LT8640 datasheet(16/28 Pages) AD

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LT8638S

Rev. 0

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APPLICATIONS INFORMATION

Burst Mode Operation

To enhance efficiency at light loads, the LT8638S oper-

ates in 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 LT8638S 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 LT8638S consumes 125µA.

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

rent pulses decreases (see Figure 2) and the percentage

of time the LT8638S is in sleep mode increases, resulting

in much higher light load efficiency than for typical con-

verters. By maximizing the time between pulses, the qui-

escent current approaches 90µA 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.

FRONT PAGE APPLICATION

VIN = 12V

VOUT = 5V

LOAD CURRENT (A)

0.2

0.4

0.6

0.8

200

400

600

800

1000

1200

8638S F02

Figure 2. SW Frequency vs Load Information in

Burst Mode Operation

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

LT8638S can stay in sleep mode longer between each

pulse. This can be achieved by using a larger value induc-

tor (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.

While in Burst Mode operation the current limit of the

top switch is approximately 2A (as shown in Figure 3),

resulting in low output voltage ripple. Increasing the out-

put capacitance will decrease output ripple proportionally.

As load ramps upward from zero the switching frequency

will increase but only up to the switching frequency pro-

grammed by the resistor at the RT pin as shown in Figure 2.

FRONT PAGE APPLICATION

12VIN to 5VOUT AT 10mA

Burst Mode OPERATION

20µs/DIV

1A/DIV

VSW

5V/DIV

8638S F03

Figure 3. Burst Mode Operation

The output load at which the LT8638S reaches the pro-

grammed frequency varies based on input voltage, output

voltage and inductor choice. To select low ripple Burst

Mode operation, tie the SYNC/MODE pin below 0.7V (this

can be ground or a logic low output).

Forced Continuous Mode

The LT8638S can operate in forced continuous mode

(FCM) for fast transient response and full frequency oper-

ation over a wide load range. When in FCM, the oscillator

operates continuously and positive SW transitions are

aligned to the clock. Negative inductor current is allowed

at light loads or under large transient conditions. The

LT8638S can sink current from the output and return

this charge to the input in this mode, improving load step

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