Datasheet

LT4430
16
4430fc
For more information www.linear.com/LT4430
applicaTions inForMaTion
Figure 6a. Frequency Compensation with Opto-Coupler Common-Emitter Configuration
–
+
V
CC
V
REF
V
C
R
C
C
C
PRIMARY-SIDE
ERROR AMP
FB
ISOLATION
BARRIER
–
+
OPTO
DRIVER
OPTO
LT4430
COMP
1.1V
0.6V
–
+
ERROR
AMP
R3
R4
15k
R5
90k
4430 F06a
V
OUT
FB
C3
R1
R2
C1
C2
R
K
OPTO
C
K
Figure 5. Setting Output Voltage
4430 F05
V
OUT
75nA
FB
R2
R1
Setting Output Voltage
Figure 5 shows how to program the power supply output
voltage with a resistor divider feedback network. Connect
the top of R1 to V
OUT
, the tap point of R1/R2 to FB and
the bottom of R2 directly to GND of the LT4430. The FB
pin regulates to 600mV and has a typical input pin bias
current of 75nA flowing out of the pin.
The output voltage is set by the formula:
V
OUT
= 0.6V • (1 + R1/R2) – (75nA) • R1
Opto-Coupler Feedback and Frequency Compensation
An isolated power supply with good line and load regula-
tion generally
employs the following strategy. Sense and
compare the output voltage with an accurate reference
potential. Amplify and feed back the error signal to the
supply’s control circuitry to correct the sensed error. Have
the error signal cross the isolation barrier if the control
circuitry resides on the primary-side. Coupling this signal
requires an element that withstands the isolation potentials
and still transfers the loop error signal.
Opto-couplers remain in prevalent use because of their
ability to couple DC signals. Opto-couplers typically con-
sist of
an input infrared light emitting diode (LED) and an
output
phototransistor separated by an insulating gap.
Most
opto-coupler data sheets loosely specify the gain,
or current transfer ratio (CTR), between the input diode
and the output transistor. CTR is a strong function of the
input diode current, temperature and time (aging). Ag
-
ing degrades
the LED’s brightness and accelerates with
higher operating current. CTR variation directly affects the
overall system loop gain and the design must account for
total variation. To make an effective optical detector, the
output transistor design maximizes the base area to col
-
lect light
energy. This constraint yields a transistor with a
large
collector-to-base capacitance. This capacitance can
influence the circuit’s performance based on the output
transistor’s hookup.
The two most common topologies for the output tran
-
sistor of
the opto-coupler are the common-emitter and
common-collector
configurations. Figure 6a illustrates
the common-emitter design with the output transistor’s
collector connected to the output of the primary-side
controller’s error amplifier.