LMD18245_06 Datasheet, PDF (8/21 Pages) National Semiconductor (TI) – 3A, 55V DMOS Full-Bridge Motor Driver

Functional Descriptions (Continued)
THE LMD18245 CHOPPER AMPLIFIER
The LMD18245 incorporates all the circuit blocks needed to
implement a fixed off-time chopper amplifier. These blocks
include: an all DMOS, full H-bridge with clamp diodes, an
amplifier for sensing the load current, a comparator, a
monostable, and a DAC for digital control of the chopping
threshold. Also incorporated are logic, level shifting and drive
blocks for digital control of the direction of the load current
and braking.
THE H-BRIDGE
The power stage consists of four DMOS power switches and
associated body diodes connected in an H-bridge configura-
tion (Figure 2 ).
The time constant to charge or discharge any inductor, in this
case the motor windings, is defined as:
τ = L/R
where L is the winding inductance, and R is the sum of the
series resistance in the current path including the winding
resistance.
Turning ON a source switch and a sink switch in opposite
halves of the bridge forces the full supply voltage less the
switch drops (I x R_DS(ON)) across the motor winding. While
the bridge remains in this state, the winding current in-
creases exponentially towards a limit dictated by the supply
voltage, the switch drops (I x R_DS(ON)), and the winding
resistance. However, the winding current exponential rate of
increase will end when the current chopping circuitry be-
comes active.
Subsequently turning OFF the sink switch causes a voltage
transient that forward biases the body diode of the other
source switch. The diode clamps the transient at one diode
drop above the supply voltage and provides an alternative
current path. While the bridge remains in this state, it essen-
tially shorts the winding, the winding current recirculates and
decays exponentially towards zero at a rate that is defined
by the L/R time constant.
During a change in the direction of the winding current, both
the switches and the body diodes provide a decay path for
the initial winding current (Figure 3 ).
During actual motor operation there are many variables that
can effect the motor winding magnetic behavior and perfor-
mance. Resonance, eddy currents, friction, motor loading,
damping, temperature coefficients of the windings, are only a
few. These are all issues that are beyond the scope of the
this data sheet.
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