LT8650SPJVPBF datasheet(16/34 Pages) AD

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LT8650SP

Rev. B

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APPLICATIONS INFORMATION

Achieving Ultralow Quiescent Current

To enhance efficiency at light loads, the LT8650SP oper-

ates in low ripple Burst Mode operation, which keeps

the output capacitor charged to the desired output volt-

age while minimizing the input quiescent current and

minimizing output voltage ripple. 3.7μA is supplied by

VIN1 to common bias circuits. In Burst Mode operation

the LT8650SP 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 both channels consume a combined 6.2μA.

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

rent pulses decreases (see Figure 1) and the percentage

of time the LT8650SP 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 6.2µA 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 1.6A resulting in output voltage

rippleshowninFigure2.Increasingtheoutputcapacitance

willdecreasetheoutputrippleproportionally.Asloadramps

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 LT8650SP reaches the programmed

frequency varies based on input voltage, output voltage,

and inductor choice.

For some applications it is desirable to select forced

continuous mode (FCM) to maintain full switching fre-

quency down to zero output load. See Forced Continuous

Mode section.

Figure 1. Burst Frequency

Figure 2. Burst Mode Operation

LOAD CURRENT (A)

0.20

0.40

0.60

0.80

1.00

1.20

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

2.25

VIN1 = VIN2 = 12V

VOUT = 5V

SYNC = 0V

RT = 15k

L = 1.0µH

LT8650sp F01

2µs/DIV

5V/DIV

1A/DIV

12VIN TO 5VOUT AT 100mA

SYNC = 0V

LT8650sp F02

FB Resistor Network

The output voltage is programmed with a resistor divider

betweentheoutputandtheFBpin(R1-2forchannel1,R3-4

for channel 2). Choose the resistor values according to:

=R2

VOUT1

0.8V

–1

⎛

⎝⎜

⎞

⎠⎟

ReferencedesignatorsrefertotheBlockDiagram.1%resis-

torsarerecommendedtomaintainoutputvoltage accuracy.

Iflowinputquiescentcurrentandgoodlight-loadefficiency

are desired, use large resistor values for the FB resistor

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