LT8646SEVPBF datasheet(25/30 Pages) AD

25 / 30 page

LT8645S/LT8646S

Rev. B

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APPLICATIONS INFORMATION

Figure 7. Case Temperature Rise

Frequency foldback behavior depends on the state of the

SYNC pin: If the SYNC pin is low the switching frequency

will slow while the output voltage is lower than the pro-

grammed level. If the SYNC pin is connected to a clock

source,floated,ortiedhigh,theLT8645S/LT8646Swillstay

at the programmed frequency without foldback and only

slow switching if the inductor current exceeds safe levels.

Thereisanothersituationtoconsiderinsystemswherethe

output will be held high when the input to the LT8645S/

LT8646S is absent. This may occur in battery charg-

ing applications or in battery-backup systems where

a battery or some other supply is diode ORed with the

LT8645S/LT8646S’s output. If the VIN pin is allowed to

float and the EN pin is held high (either by a logic signal

or because it is tied to VIN), then the LT8645S/LT8646S’s

internal circuitry will pull its quiescent current through

its SW pin. This is acceptable if the system can tolerate

several μA in this state. If the EN pin is grounded the SW

pin current will drop to near 1µA. However, if the VIN pin

is grounded while the output is held high, regardless of

EN, parasitic body diodes inside the LT8645S/LT8646S

can pull current from the output through the SW pin and

the VIN pin. Figure 6 shows a connection of the VIN and

EN/UV pins that will allow the LT8645S/LT8646S to run

only when the input voltage is present and that protects

against a shorted or reversed input.

the LT8645S/LT8646S. Placing additional vias can reduce

thermal resistance further. The maximum load current

should be derated as the ambient temperature approaches

themaximumjunctionrating.Powerdissipationwithinthe

LT8645S/LT8646Scanbeestimatedbycalculatingthetotal

power loss from an efficiency measurement and subtract-

ing the inductor loss. The die temperature is calculated by

multiplying the LT8645S/LT8646S power dissipation by

the thermal resistance from junction to ambient.

Theinternalovertemperatureprotectionmonitorsthejunc-

tion temperature of the LT8645S/LT8646S. If the junction

temperature reaches approximately 180°C, the LT8645S/

LT8646S will stop switching and indicate a fault condition

until the temperature drops about 10°C cooler.

Temperature rise of the LT8645S/LT8646S is worst when

operating at high load, high VIN, and high switching fre-

quency. If the case temperature is too high for a given

application, then either VIN, switching frequency, or load

current can be decreased to reduce the temperature to

an acceptable level. Figure 7 shows examples of how

case temperature rise can be managed by reducing VIN,

switching frequency, or load.

Figure 6. Reverse VIN Protection

VIN

LT8645S/

LT8646S

EN/UV

8645S F06

GND

DC2468A DEMO BOARD

VIN = 12V, fSW = 500kHz

VIN = 24V, fSW = 500kHz

VIN = 12V, fSW = 2MHz

VIN = 24V, fSW = 2MHz

LOAD CURRENT (A)

8645S F07

Thermal Considerations

For higher ambient temperatures, care should be taken

in the layout of the PCB to ensure good heat sinking of

the LT8645S/LT8646S. The ground pins on the bottom of

the package should be soldered to a ground plane. This

ground should be tied to large copper layers below with

thermal vias; these layers will spread heat dissipated by

TheLT8645S/LT8646S’stopswitchcurrentlimitdecreases

with higher duty cycle operation for slope compensation.

This also limits the output current the LT8645S/LT8646S

can deliver for a given application. See curve in the Typical

Performance Characteristics section.

LT8646SEVPBF 数据表(PDF) 25 Page - Analog Devices

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