LT8640 datasheet(17/32 Pages) AD

17 / 32 page

LT8640S/LT8643S/

LT8640SA/LT8643SA

Rev. D

For more information www.analog.com

APPLICATIONS INFORMATION

load efficiency than for typical converters. By maximizing

the time between pulses, the LT8640S/LT8640SA’s qui-

escent 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.

In order to achieve higher light load efficiency, more

energy must be delivered to the output during the

single small pulses in Burst Mode operation such that

the LT8640S/LT8643S/LT8640SA/LT8643SA can stay

in sleep mode longer between each pulse. This can be

achieved by using a larger value inductor (i.e., 4.7µH),

and should be considered independent of switching

frequency when choosing an inductor. For example,

while a lower inductor value would typically be used

for a high switching frequency application, if high

light load efficiency is desired, a higher inductor value

should be chosen. See curve in Typical Performance

Characteristics.

While in Burst Mode operation the current limit of the top

switch is approximately 900mA (as shown in Figure 3),

resulting in low output voltage ripple. Increasing the out-

put capacitance will decrease output ripple proportionally.

As load ramps upward from zero the switching frequency

will increase but only up to the switching frequency

programmed by the resistor at the RT pin as shown in

Figure 2.

Figure 2. SW Frequency vs Load Information

in Burst Mode Operation

Burst Frequency

Figure 3. Burst Mode Operation

FRONT PAGE APPLICATION

VIN = 12V

VOUT = 5V

LOAD CURRENT (mA)

100

200

300

400

500

600

200

400

600

800

1000

1200

8640S F02

FRONT PAGE APPLICATION

12VIN TO 5VOUT AT 10mA

VSYNC = 0V

5µs/DIV

VSW

5V/DIV

500mA/DIV

8640S F03

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 GND 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 LT8640S/LT8643S/

LT8640SA/LT8643SA 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 minimizing output voltage ripple. In Burst Mode

operation the LT8640S/LT8643S/LT8640SA/LT8643SA

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 LT8640S/

LT8640SA consumes 1.7µA and the LT8643S/LT8643SA

consumes 230µA.

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

rent pulses decreases (see Figure 2) and the percentage

of time the LT8640S/LT8643S/LT8640SA/LT8643SA is

in sleep mode increases, resulting in much higher light

LT8640 Datasheet(PDF) 17 Page - Analog Devices

LT8640 Datasheet(HTML) 17 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 23 24 25 26 27 28 29 30 31 32

Datasheet Download

Link URL