LT8640 datasheet(17/32 Pages) AD

17 / 32 page

LT8636/LT8637

Rev. C

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While in Burst Mode operation the current limit of the top

switch is approximately 900mA (as shown in Figure 3),

resulting in low output voltage ripple. Increasing the out-

put capacitance will decrease output ripple proportion-

ally. As load ramps upward from zero the switching fre-

quency will increase but only up to the switching frequency

programmed by the resistor at the RT pin as shown in

Figure 2.

The exposed pads on the bottom of the package should be

soldered to the PCB to reduce thermal resistance to ambi-

ent. To keep thermal resistance low, extend the ground

plane from GND as much as possible, and add thermal

vias to additional ground planes within the circuit board

and on the bottom side.

Burst Mode Operation

To enhance efficiency at light loads, the LT8636/LT8637

operates in low ripple Burst Mode operation, which keeps

the output capacitor charged to the desired output voltage

while minimizing the input quiescent current and minimiz-

ing output voltage ripple. In Burst Mode operation the

LT8636/LT8637 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 LT8636 consumes 1.7µA, and the LT8637

consumes 230µA.

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

rent pulses decreases (see Figure 2) and the percentage

of time the LT8636/LT8637 is in sleep mode increases,

resulting in much higher light load efficiency than for typi-

cal converters. By maximizing the time between pulses,

the LT8636/LT8637’s quiescent current approaches

2.5µA for a typical application when there is no output

load. Therefore, to optimize the quiescent current perfor-

mance at light loads, the current in the feedback resistor

divider must be minimized as it appears to the output as

load current.

In order to achieve higher light load efficiency, more

energy must be delivered to the output during the sin-

gle small pulses in Burst Mode operation such that the

LT8636/LT8637 can stay in sleep mode longer between

each pulse. This can be achieved by using a larger

value inductor (i.e., 4.7µH), and should be considered

independent of switching frequency when choosing

an inductor. For example, while a lower inductor value

would typically be used for a high switching frequency

application, if high light load efficiency is desired, a

higher inductor value should be chosen. See curve in

Typical Performance Characteristics.

Figure 2. SW Frequency vs Load Information

in Burst Mode Operation

Figure 3. Burst Mode Operation

FRONT PAGE APPLICATION

VIN = 12V

VOUT = 5V

LOAD CURRENT (mA)

100

200

300

400

500

600

200

400

600

800

1000

1200

8636 F02

APPLICATIONS INFORMATION

FRONT PAGE APPLICATION

12VIN TO 5VOUT AT 10mA

VSYNC = 0V

5µs/DIV

VSW

5V/DIV

500mA/DIV

8636 F03

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