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LT8311

8311f

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directly proportional to the output inductor current (M1

Current • NP/NS= ILOUT), an increase in M1’s peak current

translates into an increase in the output inductor’s peak

current. In essence, the feedback loop is commanding

the output inductor peak current to meet the demands

of the increased load current, with the ultimate goal of

helping the output voltage recover from a load step and

stay regulated.

Opto-Control Operation at Start-Up

For applications connecting the LT8311’s VIN pin directly

to the converter output, the LT8311 includes intelligent

circuitry to ensure no interruption in the switching of

the primary-side MOSFET upon the LT8311’s turn-on.

The LT8311 turns on when its VIN pin (and therefore the

converter output voltage when VIN is directly connected to

the output) exceeds 3.7V. Without intelligent circuitry, this

VOUT level will cause the FB pin voltage of the LT8311 to

be greater than the voltage on the LT8311’s SS pin (which

is typically at 0V upon turn-on of the IC), causing ampli-

fier A1 to drive the COMP pin low. This drives the OPTO

pin high, which causes full current into the opto-coupler

and terminates switching of the primary-side MOSFET.

Termination of the primary-side MOSFET’s switching can

lead to the converter’s output voltage dropping, which

could cause the LT8311 to lose power and shut off. The

LT8311’s intelligent circuitry prevents this situation using

two unique features. It has a built-in 100mV hysteresis on

Figure 10. Opto Control Operation at Start-Up

Figure 11. Power Good Activates (PGOOD = Low) When the

LT8311’s FB Pin Voltage Is Within ±7% of Its Regulated Target

(1.227V). The PGOOD Pin Is Pulled Up Externally to a 12V

Housekeeping Supply Through a 100k External Resistor

its VIN UVLO voltage, so that upon getting power, it can

tolerate up to a 100mV drop on its VIN pin before losing

power again. Even more importantly, the LT8311 has an

opto-control start-up system that keeps the LT8311’s

“opto-control brains” turned off until all relevant node

voltages within the voltage loop are prebiased to a state

where they will not cause switching activity to cease when

the loop is eventually enabled.

As shown in Figure 9 and the scope shot in Figure 10,

the LT8311’s opto-control operation at start-up involves

slewing the SS pin voltage close to the FB pin voltage,

slewing the COMP pin voltage to its high clamp voltage,

and keeping the OPTO pin voltage held low. During this

phase, the inductor current (and by extension, the output

voltage)iscontrolledbythesoft-startfunctionprovidedby

theprimary-sideIC.Uponcompletionofthestatemachine,

theLT8311allowsthefeedbacklooptobefunctionalagain,

and the FB pin voltage tracks the LT8311’s SS pin voltage

until FB finally gets to its regulation target of 1.227V.

Power Good

The LT8311 offers output power good monitoring to

assist with system level design. The LT8311’s PGOOD

pin is pulled low internally when the FB pin voltage stays

within a ±7% window of the 1.227V reference for a period

of 175µs. Waiting for 175µs to elapse prevents the PGOOD

pin from indicating false positives during transient events.

OPERATION

COMP

1V/DIV

200mV/DIV

OPTO

500mV/DIV

8311 F10

5ms/DIV

PGOOD

5V/DIV

500mV/DIV

8311 F11

2ms/DIV

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