LT8640 datasheet(14/26 Pages) AD

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LT8642-1

Rev. 0

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APPLICATIONS INFORMATION

that layer. Place a local, unbroken ground plane under the

application circuit on the layer closest to the surface layer.

The SW and BOOST nodes should be as small as possi-

ble. Finally, keep the FB and RT nodes small so that the

ground traces will shield them from the SW and BOOST

nodes. The exposed pads on the bottom of the package

should be soldered to the PCB to reduce thermal resis-

tance to ambient. 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 LT8642-1 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 LT8642-1 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 LT8642-1 consumes 230µA.

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

rent pulses decreases (see Figure 2) and the percentage

of time the LT8642-1 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 240µA for a typ-

ical application when there is no output load. Note that

the current in the feedback resistor divider appears to the

output as load current.

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

output capacitance will decrease output ripple propor-

tionally. As load ramps upward from zero the switch-

ing frequency will increase but only up to the switching

frequency programmed by the resistor at the RT pin as

shown in Figure 2.

Figure 2. SW Frequency vs Load Information

in Burst Mode Operation

FRONT PAGE APPLICATION

VIN = 12V

VOUT = 1.2V

LOAD CURRENT (A)

0 0.25 0.50 0.75 1 1.25 1.50 1.75 2

0.5

1.0

1.5

2.0

2.5

86421 F02

Burst Frequency

Figure 3. Burst Mode Operation

FRONT PAGE APPLICATION

12VIN TO 1.2VOUT AT 100mA

VSYNC = 0V

5µs/DIV

2A/DIV

VSW

5V/DIV

86421 F03

The output load at which the LT8642-1 reaches the

programmed frequency varies based on input voltage,

output voltage and inductor choice. To select low ripple

Burst Mode operation, tie the SYNC/MODE pin below 0.4V

(this can be ground or a logic low output).

Forced Continuous Mode

The LT8642-1 can operate in forced continuous mode

(FCM) for fast transient response and full frequency

operation over a wide load range. When in FCM, the

oscillator operates continuously and positive SW tran-

sitions are aligned to the clock. Negative inductor cur-

rent is allowed at light loads or under large transient

conditions. The LT8642-1 can sink current from the

output and return this charge to the input in this mode,

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