Page 80 - CNC Robotics

Chapter 2 / Electronics

APPLICATI ON NOTE

Figure 11: Spikeson the senseresistor caused byreverse recoverycurrents andnoisecantrick the
currentsensingcomparator.

SYNC
/ - -
Motor / -
Current - ./
Expected
Actual

ever, in applications using monolithic bridge driv- DMOS drivers, like the L6202, the reverse recov-
ers, like the L29SN, internal parasitic structures ery current always flows through the senseresis-
often produce recovery current spikes similar in tor since the internal diode in parallel with the
nature to the diodereverse recovery current and lower transistor is connectedto the sourceof the
these mayflowthroughthe emitter lead ofthe de- DMOS device and notto ground.
vice and hence the sense resistor. When using In constant off time FM control circuits, like the

CALCULA TlNG POWER DISSIPATION IN BRIDGE DRIVER IC'S

The power dissipated in a monolithic driver IC like the L298N or L6202 is the
sum of three elements: 1) the quiescent dissipation, 2) the saturation losses
and 3) the switching losses.
The quiescent dissipation is basically the dissipation of the bias circuitry in the
device and can be calculaled as Vs · is where Vs is Ihe power supply vol/age
and Is is the bias current or quiscent current from the supply. When a device
has Iwo supply voltages, tike the L298N, the dissipation for each must be cal-
cual/ed then added to get the total quiescent dissipation. Generally the quies-
cent current for most monolithic IC's is constant over a vide range of input
voltages and the maximum value given on the data sheet can De used for
most supply vol/ages within the allowable range.
The saturation loss is basically the sum of the vol/age drops times the current
in each of the output transistors. for Bipolar devices, L298N, this is Vsat . I.
For DMOS power devices this is I . Rosan.
The third main component of dissipation is the switching loss associated with
the output devices. In general the switching loss can be calculatedas:
Vsupply . 1I0ad . tcross . fswitch
To cetcutete the total power dissipation these three compnents are each cal-
culated, mul/ipled by their respective duty cycle then added togther. Obviously
the duty cycle for the quiescent current is equal to 100%.

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