LT8331 datasheet(12/30 Pages) LINER

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LT8331

8331fa

For more information www.linear.com/LT8331

APPLICATIONS INFORMATION

is close to GND (low VOUT levels). This frequency foldback

provides a larger switch-off time, allowing inductor cur-

rent to fall enough each cycle (see Normalized Switching

Frequency vs FBX Voltage in the Typical Performance

Characteristics section).

THERMAL LOCKOUT

If the LT8331 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

The LT8331 is internally compensated. The decision to

use either low ESR (ceramic) capacitors or the higher ESR

(tantalum or OS-CON) capacitors, for the output capacitor,

can affect the stability of the overall system. The ESR of

any capacitor, along with the capacitance itself, contrib-

utes a zero to the system. For the tantalum and OS-CON

capacitors, this zero is located at a lower frequency due

to the higher value of the ESR, while the zero of a ceramic

capacitor is at a much higher frequency and can generally

be ignored.

Aphaseleadzerocanbeintentionallyintroducedbyplacing

a capacitor in parallel with the resistor between VOUT and

FBX.Bychoosingtheappropriatevaluesfortheresistorand

capacitor, the zero frequency can be designed to improve

the phase margin of the overall converter. The typical target

value for the zero frequency is between 5kHz to 20kHz.

A practical approach to compensation is to start with one

of the circuits in this data sheet that is similar to your ap-

plication. Optimize performance by adjusting the output

capacitor and/or the feed forward capacitor (connected

across the feedback resistor from output to FBX pin).

THERMAL CONSIDERATIONS

CareshouldbetakeninthelayoutofthePCBtoensuregood

heat sinking of the LT8331. The package has an exposed

pad (Pin 17) underneath the IC which is the best path for

heat out of the package. Pin 17 should be soldered to a

continuouscoppergroundplaneunderthedevicetoreduce

die temperature and increase the power capability of the

LT8331. The ground plane should be connected to large

copper layers to spread heat dissipated by the LT8331.

Power dissipation within the LT8331 (PDISS_LT8331) 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 LT8331 can then be estimated by:

TJ(LT8331) = TA + θJA • PDISS_LT8331

APPLICATION CIRCUITS

TheLT8331canbeconfiguredfordifferenttopologies.The

first topology to be analyzed will be the boost converter,

followed by the flyback, SEPIC and inverting converters.

Boost Converter: Switch Duty Cycle

The LT8331 can be configured as a boost converter for

the applications where the converter output voltage is

higher than the input voltage. Remember that boost con-

verters are not short-circuit protected. Under a shorted

output condition, the inductor current is limited only by

the input supply capability. For applications requiring a

step-up converter that is short-circuit protected, please

refer to the Applications Information section covering

SEPIC converters.

The conversion ratio as a function of duty cycle is:

VOUT

VIN

− D

in continuous conduction mode (CCM).

For a boost converter operating in CCM, the duty cycle

of the main switch can be calculated based on the output

voltage (VOUT) and the input voltage (VIN). The maximum

duty cycle (DMAX) occurs when the converter has the

minimum input voltage:

DMAX =

VOUT − VIN(MIN)

VOUT

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