L6223 datasheet(19/33 Pages) STMICROELECTRONICS

19 / 33 page

2) Back EMF (BEMF) equal to 80% of its peak

during the phase change and equal to 50% of its

peak during the chopping period.

3) Constant slope of the current during tON,tOFF

and for power calculation during the phase

change (See t1 in Fig. 27).

4) Current imbalance supposed to be zero.

5) Current ripple during the chopping neglegible.

As was previously stated, the current chopping is

obtained by means of one PWM Loop that con-

trols the charge time tON of the inductance of the

windings, A and B for example in fig.22.

This time starts each clock pulse and stops when

Q5 is switched OFF because of the condition:

Vref =2 RSIp.

A factor 2 is required because the single sensing

resistor RS is crossed by the peak current Ip flow-

ing through each of the two energized windings

(A of MA; B of MB).

This configuration can produce an imbalance be-

tween the two peak currents because at the

phase change the BEMF of one winding (MA) can

be out of phase with respect to the BEMF of the

other one (MB); in addition, an imbalance may

also occur at the phase change when the Power

Supply Voltage selected is too low and/or when

one motor is driven with too large Lm/Rm ratio.

Nevertheless in most of the applications the dissi-

pated power is not increased and there is no sig-

nificant change in torque.

During tON the current Ip, flowing through the

phase A (seg. Fig. 24), is defined by VON

VON

≅ VS -Rtot Ip - BEMF

where Rtot =RS +Rm +RDSON tot

in which Rm is the winding resistance of the

phase A and RDSON tot is the sum of the RDSON of

Q1 and Q5: Rm and BEMF are not shown on the

Figure 24.

At the end of tON, the current starts its slow decay

and jumps to Ip/2 (see Fig. 25) since the total in-

ductance becomes four times Lm (perfect cou-

pling) that is the inductance of the phase A alone.

The recirculation time tOFF is defined by:

VOFF

≅ 2BEMF + IP (Rm +RDSONQ1)

since RDSONQ1 =RDSONOQ2.

The current through Q1 is shown in Fig. 26: the

current ripple is on lp and IP/2 during tON and tOFF

respectively. It can be obtained the Duty Cycle:

DC = VOFF / (2VON +VOFF)

since 2VON tON =VOFF tOFF

The slow decay allows a small current ripple as

earlier It is considered equal to zero. The current

through the phases A and B can be seen in Fig.

27 where the InA and InB signals (see Fig. 22)

are shown as well.

These two signals are 90

° out of phase with each

other and they are 180

° out of phase with the cor-

responding inputs of the IC. In A and In B are not

shown in the Figure.

During the time Tp the motor goes through four

steps and the rotation speed Vrot (step/sec) can

be given by:

Vrot = 4/Tp.

By considering what was stated above, the follow-

ing can be applied:

1) Dissipated power by the 4 sink power DMOS

(Q1 to Q4).

PdL

≅ 4RDSONQ1 Ip







+ 



+ T







+ DC







2) Dissipated power by Q5 (PdH).

PdH

≅ 4RDSONQ5Ip2







+ T1







− 4DC









where the phase change duration is:

− Lm

Rtot

loge







−

2Rtot Ip

− 1.6 BEMF + Rtot IP







The chopping produces little power dissipation.

It’s value can be approximated by:

3) Pdch

≅ 8 ⋅ 10-3 Vs Ip

The sum of 1) + 2) + 3) gives the dissipated

power of the output stage. To obtain the total

amount of dissipated power it’s necessary to in-

clude the power dissipation produced by the qui-

escent currents IS (from the power stage) and ISS

(from the Logical circuits):

Pdo =VS IS +VSS ISS,

considering IS constat versus VS. Finally:

Ptot = PdL + PdH + Pdch + Pdo

Example

Supply Voltage

VS = 36V

Logic Voltage

VSS =5V

Peak current (per phase)Ip = 0.7A

Motor resistance

Rm =9

Ω

Motor inductance

Lm = 6mH

at Tamb =50

°C

Rotation speed

Vrot = 500 step/sec (const)

Peak of the BEMF

BEMF = 1 Vp

Max ambient temperature Tamb =50

°C

Max junction temperature Tj = 125

°C

From the Electrical Characteristics of the L6223

(Typical value):

Internal Reference Voltage Vref = 0.5V

Sink DMOS RDSON

RDSON L = 1.2

Ω

Source DMOS RDSON RDSON H = 0.7

Ω Tj= 25°C

Power Supply Current

IS = 4 mA

Worst

Logic Supply Current

ISS = 20 mA

Case

From Fig. 3 (see pag. 6) the following is obtained:

α ≅ 1.65 at Tj = 125°C.

The DMOS ON-Resistances become (worst case):

L6223

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