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LT8365

Rev. A

For more information www.analog.com

Figure 3. Soft-Start Waveforms

APPLICATIONS INFORMATION

FREQUENCY FOLDBACK

Duringstart-uporfaultconditionsinwhichVOUTisverylow,

extremely small duty cycles may be required to maintain

control of inductor peak current. The minimum on-time

limitationofthepowerswitchmightpreventtheselowduty

cycles from being achievable. In this scenario inductor

current rise will exceed inductor current fall during each

cycle, causing inductor current to “walk up” beyond the

switch current limit. The LT8365 provides protection from

this by folding back switching frequency whenever FBX

or SS pins are close to GND (low VOUT levels or start-up).

This frequency foldback provides a larger switch-off time,

allowing inductor current to fall enough each cycle (see

Normalized Switching Frequency vs FBX Voltage in the

Typical Performance Characteristics section).

THERMAL LOCKOUT

If the LT8365 die temperature reaches 170°C (typical),

the part will stop switching and go into thermal lockout.

When the die temperature has dropped by 5°C (nominal),

the part will resume switching with a soft-started inductor

peak current.

COMPENSATION

Loop compensation determines the stability and transient

performance. The LT8365 uses current mode control to

regulate the output which simplifies loop compensation.

Theoptimumvaluesdependontheconvertertopology,the

componentvaluesandtheoperatingconditions(including

the input voltage, load current, etc.). To compensate the

feedback loop of the LT8365, a series resistor-capacitor

network is usually connected from the VC pin to GND.

The Block Diagram shows the typical VC compensation

network. For most applications, the capacitor should be

in the range of 100pF to 10nF, and the resistor should

be in the range of 5k to 100k. A small capacitor is often

connected in parallel with the RC compensation network

to attenuate the VC voltage ripple induced from the output

voltageripplethroughtheinternalerroramplifier.Theparal-

lel capacitor usually ranges in value from 2.2pF to 22pF. A

practicalapproachtodesigningthecompensationnetwork

is to start with one of the circuits in this data sheet that

is similar to your application, and tune the compensation

networktooptimizetheperformance. Stabilityshouldthen

be checked across all operating conditions, including load

current, input voltage and temperature. Application Note

76 is a good reference.

THERMAL CONSIDERATIONS

CareshouldbetakeninthelayoutofthePCBtoensuregood

heatsinkingoftheLT8365.Bothpackageshaveanexposed

pad underneath the IC which is the best path for heat out

of the package. The exposed pad should be soldered to a

continuouscoppergroundplaneunderthedevicetoreduce

die temperature and increase the power capability of the

LT8365. The ground plane should be connected to large

copper layers to spread heat dissipated by the LT8365.

Power dissipation within the LT8365 (PDISS_LT8365) can

be estimated by subtracting the inductor and Schottky

diode power losses from the total power losses calculated

in an efficiency measurement. The junction temperature

of LT8365 can then be estimated by:

TJ(LT8365) = TA + θJA • PDISS_LT8365

APPLICATION CIRCUITS

TheLT8365canbeconfiguredfordifferenttopologies.The

first topology to be analyzed will be the boost converter,

followed by the SEPIC and inverting converters.

Boost Converter: Switch Duty Cycle

The LT8365 can be configured as a boost converter for the

applications where the converter output voltage is higher

thantheinputvoltage.Rememberthatboostconvertersare

10ms/DIV

VOUT

20V/DIV

200mA/DIV

8365 F03

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