Datasheet

PWM
FB
1.26V
5k
COMP
VDD
1.3V
V
OUT
LM5015
EA
LMV431A
AGND
R
O1
R
O2
LM5015
SNVS538C –NOVEMBER 2007–REVISED APRIL 2013 www.ti.com
CONVENTIONAL ISOLATED OUTPUT FEEDBACK DESIGNS
When isolation between the primary and secondary circuits is required, the internal error amplifier is usually
disabled by connecting the FB pin to AGND, and an external error amplifier is employed on the secondary side of
the isolation boundary. An opto-coupler is normally used to send the feedback signal across the isolation
boundary, as shown in Figure 21. The LM5015 provides built-in pull-up for the collector of the opto-coupler at the
COMP pin through a 5 kΩ internal resistor. Similar compensation network used in Figure 15 is applicable to the
external, secondary-side error amplifier. The LM431A shown in Figure 21 provides both the voltage reference
and error amplifier required to regulate the isolated output voltage.
Figure 21. Conventional Isolated Feedback
HIGH BANDWIDTH ISOLATED FEEDBACK DESIGN USING THE CFB PIN
The LM5015 also includes a current mirror circuit for optional high bandwidth feedback loop design. As shown in
Figure 22, the emitter of the opto-coupler transistor can be connected to the CFB pin, and the collector
connected to VCC through an external 1 kΩ resistor. The 1 kΩ resistor to VCC is used to protect the CFB pin by
limiting the opto-coupler current to less than 10 mA. The 1 kΩ resistor from the opto-coupler collector to ground
is introduced to protect the opto-coupler from excess voltage by limiting its V
CE
to less than 50% of Vcc max.
When the output voltage is below regulation, no current flows into the CFB pin and the PWM of the LM5015
operates at maximum duty cycle.
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