Datasheet

V
PWR
LM5026
CS
OUTA
AGND
V
IN
OUTB
R
F
C
F
Q1
Q2
R1
Power
Transformer
V
PWR
LM5026
CS
OUTA
AGND
V
IN
OUTB
R1
R
F
C
F
Power
Transformer
Current
Sense
Q1
Q2
LM5026
www.ti.com SNVS363D –AUGUST 2005–REVISED APRIL 2013
The SYNC pin can be used to synchronize the internal oscillator to an external clock. An open drain output is the
recommended interface from the external clock to the SYNC pin. The clock pulse width should be greater than
15 ns. The external clock must be a higher frequency than the free running frequency set by the RT resistor.
Multiple LM5026 devices can be synchronized together simply by connecting the devices SYNC pins together.
Care should be taken to ensure the ground potential differences between devices are minimized. In this
configuration all of the devices will be synchronized to the highest frequency device.
VOLTAGE FEEDBACK (COMP)
The COMP pin is designed to accept the voltage loop feedback error signal from the regulated output via an
error amplifier and (typically) an optocoupler. In a typical configuration, VOUT is compared to a precision
reference voltage by the error amplifier. The amplifier’s output drives the optocoupler, which in turn drives the
COMP pin. The parasitic capacitance of the optocoupler often limits the achievable loop bandwidth for a given
power converter. The optocoupler LED and detector junction capacitance produce a low frequency pole in the
voltage regulation loop. The LM5026 current controlled optocoupler interface (COMP) previously described,
greatly increases the pole frequency associated with the optocoupler.
CURRENT SENSE (CS)
The CS pin receives an input signal representative of the transformer primary current, either from a current sense
transformer (Figure 31) or from a resistor in series with the source of the primary switch (Figure 32). In both
cases the sensed current creates a ramping voltage across R1, while the R
F
/C
F
filter suppresses noise and
transients. R1, R
F
and C
F
should be as physically close to the LM5026 as possible, and the ground connection
from the current sense transformer, or R1, should be a dedicated track to the AGND pin. The current sense
components must provide >0.5V at the CS pin when an over-current condition exists.
Figure 31. Current Sense Using a Current Sense Transformer
Figure 32. Current Sense Using a Source Sense Resistor (R1)
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