LT8640 datasheet(19/22 Pages) AD

19 / 22 page

LT8644S

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

different voltage levels. During FCM, Spread Spectrum,

and Synchronization modes, all devices will operate at

the same frequency. Figure 6 shows a 2-phase applica-

tion where two LT8644Ss are paralleled to get one output

capable of up to 32A.

LT8644S’s internal circuitry will pull its quiescent current

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

tolerate current draw in this state. If the EN pin is grounded

the SW pin current will drop to near 6µA. However, if the

VIN pin is grounded while the output is held high, regard-

less of EN, parasitic body diodes inside the LT8644S

can pull current from the output through the SW pin and

the VIN pin. Figure 7 shows a connection of the VIN and

EN/UV pins that will allow the LT8644S to run only when

the input voltage is present and that protects against a

shorted or reversed input.

Figure 6. Paralleling Two LT8644Ss

LT8644S

8644S F06

CLKOUT

PHMODE

LT8644S

SYNC/MODE

VOUT

32A

Figure 7. Reverse VIN Protection

VIN

LT8644S

EN/UV

8644S F07

GND

Shorted and Reversed Input Protection

The LT8644S will tolerate a shorted output. Several fea-

tures are used for protection during output short-circuit

and brownout conditions. The first is the switching

frequency will be folded back while the output is lower

than the set point to maintain inductor current control.

Second, the bottom switch current is monitored such that

if inductor current is beyond safe levels switching of the

top switch will be delayed until such time as the inductor

current falls to safe levels.

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 or tied high, the LT8644S will stay at the

programmed frequency without foldback and only slow

switching if the inductor current exceeds safe levels.

There is another situation to consider in systems where the

output will be held high when the input to the LT8644S is

absent. This may occur in battery charging applications or

in battery-backup systems where a battery or some other

supply is diode ORed with the LT8644S’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

Thermal Considerations

For higher ambient temperatures, care should be taken in

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

LT8644S. 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 the LT8644S.

Placing additional vias can reduce thermal resistance

further. The maximum load current should be derated

as the ambient temperature approaches the maximum

junction rating. Power dissipation within the LT8644S

can be estimated by calculating the total power loss from

an efficiency measurement and subtracting the induc-

tor loss. The die temperature is calculated by multiplying

the LT8644S power dissipation by the thermal resistance

from junction to ambient.

The internal overtemperature protection monitors the

junction temperature of the LT8644S. If the junction

temperature reaches approximately 165°C, the LT8644S

will stop switching and indicate a fault condition until the

temperature drops about 5°C cooler.

LT8640 Datasheet(PDF) 19 Page - Analog Devices

LT8640 Datasheet(HTML) 19 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Datasheet Download

Link URL