Datasheet

Error Amplifier
(Internal or external)
V
REF
V
O
R
O1
R
O2
V
REF
=
V
O
- V
REF
R
O2
R
O1
FB
COMP
LM5015
CFB
VCC
1k
LMV431A
1k
Current
Mirror
PWM
1.26V
5k
VDD
1.3V
EA
V
OUT
R
O1
R
O2
LM5015
www.ti.com SNVS538C –NOVEMBER 2007–REVISED APRIL 2013
Figure 22. High bandwidth Isolated Feedback Using CFB Pin
The two external pull-up and pull-down resistors are effectively connected in parallel for ac signal, yielding a
collector resistance on the opto-coupler of 0.5 kΩ. This resistance is 10 times smaller than the internal pull up
resistor and the pole associated with the collector-base capacitance of the opto-coupler transistor is pushed out
to a decade higher frequency. Moving the opto-coupler pole to a higher frequency allows higher loop bandwidth
capability than conventional isolated feedback designs (see Figure 21).
OUTPUT VOLTAGE
Output voltage is normally set with a resistor divider shown in Figure 23 . To set the regulator’s output voltage to
V
O
, the two resistors must satisfy the following equation:
where
• V
REF
is the reference voltage of the error amplifier, which is 1.26V for the LM5015’s internal error amplifier (5)
Figure 23. Output Voltage Setting
In isolated designs, the reference and error amplifier are located on the secondary side. To obtain a 5V output in
an isolated feedback design using LMV431A (whose nominal V
REF
is1.24V), the feedback divider resistors ratio
specified by above equation is 0.330. Selecting 24.3 kΩ for R
O1
, then R
O2
should be 8.06 kΩ. In a similar non-
isolated converter design using the internal error amplifier, the divider’s resistors ratio is 0.337, therefore if 24.3
kΩ is selected for R
O1
, then R
O2
should be 8.20 kΩ.
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