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LT8311

8311f

For more information www.linear.com/LT8311

APPLICATIONS INFORMATION

The TIMER resistor is typically picked to set a timeout

period that is 20% higher than the forward converter’s

nominal switching period.

Timeout

1.20

fSW

where fSW is converter switching frequency in Hz.

Once a timeout period is calculated, the TIMER pin resis-

tor, RTIMER, can be calculated as follows:

RTIMER (kΩ) ~ 22.1E6 • Timeout

where Timeout has units of seconds.

The relationship between RTIMER and timeout is not per-

fectly linear. Table 1 shows RTIMERvalues(nearest1%)for

arangeoftypicalforwardconverterswitchingfrequencies:

Table 1. RTIMER 1% Resistor Values for Different Forward

Converter Switching Frequencies

SWITCHING

FREQUENCY (kHz)

TIMEOUT (µs) = 1.2/fSW

RTIMER (kΩ)

100

267

150

178

200

133

250

4.8

107

300

88.7

400

66.5

500

2.4

53.6

Setting the timeout period at 1.2 • Switching Period will

keep synchronous conduction shut off through frequency

foldback in preactive mode, until the switching frequency

approaches 80% of its final value. For 100kHz switching

applications, this means that the LT8311 is ready for

synchronous conduction in preactive mode, at 80kHz.

Although80kHzisoutsidetheLT8311’sdatasheetspecifi-

cations for preactive mode operating frequency range, the

IC is designed to operate down to 80kHz to ride through

such a frequency foldback event. Timeout may also shut

off synchronous conduction during CSW pulse-skipping

events at light output load currents.

Setting RTIMER in SYNC Mode

In SYNC mode, the functionality of the timeout period is

similar to preactive mode, except that the resetting of the

LT8311’s internal timeout signal happens every time the

SYNC pin voltage falls below –1.2V. The goal of the time-

out function in SYNC mode is primarily to limit the catch

MOSFET on-time in the event that the catch MOSFET stays

ontoolongandconductsanunsafelevelofreverse-output

inductor current (current flows from the output capacitor

back towards the drain of the catch MOSFET). Refer to

the section, "Configuring CSP/CSN Inputs of the Current

Sense Comparators in SYNC Mode,” for further informa-

tion on what constitutes an unsafe level of reverse-output

inductor current.

In SYNC mode, set the timeout period to be 20% longer

than the longest switching period of the primary-side IC.

Typically, the longest switching period of the primary-

side IC corresponds to the smallest frequency foldback

frequency (fSW_SMALLEST):

Timeout

1.2

fSW _SMALLEST

Once a timeout period is calculated, the TIMER pin resis-

tor, RTIMER, can be calculated as follows:

RTIMER(kΩ) ~ 22.1E6 • Timeout

where Timeout has units of seconds, and fSW_SMALLEST

is in units of Hz.

Configuring CSP/CSN Inputs of Current Sense

Comparator in Preactive Mode

The differential input current sense comparator in the

LT8311 is used to provide the IC with information about

the current in the catch MOSFET. Connect the CSP and

CSN pins, through series resistors, to the drain and source

of the catch MOSFET (MCG), to allow the LT8311 to sense

the drain-source voltage of MCG, and make inferences

about its current. Alternatively, CSP and CSN can be tied,

through series resistors, across a sense resistor which

is placed from the source of MCG to ground. As explained

earlier in the Operation section, the CSP and CSN pins

should be configured, in preactive mode, to trip at zero

currentinthecatchMOSFET.Sincethecurrentcomparator

internally trips at 66mV, and the CSP pin sources 40µA,

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