LT8645S/LT8646S

17

Rev. B

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APPLICATIONS INFORMATION

For more detail and PCB design files refer to the Demo

Board guide for the LT8645S/LT8646S.

Note that large, switched currents flow in the LT8645S/

LT8646S 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 output

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

ambient.Tokeepthermalresistancelow,extendtheground

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

Toenhanceefficiencyatlightloads,theLT8645S/LT8646S

operates in low ripple Burst Mode operation, which keeps

the output capacitor charged to the desired output voltage

while minimizing the input quiescent current and minimiz-

ing output voltage ripple. In Burst Mode operation the

LT8645S/LT8646S 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 consumes 1.7μA. and

the LT8646S consumes 230μA.

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

rent pulses decreases (see Figure 2a) and the percentage

of time the LT8645S/LT8646S is in sleep mode increases,

resulting in much higher light load efficiency than for typi-

cal converters. By maximizing the time between pulses,

the LT8645S’s quiescent current approaches 2.5µA for a

typicalapplicationwhenthereisnooutputload.Therefore,

to optimize the quiescent current performance at light

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