AN1451 datasheet(25/43 Pages) STMICROELECTRONICS

25 / 43 page

25/43

AN1451 APPLICATION NOTE

Figure 26. Thermal Data inputs and results.

2.14 Choosing the Decay Mode

L6208 can operate in either fast or slow decay mode, each having a specific recirculation path for the current

during off-time. In slow decay mode only the lower DMOS is turned off and the current recirculates around the

upper half of the bridge so that voltage across the coil is essentially 0. In Fast decay mode both DMOS are

turned off and the current recirculates back to the power supply rail so that voltage across the coil is essentially

power supply voltage itself.

Slow decay operation provides several advantages: for a given peak current and off-time, current ripple is min-

imized, and the same is true for acoustic noise and losses in the motor iron (achieving the same current ripple

with fast decay mode would require a shorter off-time resulting in a higher switching frequency and higher power

dissipation in the IC). As current recirculates in the upper half of the bridge and both the high side DMOS in the

same bridge are on,

synchronous rectification is realized, minimizing power dissipation in the power switches.

Also, as no output pin goes below GND (see

Supply Voltage Ratings and Operating Range section), no power

is dissipated on the sense resistor during the off-time (see

Sensing Resistors section).

On the other hand, slow decay can be undesirable in some situations, for example when current has to be reg-

ulated at very low values or motor winding L / R ratio is high. In these cases an on-time shorter than the minimum

tON (about 1.5µs) may be requested to regulate the current, and this can cause the PWM controller to loose the

regulation (refer to the

Programmable off-time Monostable section).

Another situation where fast decay is to be preferred to slow decay is with regulated current expected to vary

over time with a given profile (enforced providing a variable voltage on the Vref pins, see also Microstepping

section). Here fast decay helps following fast decreasing edges in the desired profile.

Input D ata

P ackage

SO 24

Copper Area

4.0

1÷10 sq. cm

Copper A rea is on

S am e side of the device

G round Layer

N/A

Am bient Tem perature

-25 ÷ 100 ºC

R esults

Therm al Resistance

Junction to A m bient

53.36

ºC / W

Therm al Resistance

Junction to P ins / Slug

14.00

ºC / W

Estim ated Junction

Tem perature

122.66

ºC

Estim ated

Pins / Slug Tem perature

103.60

AN1451 Datasheet(PDF) 25 Page - STMicroelectronics

AN1451 Datasheet(HTML) 25 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43

Datasheet Download

Link URL