LT8607 datasheet(12/24 Pages) AD

12 / 24 page

LT8607/LT8607B

Rev. D

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APPLICATIONS INFORMATION

Achieving Ultralow Quiescent Current

To enhance efficiency at light loads, the LT8607 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 mini-

mizing output voltage ripple. In Burst Mode operation the

LT8607 delivers single small pulses of current to the out-

put capacitor followed by sleep periods where the output

power is supplied by the output capacitor. While in sleep

mode the LT8607 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 LT8607 is in sleep mode increases, result-

ing in much higher light load efficiency than for typical

converters. By maximizing the time between pulses, the

converter quiescent current approaches 3.0µA for a typi-

cal 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 250mA resulting in output

voltage ripple shown in Figure 3. 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 LT8607 reaches the

programmed frequency varies based on input voltage,

output voltage, and inductor choice.

For some applications it is desirable for the LT8607 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 opera-

tion as shown in Figure 2. To enable pulse-skipping mode

the SYNC pin is floated. To achieve spread spectrum

modulation with pulse-skipping mode, the SYNC pin is

tied high. While a clock is applied to the SYNC pin the

LT8607 will also operate in pulse-skipping mode. The

Figure 1. Burst Frequency vs Output Current

RT = 18.2kΩ

VIN = 12V

L = 2.2µH

VOUT = 3.3V

INPUT VOLTAGE (V)

10 15 20 25 30 35 40 45

100

125

150

8607 F02

2µs/DIV

VOUT

20mV/DIV

ILOAD

200mA/DIV

VSW

10V/DIV

8607 F03

LT8607 DFN is programmed for Burst Mode operation

and cannot enter pulse-skipping mode. The LT8607B DFN

is programmed for pulse-skipping mode and cannot enter

Burst Mode operation.

OUTPUT CURRENT (mA)

100

125

250

500

750

1000

1250

1500

1750

2000

2250

2500

8607 F01

VIN = 12V

L = 2.2µH

VOUT = 3.3V

SYNC = 0V OR LT8607 DFN

Figure 2. Minimum Load to Full Frequency

(SYNC Float to 1.9V) (MSOP or LT8607B DFN)

Figure 3. Burst Mode Operation

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