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LT8619/LT8619-5

8619f

For more information www.linear.com/LT8619

APPLICATIONS INFORMATION

Achieving Ultralow Quiescent Current

To enhance efficiency at light loads, the LT8619 enters into

Burst Mode operation, which keeps the output capacitor

charged to the desired output voltage while minimizing the

input quiescent current and output ripple voltage. In Burst

Mode operation the LT8619 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 LT8619 consumes less than 6μA.

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

rent pulses decreases (see Figure 1) and the percentage

of time the LT8619 is in sleep mode increases, result-

ing in much higher light load efficiency than for typical

converters. For a typical application, when the output is

not loaded, by maximizing the time between pulses, the

regulator quiescent approaches 6µA. Therefore, to opti-

mize the quiescent current performance at light loads,

the current in the feedback resistor divider must be mini-

mized as it appears to the output as load current (See FB

Resistor Network section).

While in Burst Mode operation, the current limit of the

top switch is approximately 380mA resulting in output

voltage ripple 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 1. The output load at which the LT8619 reaches

the programmed frequency varies based on input voltage,

output voltage, and inductor choice.

For some applications it is desirable for the LT8619 to

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

ferences from Burst Mode operation. First, the minimum

inductor current clamp present in Burst Mode operation

is removed, providing a smaller packet of charge to the

output capacitor and reduce the output ripple voltage.

For a given load, the chip awake more often, resulting in

higher supply current compared to Burst Mode opera-

tion. Second is that full switching frequency is reached

at lower output load than in Burst Mode operation (see

Figure 3). To enable pulse-skipping mode, leave the SYNC

pin floating. Tying the SYNC pin to INTVCC node enables

the programmed switching frequency at no load.

Figure 1. Burst Frequency vs Load Current

8619 F02

VOUT (AC)

10mV/DIV

200mA/DIV

10V/DIV

SW (ZOOM IN)

10V/DIV

IL (ZOOM IN)

200mA/DIV

VOUT (AC, ZOOM IN)

10mV/DIV

TOP = 20ms/DIV, BOT = 1s/DIV

Figure 2. Burst Mode Operation Waveform with

VIN = 12V, VOUT = 3.3V at No Load, RT = 66.5k,

L = 10μH, COUT = 22μF

VOUT = 3.3V

fOSC = 700kHz

L = 10µH

VIN (V)

100

150

200

250

300

350

400

8619 F03

PULSE-SKIPPING MODE

Burst Mode OPERATION

VIN = 12V

VOUT = 3.3V

fOSC = 700kHz

L = 10µH

Burst Mode OPERATION

LOAD CURRENT (mA)

0.001 0.01

0.1

100

0.01

0.1

100

8619 F01

Figure 3. Minimum Load for Full Frequency Operation

vs VIN in Burst Mode Operation and Pulse-Skipping

Mode Setting

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