Philips Semiconductors
Product specification
SMPS control circuit
SG3524
One final point on the compensation terminal is that this is also a
convenient place to insert any programming signal which is to
override the error amplifier. Internal shutdown and current limit
circuits are connected here, but any other circuit which can sink
200µA can pull this point to ground, thus shutting off both outputs.
While feedback is normally applied around the entire regulator, the
error amplifier can be used with conventional operational amplifier
feedback and is stable in either the inverting or non-inverting mode.
Regardless of the connections, however, input common-mode limits
must be observed or output signal inversions may result. For
conventional regulator applications, the 5V reference voltage must
be divided down as shown in Figure 8. The error amplifier may also
be used in fixed duty cycle applications by using the unity gain
configuration shown in the open-loop test circuit.
V
REF
5k
R
2
POSITIVE
OUTPUT
VOLTAGES
2
1
+
–
5k
GND
R
1
V
REF
R
1
5k
2
1
5k
GND
R
2
+
–
NEGATIVE
OUTPUT
VOLTAGES
Current Limiting
The current limiting circuitry of the SG3524 is shown in Figure 9.
By matching the base-emitter voltages of Q1 and Q2, and assuming
a negligible voltage drop across R
1
:
Threshold=V
BE
(Q1)+I
1
R
2
-V
BE
(Q2)
=I
1
R
2
≅
200mV
Although this circuit provides a relatively small threshold with a
negligible temperature coefficient, there are some limitations to its
use, the most important of which is the
±1V
common-mode range
which requires sensing in the ground line. Another factor to consider
is that the frequency compensation provided by R
1
C
1
and Q1
provides a roll-off pole at approximately 300Hz.
Since the gain of this circuit is relatively low, there is a transition
region as the current limit amplifier takes over pulse width control
from the error amplifier. For testing purposes, threshold is defined as
the input voltage required to get 25% duty cycle with the error
amplifier signaling maximum duty cycle.
In addition to constant current limiting, Pins 4 and 5 may also be
used in transformer-coupled circuits to sense primary current and to
shorten an output pulse, should transformer saturation occur.
Another application is to ground Pin 5 and use Pin 4 as an additional
shutdown terminal: i.e., the output will be off with Pin 4 open and on
when it is grounded. Finally, foldback current limiting can be
provided with the network of Figure 10. This circuit can reduce the
short-circuit current (I
SC
) to approximately one-third the maximum
available output current (I
MAX
).
9
SL00181
Figure 8. Error Amplifier Biasing Circuits
RAMP
t
1
R
1
ERROR
AMPLIFIER
C
1
R
1
Q1
Q2
COMPARATOR
5
–
SENSE
+
4
SL00182
Figure 9. Current Limiting Circuitry of the SG3524
V
O
= 5V
S
A
/S
B
R
1
R
2
R
S
–
SENSE
+
5
4
I MAX
+
+
1
R
S
V
)
V 0R 2
R1
)
R2
TH
I
V TH
R
S
where
SC
NOTE:
V
TH
= 200mV
Foldback current limiting can be used to reduce power dissipation
under shorted output conditions.
SL00183
Figure 10. Foldback Current Limiting
1994 Aug 31
5
PHILIPS [ NXP SEMICONDUCTORS ]
