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LT8610E

42V, 2.5A Synchronous Step-Down Regulator with 2.5µA Quiescent Current

Linear Technology 

LT8610

APPLICATIONS INFORMATION

Achieving Ultralow Quiescent Current

To enhance efficiency at light loads, the LT8610 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 LT8610

delivers single small pulses of current to the output capaci-

tor followed by sleep periods where the output power is

supplied by the output capacitor. While in sleep mode the

LT8610 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 LT8610 is in sleep mode increases, resulting in

Burst Frequency

800

VIN = 12V

700 VOUT = 3.3V

600

500

400

300

200

100

0

0

50

100

150

200

LOAD CURRENT (mA)

(1a)

8610 F01a

Minimum Load to Full Frequency (SYNC DC High)

100

5VOUT

700kHz

80

60

much higher light load efficiency than for typical convert-

ers. 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.

While in Burst Mode operation the current limit of the top

switch is approximately 400mA 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 1a. The out-

put load at which the LT8610 reaches the programmed

frequency varies based on input voltage, output voltage,

and inductor choice.

For some applications it is desirable for the LT8610 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 LT8610

will also operate in pulse-skipping mode.

IL

200mA/DIV

40

20

0

5 10 15 20 25 30 35 40 45

INPUT VOLTAGE (V)

(1b)

8610 F01b

Figure 1. SW Frequency vs Load Information in

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

VOUT

10mV/DIV

VSYNC = 0V

5µs/DIV

8610 F02

Figure 2. Burst Mode Operation

For more information www.linear.com/LT8610

8610fa

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