LT4430HS6 LINER [Linear Technology], LT4430HS6 Datasheet - Page 16

• Manufacturer Part Number: LT4430HS6
• Description: Secondary-Side Opto-Coupler Driver Ground-Referenced Opto-Coupler Drive
• Manufacturer: LINER [Linear Technology]

LT4430
APPLICATIONS INFORMATION
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
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 fl owing out of the pin.
The output voltage is set by the formula:
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
16
V
OUT
= 0.6V • (1 + R1/R2) – (75nA) • R1
V
REF
FB
PRIMARY-SIDE
Figure 5. Setting Output Voltage
ERROR AMP
–
+
OUT
FB
Figure 6a. Frequency Compensation with Opto-Coupler Common-Emitter Confi guration
, the tap point of R1/R2 to FB and
75nA
4430 F05
V
C
V
OUT
R1
R2
C
C
V
CC
ISOLATION
BARRIER
R
C
R
OPTO
K
C
K
OPTO
90k
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
infl uence 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 confi gurations. Figure 6a illustrates
the common-emitter design with the output transistor’s
collector connected to the output of the primary-side
controller’s error amplifi er.
R5
DRIVER
OPTO
+
–
1.1V
15k
R4
LT4430
COMP
C3
ERROR
C2
AMP
+
–
R3
FB
0.6V
4430 F06a
V
OUT
R1
R2
C1
4430fb