L6238S datasheet(23/31 Pages) STMICROELECTRONICS

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Coff to discharge to dischargte through Rslew, initi-

ating the Constant-OFF time-out. When the volt-

age on Coff reaches 1.2V, comparator A1switches

state toggling the latch in the opposite state, turn-

ing the upper driver back ON. SW1 also closed

quickly charging up Coff for the next cycle.

5.1 PWM Design Considerations

In order to select the parameters associated with

PWM operation, the following factors must be

taken into consideration:

1. PWM Switching Frequency

2. Duty Cycle

3. Motor Currents

4. Minimum ON Time

5. Noise Blanking

6. Bemf Masking/Sampling

5.1.1. PWM Switching Frequency

The PWM switching frequency Fpwm is found

from:

Fpwm =

Ton

+ Toff

(5.1.1)

where:

Ton = The time required for the motor current

to reach the commanded level.

Toff = The programmed OFF time.

The two main considerations for this parameter

are the minimum and maximum switching fre-

quency.

The maximum switching frequency occurs during

the Start-up and should be kept below 50KHz due

tointentional bandwidth limitations and output

switching losses.

5.1.2 Duty Cycle

Besides reducing the power dissipation of the

controller output stage, running in PWM offers 2

additional ”free” benefits:

A. Reduced Powe Supply Current at Start Up

B. Lower Power Rating for the Motor Current

Sense Resistor.

Figure 5-2 is the current path during the ON time

of a phase period. The current from the supply

passes through the upper sourcing DMOS, Q3

transistor through the two driven winding, the

lower DMOS, Q2 and finally through the current

sensing resistor Rsns. Since both Q3 and Q4 are

ON, while Q3 is turned OFF. The voltage, causing

the current to continue to flow through Q2, and

Q4.

If the duty cycle is nearor at 50%, then for 1/2 the

PWM cycle, no current is flowing from the power

supply or the sense resistor while current is still

flowing in the motor. This lowers the requirement

for both the Power Supply and the Power Rating

for the sensing resistor.

5.1.3 Motor Currents

Note: It is not the objective of this section to describe the principles

of brushless DC motor, but to provide sufficient information about the

parameters associated with PWM operation in order to optimize an

application.

A simplified model of a motor is shown in figure 5-

4. For this discussion, lower order effects due to

mutual inductance between windings, resistance

due to losses in the magnetic circuit, etc. are not

shown.

The motor at stall is equal to a resistance, Rmtr,

in series with an inductance, Lmtr. When the mo-

tor is rotating, there is an induced emf that ap-

pears across the armaure terminals and is shown

in figure 5-4 as an internally generated voltage

Ibemf), Eg.

OUTPUTA

OUTPUTB

VPOWER

RSENSE

RSNS

D95IN319

Figure 5-2

OUTPUTA

OUTPUTB

VPOWER

RSENSE

RSNS

D95IN320

Figure 5-3

L6238S

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