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LT8613

8613f

applicaTions inForMaTion

Achieving Ultralow Quiescent Current

To enhance efficiency at light loads, the LT8613 operates

in low ripple Burst Mode operation, which keeps the out-

put capacitor charged to the desired output voltage while

minimizing the input quiescent current and minimizing

output voltage ripple. In Burst Mode operation the LT8613

deliverssinglesmallpulsesofcurrenttotheoutputcapaci-

tor followed by sleep periods where the output power is

supplied by the output capacitor. While in sleep mode the

LT8613 consumes 1.7μA.

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

rent pulses decreases (see Figure 1a) and the percentage

of time the LT8613 is in sleep mode increases, resulting in

much higher light load efficiency than for typical convert-

ers. By maximizing the time between pulses, the converter

quiescentcurrentapproaches2.5µAforatypicalapplication

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.

While in Burst Mode operation the current limit of the top

switchisapproximately1Aresultinginoutputvoltageripple

shown in Figure 2. Increasing the output capacitance will

decrease the 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 1a. The out-

put load at which the LT8613 reaches the programmed

frequency varies based on input voltage, output voltage,

and inductor choice.

For some applications it is desirable for the LT8613 to

operate in pulse-skipping mode, offering two major differ-

ences from Burst Mode operation. First is the clock stays

awake at all times and all switching cycles are aligned to

the clock. In this mode much of the internal circuitry is

awake at all times, increasing quiescent current to several

hundred µA. Second is that full switching frequency is

reached at lower output load than in Burst Mode operation

(see Figure 1b). To enable pulse-skipping mode, the SYNC

pin is tied high either to a logic output or to the INTVCC

pin. When a clock is applied to the SYNC pin the LT8613

will also operate in pulse-skipping mode.

Figure 1. SW Frequency vs Load Information in

Burst Mode Operation (1a) and Pulse-Skipping Mode (1b)

Figure 2. Burst Mode Operation

Minimum Load to Full Frequency (SYNC DC High)

Burst Frequency

(1a)

(1b)

LOAD CURRENT (mA)

500

600

800

8613 F01a

400

300

200

100

700

100

200

500

300

400

VIN = 12V

VOUT = 5V

L = 3.9µH

INPUT VOLTAGE (V)

8613 F01b

FRONT PAGE

APPLICATION

1A/DIV

VSW

5V/DIV

5µs/DIV

12VIN TO 5VOUT AT 20mA; FRONT PAGE APP

VSYNC = 0V

8613 F02

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