LT8613 datasheet(20/26 Pages) LINER

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LT8619/LT8619-5

8619f

For more information www.linear.com/LT8619

RT = 20kΩ

VFB (V)

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

0.5

1.0

1.5

2.0

2.5

8619 F08

Figure 8. Frequency Foldback Transfer Function

Figure 9. Short-Circuit Waveform with VIN = 12V,

VOUT = 3.3V, fOSC = 2MHz, L = 4.7μH, COUT = 22μF

VIN

LT8619

EN/UV

GND

8619 F10

PCB Layout

For proper operation and minimum EMI, care must be

taken during printed circuit board (PCB) layout. Figure 11

and Figure 12 show the recommended component place-

ment with trace, ground plane and via locations. Note that

large, switched currents flow in the LT8619’s VIN, SW,

GND pins, and the input capacitor. The loop formed by

these components should be as small as possible by plac-

ing the capacitor adjacent to the VIN and GND pins. When

using a physically large input capacitor, the resulting loop

may become too large in which case using a small case/

value capacitor placed close to the VIN and GND pins plus

a larger capacitor further away is preferred. These com-

ponents, along with the inductor and output capacitor,

should be placed on the same side of the circuit board,

and their connections should be made on that layer. Place

a local, unbroken ground plane under the application cir-

cuit on the layer closest to the surface layer. The SW and

BST nodes should be as small as possible. Finally, keep

the FB and RT nodes small so that the ground traces will

shield them from the SW and BST nodes. The exposed

pad on the bottom of the package must be soldered to

ground so that the pad is connected to ground electrically

and also acts as a heat sink thermally. To keep thermal

resistance low, extend the ground plane as much as pos-

sible, and add thermal vias under and near the LT8619 to

additional ground planes within the circuit board and on

the bottom side.

High Temperature Output Current Considerations

The maximum practical continuous load that the LT8619

can drive, while rated at 1.2A, actually depends upon both

the internal current limit (refer to the Typical Performance

Characteristics section) and the internal temperature

which depends on operating conditions, PCB layout and

airflow.

APPLICATIONS INFORMATION

5s/DIV

VOUT

1V/DIV

ISHORT

10A/DIV

0.5A/DIV

10V/DIV

8619 F09

Reversed Input Protection

Load protection may be necessary in systems where the

output will be held high when the input to the LT8619 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 LT8619’s output. If the VIN

pin is allowed to float and the EN/UV pin is held high (either

by a logic signal or because it is tied to VIN), then the

LT8619’s internal circuitry will pull its quiescent current

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

erate several μA in this state. If the EN/UV 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, regard-

less of EN/UV, parasitic body diodes inside the LT8619

can pull current from the output through the SW pin and

the VIN pin. Figure 10 shows a connection of the VIN and

EN/UV pins that will allow the LT8619 to run only when

the input voltage is present and that protects against a

shorted or reversed input.

Figure 10. Reverse VIN Protection

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