LT4430HS6 LINER [Linear Technology], LT4430HS6 Datasheet - Page 9

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

APPLICATIONS INFORMATION
Block Diagram Operation
A precision voltage reference, a high-bandwidth error
amplifi er, an inverting opto-coupler driver and an overshoot
control amplifi er comprise the LT4430. Referring to the
block diagram, a start-up circuit establishes all internal
current and voltage biasing for the IC. A precision-trimmed
bandgap generates the 600mV reference voltage and a
1.1V bias voltage for the opto-coupler driver. Room tem-
perature reference voltage accuracy is specifi ed at ±0.75%
and operating temperature range tolerance is specifi ed at
±1.25%. The 600mV reference ties to the noninverting
input of the error amplifi er.
The LT4430 error amplifi er senses the output voltage
through an external resistor divider and regulates the
FB pin to 600mV. The FB pin ties to the inverting input
of the error amplifi er. The error amplifi er’s open loop DC
gain is 80dB and its unity-gain crossover frequency of
9MHz provides negligible phase shift at typical feedback
loop crossover frequencies. The error amplifi er is a true
voltage-mode amplifi er and frequency compensation con-
nects around the amplifi er. Typical LT4430 compensation
schemes use series R-C in parallel with C networks from
the COMP pin to the FB pin.
The opto-coupler driver amplifi es the voltage difference
between the COMP pin and the 1.1V bias potential applied
to its noninverting terminal with an inverting gain of 6. This
signal drives the opto-coupler referenced to GND. Driving
the opto-coupler referenced to GND accommodates low
output voltages and simplifi es loop frequency compen-
sation as the secondary feedback path with a traditional
“431” topology is eliminated. A resistor in series with the
opto-coupler sets the opto-coupler’s DC bias current. The
opto driver amplifi er sources a guaranteed maximum of
10mA, sinks 350μA typically and is short-circuit protected.
The opto-coupler driver amplifi er’s typical –3dB bandwidth
is 600kHz. The opto-coupler’s output crosses the galvanic
isolation barrier and closes the feedback loop to the pri-
mary-side controller.
The LT4430 incorporates a unique overshoot control
function that allows the user to ramp the output voltage
on start-up and recovery from short-circuit conditions,
preventing overshoot. A capacitor, connected from the OC
pin to GND and charged by internal 8.5μA current source
I
OC capacitor low until V
cal undervoltage lockout threshold of 2.2V. Q1 then turns
off and the OC capacitor charges linearly. Q2 and Q3 OR
the OC pin voltage and the 600mV reference voltage at
the noninverting terminal of the error amplifi er. The OC
pin voltage is the reference voltage for the error amplifi er
until it increases above 600mV. If the feedback loop is in
control, the FB pin voltage follows and regulates to the OC
pin voltage. As the OC pin voltage increases past 600mV,
the reference voltage takes control of the error amplifi er
and the FB pin regulates to 600mV. The OC pin voltage
increases until it is internally clamped by R2, Q6 and V1.
The OC pin’s typical clamp voltage of 0.93V ensures that
Q3 turns off. All of I1’s current fl ows in Q2, matching I2’s
current in Q4.
In a short-circuit condition, the output voltage decreases to
something well below the regulated level. The error ampli-
fi er reacts by increasing the COMP pin voltage, thereby
decreasing the drive to the opto-coupler. The decreased
opto-coupler bias signals the primary-side controller to
increase the amount of power it delivers in an attempt to
raise the output voltage back to its regulated value. As
long as the fault persists, the output voltage remains low.
The error amplifi er’s COMP pin voltage increases until it
reaches a clamp level set by Q7 and V2. Q7’s resultant
collector current drives internal logic that closes normally
open switch S1. This action activates the overshoot control
amplifi er which employs a unity-gain follower confi gura-
tion. The overshoot control amplifi er monitors the FB pin
voltage and, on S1’s closing, pulls the OC pin voltage
down to the FB pin voltage plus a built-in offset voltage
of typically 48mV. The built-in offset voltage serves two
purposes. First, the offset voltage prevents the overshoot
control amplifi er from interfering with normal transient
operating conditions. Second, the offset voltage biases
the feedback loop so that if the short-circuit condition
ends, the feedback loop immediately starts to increase
the output voltage to its regulated value.
OC
, sets the ramp rate. On start-up, Q1 actively holds the
IN
of the LT4430 reaches its typi-
LT4430
4430fb
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