AN904 Datasheet, PDF (2/3 Pages) STMicroelectronics – MAGNETIC AMPLIFIER WITH LPR30 CONTROLLER

AN904 APPLICATION NOTE
Figure 2.
FEEDBAK
V+
CR2
+
MAIN Vout
-
PWM
DRIVER
Figure 3.
SR
CR3
CR1
+6
L4
CR2
Rsense
8
+
VIN
1 ^LPR30
3 LIM
CURR
LM
2 LIM+
7 ERR
ERR
AM P
6 ERR+
RE F
5
GND
4 VREF
+
AUXILIARY Vout
-
V+
CURRENT
SR
CONTROL
CR1
L4
Rsense
+
CR2
Vout
+6
-
PWM
CONTROLLER
CR3
CR4
8
+
VIN
1 ^LPR30
3 LIM
C URR
LM
2 LIM+
7 ERR
ER R
AMP
6 ERR+
4 VREF
REF
5
GND
LPR30 CONTROLLER
Using the LPR30 like controller (as shown in fig.2) , in the PWM (Auxiliary Outputs) the pulse width is
controlled by sensing the output voltage comparing it (pin 6) to the internal reference (pin 4) and using
the Error signal to adjust the pulse duration. Without any MAG AMP plus controller, in the Auxiliary cir-
cuits the outputs would be " semi - regulated ", since the primary control loop would provide line regula-
tion. But the other output would vary with load and temperature.
Besides, the LPR30 controller plus MAG AMP is particularly advantageous when independent current
limiting for the auxiliary outputs is desired, due to the fact the LPR30 is able to monitor even the current.
It is always active and together with MAG AMP provides an exceptionaly smooth transition as the output
loaded beyond the current limit and then returned to normal load condition.
To do this job, the LPR30 uses its second internal comparator ( CURRENT LIMIT AMPLIFIER , pin 2
and pin 3 see fig.3) that has an artificial offset of 50mV.
This means that when the voltage to the RSENSE is more than 50mV, the CURRENT LIMIT AMPLIFIER
recognises that the current is more than the nominal one generating then an error signal that is applied
to the MAG AMP RESET TRANSISTOR.
The increase in reset current decreases the pulses width at the output of the MAG AMP and thus op-
poses the increase in current which was sensed by RSENSE .
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