LT3752 datasheet(22/48 Pages) LINER

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LT8311

8311f

For more information www.linear.com/LT8311

With SS charged up to 2V, the transconductance error

amplifier, A1, sinks or sources current from its output,

COMP, if there is any voltage difference between the FB pin

voltage and the 1.227V reference. The COMP pin, offset

by 0.9V, serves as the input to the opto-driver, A2. If an

increase in output load current causes the FB pin voltage

to be lower than 1.227V, A1 drives the COMP pin high.

COMP going high forces A2 to drive OPTO low, sourcing

less current through RD into the opto-coupler.

Since an opto-coupler’s output current is directly propor-

tional to its input current, this decreased input current for

theopto-couplerwillcauseitsoutputcurrent,andtherefore

its emitter voltage at RE, to decrease as well. The drop

in RE voltage causes A3, through its inverting action, to

drive its output, VC, higher. An increase in the VC voltage

causes the comparator, A4, to command a higher sense

voltage across the RSNS resistor, commanding M1 to run

at a higher peak current. Since the current through M1 is

Figure 9. Flowchart for LT8311 Opto Control Operation at Start-Up

COMP < 2.2V

SS PULL-DOWN AMPLIFIER ENABLED

OPTO-DRIVER ACTIVATION

8311 F09

1. ERROR AMP DISABLED: COMP PIN VOLTAGE CHARGED UP TO COMP HI CLAMP = 2.2V; tRISE ∼ 1.1 • 10kΩ • CC

2. SS PULL-DOWN AMPLIFIER DISABLED

3. SS PULL-UP AMPLIFIER ACTIVATED. THIS AMPLIFIER ONLY HAS SOURCING CAPABILITY (1mA SLEW CURRENT), AND

WILL DRIVE SS PIN VOLTAGE CLOSE TO FB PIN VOLTAGE (VFB – VSS ≅ 16mV)

4. SS 10µA CHARGE CURRENT ACTIVATED

5. OPTO-DRIVER DISABLED: OPTO PIN VOLTAGE HELD AT 0V

OPTO-DRIVER DEACTIVATION

VIN > 3.7V

1. SS PULL-UP AMPLIFIER DISABLED

2. SS PULL-DOWN AMPLIFIER ENABLED: THIS AMPLIFIER ONLY ACTIVATED WHEN FB PIN VOLTAGE IS LESS THAN 50%

OF FB REFERENCE VOLTAGE. THIS AMPLIFIER ONLY HAS SINKING CAPABILITY (12mA SLEW CURRENT) AND WILL

DRIVE SS PIN VOLTAGE TO BE NO HIGHER THAN 90mV ABOVE FB

1. ERROR AMP ENABLED: ERROR AMP CAN NOW DRIVE COMP BASED ON COMPARING FB VOLTAGE WITH SS VOLTAGE

OR 1.227V REFERENCE

2. OPTO-DRIVER ENABLED: OPTO-DRIVER CAN NOW DRIVE OPTO PIN AS A FUNCTION OF COMP PIN VOLTAGE

SS > FB – 16mV

YES

OPERATION

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