Datasheet

LMD18245
www.ti.com SNVS110E –APRIL 1998–REVISED APRIL 2013
(e) (f)
(g)
Figure 9. Chopper Amplifier Chopping States: Full V
CC
Applied Across the Winding (a) and (d), Shorted
Winding (b) and (e), Winding Current Decays During a Change in the Direction of the Winding Current (c)
and (f), and the Chopped Winding Current (g)
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
configuration (Figure 10 ).
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
• R is the sum of the series resistance in the current path including the winding resistance (1)
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 increases 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 becomes 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 essentially 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 11 ).
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