motor not to start-up. If fixed off-time regula-
tion with fast decay is used, select TOFF carefully, based
on the motor’s electrical characteristics.
During fast decay, the motor’s inductive current recircu-
lates through the external VDD supply, which charges up
the VDD bypass capacitor. Thus the voltage seen across
RSENSE is negative during the tOFF delay.
25% Ripple Regulation
25% ripple regulation is based on the H-bridge switching
to fast decay period until the motor current falls by 25%.
When IM reaches the regulation limit, the bridge enters
fast decay until the IM falls to 75% of the current limit. The
H-bridge polarity is then turned back to normal drive. Thus
the motor current ramps up and down between 75% and
100% of the set-point current.
25% ripple regulation eliminates t_OFF time tuning and the
TCOFF capacitor, allowing motors to be exchanged without
redesign.
Since 25% ripple regulation uses fast decay, the voltage
seen across RSENSE is negative during the time period
that the H-bridge polarity is reversed.
Select 25% ripple regulation mode by setting 0.5V <
VMODE < 1.0V. Leave TCOFF unconnected when 25%
ripple is used.
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 MAX14871 driver can generate more power than the
package for the device can safely dissipate. Total power
dissipation for the device is calculated using the following
equation:
PTOTAL = PDRIVER + PSW + PD
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