LT8607 datasheet(12/24 Pages) AD

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LT8609S

Rev. D

For more information www.analog.com

APPLICATIONS INFORMATION

Achieving Ultralow Quiescent Current

To enhance efficiency at light loads, the LT8609S enters

into low ripple Burst Mode operation, which keeps the

output capacitor charged to the desired output voltage

while minimizing the input quiescent current and min-

imizing output voltage ripple. In Burst Mode operation

the LT8609S 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 LT8609S consumes 1.7μA.

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

rent pulses decreases (see Figure 1) and the percentage

of time the LT8609S is in sleep mode increases, result-

ing in much higher light load efficiency than for typical

converters. By maximizing the time between pulses, the

INPUT VOLTAGE (V)

8609S F01b

100

VOUT = 5V

fSW = 700kHz

SYNC = FLOAT

Figure 1b. Full Switching Frequency Minimum

Load vs VIN in Pulse Skipping Mode

Figure 2. Burst Mode Operation

2.00µs/DIV

200mA/DIV

10mV/DIV

8609S F02

converter quiescent current approaches 2.5µA for a typ-

ical 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.

While in Burst Mode operation the current limit of the

top switch is approximately 600mA 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

Table 1. The output load at which the LT8609S reaches

the programmed frequency varies based on input voltage,

output voltage, and inductor choice.

For some applications it is desirable for the LT8609S 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

as shown in Figure 1b. To enable pulse-skipping mode the

SYNC pin is floated. To achieve spread spectrum modula-

tion with pulse-skipping mode, the SYNC pin is tied high.

While a clock is applied to the SYNC pin the LT8609S will

also operate in pulse-skipping mode.

LOAD CURRENT (mA)

2500

1000

1500

500

2000

600

200

400

L = 2.2µH

VOUT = 3.3V

VIN = 12V

SYNC = 0V

8609S F01a

Figure 1a. SW Burst Mode Frequency vs Load

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