Data Sheet

LT8641A

analog.com

Rev 0

20 of 35

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 shield them from the SW and BOOST nodes. Solder the exposed pad on the bottom

of the package to SW to reduce thermal resistance to ambient. To keep thermal resistance low, extend the ground

plane from GND1 and GND2 as much as possible, and add thermal vias to additional ground planes within the circuit

board and on the bottom side.

Achieving Ultralow Quiescent Current

To enhance efficiency at light loads, the LT8641A 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 LT8641A 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 LT8641A consumes 1.7μA.

As the output load decreases, the frequency of single current pulses decreases (see Figure 47 ) and the percentage of

time the LT8641A is in sleep mode increases, resulting in much higher light load efficiency than for typical converters.

By maximizing the time between pulses, the converter 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.

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 LT8641A can stay in sleep mode longer between each pulse. This can be

achieved 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 is typically used for a high switching frequency

application, if high light load efficiency is desired, choose a higher inductor value. See the curve in Typical

Performance Characteristics.

While in Burst Mode operation, the current limit of the top switch is approximately 950mA (see Figure 48), resulting

in low output voltage ripple. Increasing the output capacitance decreases output ripple proportionally. As load

ramps upward from zero, the switching frequency increases, but only up to the switching frequency programmed by

the resistor at the RT pin, as shown in Figure 47. The output load at which the LT8641A reaches the programmed

frequency varies based on input voltage, output voltage, and inductor choice.

(47a)

FRONT PAGE APPLICATION

VIN = 12V

VOUT = 5V

LOAD CURRENT (mA)

0

200

400

600

800

0

200

400

600

800

1000

1200