LT8335 datasheet(13/20 Pages) LINER

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LT8335

8335f

For more information www.linear.com/LT8335

APPLICATIONS INFORMATION

THERMAL CONSIDERATIONS

Care should be taken in the layout of the PCB to ensure

good heat sinking of the LT8335. The exposed pad (Pin 9)

must be soldered to a ground plane. Pin 5 should also be

connected to a ground plane. The ground plane should be

connected to large copper layers to spread heat dissipated

by the LT8335 and to further reduce the thermal resis-

tance (θJA) values listed in the Pin Configuration section.

Power dissipation within the LT8335 (PDISS_LT8335) 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 LT8335 can then be estimated by,

TJ (LT8335) = TA + θJA • PDISS

ADDITIONAL TOPOLOGIES : SEPIC AND INVERTING

In addition to the Boost topology, the LT8335 can be

configured in a SEPIC or Inverting topology. SEPIC and

Inverting converters are analyzed below.

SEPIC CONVERTER APPLICATIONS

The LT8335 can be configured as a SEPIC (single-ended

primary inductance converter), as shown in Figure 5. This

topology allows for the input to be higher, equal, or lower

than the desired output voltage. The conversion ratio as

a function of duty cycle is:

VOUT + VD

VIN

1−D

in continuous conduction mode (CCM).

In a SEPIC converter, no DC path exists between the input

and output. This is an advantage over the boost converter

for applications requiring the output to be disconnected

from the input source when the circuit is in shutdown.

SEPIC Converter: Switch Duty Cycle and Frequency

For a SEPIC converter operating in CCM, the duty cycle

of the main switch can be calculated based on the output

voltage (VOUT), the input voltage (VIN) and the diode

forward voltage (VD).

Themaximumdutycycle(DMAX)occurswhentheconverter

operates at the minimum input voltage:

DMAX =

VOUT + VD

VIN(MIN)+ VOUT + VD

Conversely, the minimum duty cycle (DMIN) occurs when

the converter operates at the maximum input voltage:

DMIN =

VOUT + VD

VIN(MAX)+ VOUT + VD

Be sure to check that DMAX and DMIN obey:

DMAX < 1-Minimum Off-Time(MAX) • fOSC(MAX)

and

DMIN > Minimum On-Time(MAX) • fOSC(MAX)

where Minimum Off-Time, Minimum On-Time and fOSC

are specified in the Electrical Characteristics table.

SEPIC Converter: The Maximum Output Current

Capability and Inductor Selection

As shown in Figure 5, the SEPIC converter contains two

inductors:L1andL2.L1andL2canbeindependent,butcan

also be wound on the same core, since identical voltages

are applied to L1 and L2 throughout the switching cycle.

Figure 5. LT8335 Configured in a SEPIC Topology

VOUT

VIN

FBX

GND

EN/UVLO

LT8335

VIN

VCC

INT

CIN

COUT

CDC

8335 F05

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