LT8580 Ver la hoja de datos (PDF) - Linear Technology

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componentes Descripción

Fabricante

LT8580

Boost/SEPIC/Inverting DC/DC Converter with 1A, 65V Switch, Soft-Start and Synchronization

Linear Technology 

LT8580

Applications Information

Thermal Lockout

If the die temperature reaches approximately 165°C, the

part will go into thermal lockout, the power switch will be

turned off and the soft-start capacitor will be discharged.

The part will be enabled again when the die temperature

has dropped by ~5°C (nominal).

Thermal Calculations

Power dissipation in the LT8580 chip comes from four

primary sources: switch I2R loss, NPN base drive (AC),

NPN base drive (DC), and additional input current. The

following formulas can be used to approximate the power

losses. These formulas assume continuous mode opera-

tion, so they should not be used for calculating efficiency

in discontinuous mode or at light load currents.

Average Input Current:

IIN

=

VOUT • IOUT

VIN • h

Switch I2R Loss: PSW = (DC)(IIN)2(RSW )

Base Drive Loss (AC): PBAC = 20ns(IIN)(VOUT )(f)

Base Drive Loss (DC):

PBDC

=

(VIN )(IIN )(DC)

40

Input Power Loss: PINP = 6mA (VIN)

where:

RSW = switch resistance (typically 530mΩ at 0.75A)

DC = duty cycle (see the Power Switch Duty Cycle sec-

tion for formulas)

h = power conversion efficiency (typically 85% at high

currents)

Example: boost configuration, VIN = 5V, VOUT = 12V,

IOUT = 0.2A, f = 1.25MHz, VD = 0.5V:

IIN = 0.56A

DC = 62.0%

PSW = 105mW

PBAC = 169mW

PBDC = 44mW

PINP = 30mW

Total LT8580 power dissipation (PTOT) = 348mW

Thermal resistance for the LT8580 is influenced by the pres­

ence of internal, topside or backside planes. To calculate

die temperature, use the appropriate thermal resistance

number and add in worst-case ambient temperature:

TJ = TA + θJA • PTOT

where TJ = junction temperature, TA = ambient temperature,

and θJA is the thermal resistance from the silicon junction

to the ambient air.

The published θJA value is 43°C/W for the 3mm × 3mm

DFN package and 35°C/W to 40°C/W for the MSOP ex-

posed pad package. In practice, lower θJA values can be

obtained if the board layout uses ground as a heat sink.

For instance, thermal resistances of 34.7°C/W for the

DFN package and 22.5°C/W for the MSOP package were

obtained on a board designed with large ground planes.

VIN Ramp Rate

While initially powering a switching converter application,

the VIN ramp rate should be limited. High VIN ramp rates can

cause excessive inrush currents in the passive components

of the converter. This can lead to current and/or voltage

overstress and may damage the passive components or

the chip. Ramp rates less than 500mV/µs, depending on

component parameters, will generally prevent these issues.

Also, be careful to avoid hot-plugging. Hot-plugging occurs

when an active voltage supply is “instantly” connected or

switched to the input of the converter. Hot-plugging results

in very fast input ramp rates and is not recommended.

Finally, for more information, refer to Linear application

note AN88, which discusses voltage overstress that can

occur when an inductive source impedance is hot-plugged

to an input pin bypassed by ceramic capacitors.

8580f

18

For more information www.linear.com/LT8580

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