Datasheet

The speed n of a k pole motor results :
n =
10.435
C
T
kR
P
and becomes independent of the resistor RT
which only determines the current level and the
duty cycle which should be 1 : 1 at the nominal
speed for minimum torque ripple.
The second fast loop consists of a voltage to cur-
rent converter which is driven at pin 8 by the low
pass filter R
L
,C
L
. The output current at this stage
is injected by a PNP current mirror into the inner
resistor R
B
. So the driving voltage of the output
stage consists of the integrator output voltage
plus the fast loop voltage contribution across R
B
.
The power output stage realizes different modes
dependingon the logic status at pin 18 and 19.
- Normal operation for left and right mode :
each upper TR of the bridge is used as
voltage follower whereas the lower acts as
a switch.
- Stop mode where the upper half is open
and the lower is conductive.
- High impedance status where all power
elements are switched-off.
The high impedance status is also generated
when the supply voltage overcomes the 5 V to 20
V operating range or when the chip temperature
exceeds150 °C.
A short circuit protection limits the output current
at 1.5 A. Integrated diodes clamp spikes from the
inductive load both at V
CC
and ground.
The reference voltages are derived from a com-
mon bandgap reference. All blocks are widely
supplied by an internal 3.5 V regulator which pro-
vides a maximum supply voltage rejection.
PIN FUNCTION AND APPLICATION INFORMA-
TION
PIN 1
Trigger input. Receives a proper voltage which
contains the information of the motor speed. The
waveform can be derived directly by the motor
current (fig. 3). The external resistor generates a
proper voltage drop. Togetherwith the input resis-
tance at pin 1 [R
IN
(1) = 500 Ω ] the external ca-
pacitor C
D
realizea high pass filter which differen-
tiates the commutation spikes of the motor
current. The trigger level is 0V.
The biasing of the pin 1 is 20 mV with a hystere-
sis of 10 mV. So the sensing resistance must be
chosen high enough in order to obtain a negative
spike of the least 30 mV on pin 1, also with mini-
mum variation of motor current :
R
S
≥
30mV
∆I
MOT
min.
Such value can be too much high for the preregu-
lation stage V-I and it could be necessary to split
them into 2 series resistors R
S
=R
S1
+R
S2
(see
fig. 4) as explained on pin 8 section.
The information can be taken also from an exter-
nal tachogenerator.Fig. 5 shows various sources
connections:
the input signal mustn’t be lower than 0.7 V.
Figure 3.
Figure 4.
TDA7272A
6/16