LT8645S-2

16

Rev. A

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APPLICATIONS INFORMATION

For more detail and PCB design files refer to the Demo

Board guide for the LT8645S.

Note that large, switched currents flow in the LT8645S-2

VIN and GND pins and the input capacitors. The loops

formed by the input capacitors should be as small as

possible by placing the capacitors adjacent to the VIN and

GND pins. Capacitors with small case size such as 0603

or 0805 are optimal due to lowest parasitic inductance.

The input capacitors, along with the inductor and out-

put capacitors, 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 circuit on the layer closest to the surface layer.

The SW and BOOST 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 BOOST nodes.

The exposed pads on the bottom of the package should be

soldered to the PCB to reduce thermal resistance to ambi-

ent. To keep thermal resistance low, extend the ground

plane from the GND pins as much as possible, and add

thermal vias to additional ground planes within the circuit

board and on the bottom side.

Burst Mode Operation

To enhance efficiency at light loads, the LT8645S-2 oper-

ates in low ripple Burst Mode operation, which keeps the

output capacitor charged to the desired output voltage

while minimizing the input quiescent current and min-

imizing output voltage ripple. In Burst Mode operation

the LT8645S-2 delivers single small pulses of current to

the output capacitor followed by sleep periods where the

output power is supplied by the output capacitor. While in

sleep mode the LT8645S-2 consumes 1.7μA.

As the output load decreases, the frequency of single cur-

rent pulses decreases (see Figure 2a) and the percent-

age of time the LT8645S-2 is in sleep mode increases,

resulting in much higher light load efficiency than for typ-

ical converters. By maximizing the time between pulses,

the LT8645S-2’s quiescent current approaches 2.5µA

for a typical application when there is no output load.

Therefore, to optimize the quiescent current performance

at light loads, the current in the feedback resistor divider

must be minimized as it appears to the output as load

current.

Figure 2. SW Frequency vs Load Information in Burst Mode Operation (2a) and Pulse-Skipping Mode (2b)

(2a)

(2b)

Burst Frequency

FRONT PAGE APPLICATION

VIN = 12V

VOUT = 5V

LOAD CURRENT (mA)

0

100

200

300

400

500

600

0

200

400

600

800

1000

1200

8645S F02a

Minimum Load to Full Frequency

(Pulse-Skipping Mode)

FRONT PAGE APPLICATION

VOUT = 5V

fSW = 1MHz

VSYNC = Float

INPUT VOLTAGE (V)

5

15

25

35

45

55

65

0

50

100

150

200

250

300

350

400

8645S F02b