lm5015mhx National Semiconductor Corporation, lm5015mhx Datasheet - Page 17

• Manufacturer Part Number: lm5015mhx
• Description: High Voltage Monolithic Two-switch Forward Dc-dc Regulator
• Manufacturer: National Semiconductor Corporation

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 ampli-
fier is employed on the secondary side of the isolation bound-
ary. An opto-coupler is normally used to send the 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 10,
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
FIGURE 9. Conventional Isolated Feedback
17
signal across the isolation boundary, as shown in Figure 9.
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 3 is applicable
to the external, secondary-side error amplifier. The LM431A
shown in Figure 9 provides both the voltage reference and
error amplifier required to regulate the isolated output voltage.
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 ex-
cess voltage by limiting its V
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.
CE
to less than 50% of Vcc max.
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