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L7250 Datasheet(PDF) 27 Page - STMicroelectronics

Part # L7250
Description  5V & 12V SPINDLE AND VCM MOTORS DRIVER
PDF  46 Pages
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Manufacturer  STMICROELECTRONICS [STMicroelectronics]
Direct Link  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

L7250 Datasheet(HTML) 27 Page - STMicroelectronics

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L7250
3
VOICE COIL MOTOR DRIVER
The VCM driver is configured as a transconductance amp, with an n-channel DMOS H-bridge power output,
current sense amp, error amp, and 15 bit linear DAC for command input. The power stage is a class AB voltage
amp. The error amp closes the transconductance loop around the power amp, using feedback from the current
sense amp. The VCM block is shown below.
Figure 7. VCM Driver Block Diagram
The current flowing into the voice coil is equal to:
Where Gs is the sensing opamp gain (programmable via serial port
Considering a typical application where Rf = 5.6k, Ri = 2.5k, Rs = 0.25
Ω and Gs = 4.5V/V we obtain a maximum
current equal to about 2A for 1V DAC output (Vin). The sense amplifier input range is about 0.55V. The power
stages assure this current requirement and they have a differential gain of 16.
The loop is compensated through the RC network Rc and Cc that cancels out the motor pole Lm/Rm.
This graphic shows the theoretic Gloop Bode diagram and put in evidence the second pole of the loop that is
strictly related to the error amplifier bandwidth.
Figure 8. Gloop
VM/2
VM/2
38
39
37
VM
VM
3/4
5/6
42
41
40
DAC 15
DACREF
AGND
VCMP
VCMN
DACREF
DACREF
DACREF
DACREF
VCM GND
SenseAmp
ErrorAmp
VCV
VCM
GND
43/44
45/46
52/53
Tristate
Tristate
S1
S2
1/2/54/55
POR
Rc
Cc
Ri
Rf
Gs
Gpow
Gpow
Rs
Rm
Lm
I
coil
R
ƒ
R
i
------
–
1
R
s
G
s
⋅
----------------- V
in
⋅⋅
=
F dt error
closed loop
R
c
R
f
------
1
R
c
C
c
⋅
-----------------
1
R
f
C
c
⋅
----------------
ω
t
R
f
R
i
⋅
R
c
R
f
R
i
+
()
⋅
---------------------------------
⋅
2G
pow
R
s
G
s
⋅⋅
R
s
R
m
+
--------------------------------------
1
R
f
C
c
⋅
----------------
⋅
Gloo p
A
0
R
i
R
i
R
f
+
-----------------
⋅
A
0
2G
po w
R
s
G
s
⋅⋅
⋅
R
s
R
m
+
-------------------------------------------------
R
i
R
i
R
f
+
-----------------
⋅
ω
t
R
i
R
i
R
f
+
-----------------
⋅



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