L6238S View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L6238S' PDF : 31 Pages View PDF
L6238S
sufficient response, commensurate with the
switching speed of the output drivers. At higher
frequencies the switching losses inherent in the
drivers start to negative any of the power dissipa-
tion savings gained with PWM operation.
The current sense amplifier has a minimum slew
rate of 0.31V/µs. With a worst case Motor peak
start-up current of 2.5A and Sense Resistor of
0.33, the resultant Rsense voltage would be equal
to 825mV. With a minimum gain of 3.8V/V, the
CSA output voltage would have to slew to 3.14V.
Therefore it would require approximately 10µs for
the output voltage to reach the required com-
manded level.
If an ON time were selected that was less than
this time, the motor current would overshoot the
desired level resulting in incorrect current control
possibly exceeding the output capabilities of the
drivers.
5.1.5 Noise Blanking
Referring to Figure 5-8, when operating with
lower levels of current (i.e. < 700mA, with Rsense
= 0.33Ω), the possibility exiss where the noise
due to output Turn-ON can exceed the Com-
manded Current Level causing prematire Turn-
OFF.
In order to provide noise immunity from this
switching noise, a blanking circuit automatically
rejects any signal appearing at the output of the
CSA for a 3µs period.
Figure 5-9
Figure 5-9 is an additional block diagram of the
PWM control loop including the noise blanking cir-
cuit. The output of A3 goes high when ever the
voltage at the CSA input is more positive then the
Control Voltage.
This is the case when either the motor current or
the turn-ON transient has reached the com-
manded level. The output of A3 is gates by N11.
In order to provide a blanking period, Q1 is turned
Figure 5-8
3µs BLANKING PULSE
COMMANDED
CURRENT LEVEL
Vrsense
D95IN325
1
10µs/DIV
N1
Q1
C1
N6
8pF
I1
5µA
N7
N8
DELAY
N9
N10
N11
CLK_BEMF
N2
N3
TO OUPUT
DRIVERS
N4
N5
PWM/LIN
I2
20µA
Q2
N12
RUN/BRAKE
2.4V
SW1
N12
Q3
+
A2
- 1.2V
X4
+
A3
-
D95IN326
PWM_SLEW
R1
C3
CSA INPUT
VCONTROL
PWM COMP
PWM TIMER
C4
26/31
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