Data Sheet

LT8615

analog.com

Rev 0

18 of 31

Figure 38. Burst Mode Operation

While in Burst Mode operation, the current limit of the top switch is approximately 1A, resulting in the output voltage

ripple shown in Figure 38. Increasing the output capacitance decreases the output ripple proportionally. As load

ramps upward from zero, the switching frequency increases, but only up to the switching frequency programmed by

the resistor at the RT pin, as shown in Table 4. The output load at which the LT8615 reaches the programmed

frequency varies based on input voltage, output voltage, and inductor choice.

For some applications, it is desirable for the LT8615 to operate in pulse-skipping mode. Pulse-skipping mode offers

two major differences from Burst Mode operation. First, the clock stays awake always and all switching cycles are

aligned to the clock. In this mode, much of the internal circuitry is awake always, increasing quiescent current to

several hundred µA. Second, full switching frequency is reached at lower output load than in Burst Mode operation,

as shown in Figure 37. To enable pulse-skipping mode, the SYNC pin is floated. To achieve spread-spectrum

modulation with pulse-skipping mode, the SYNC pin is tied high. While a clock is applied to the SYNC pin, the LT8615

also operates in pulse-skipping mode.

FB Resistor Network

The output voltage is programmed with a resistor divider between the output and the FB pin. Choose the resistor

values according to following equation.

R1 = R2 × (

VOUT

0.787V

− 1)

Reference designators refer to the Block Diagram 1% resistors are recommended to maintain output voltage

accuracy.

For low input quiescent current and good light-load efficiency, use large resistor values for the FB resistor divider.

The current flowing in the divider acts as a load current, and increases the no-load input current to the converter,

which is approximately given by the following equation.

IQ = 1.7µA + (

VOUT

R1 + R2

) × (

VOUT

VIN

) × (

1

n

)

where, 1.7μA is the quiescent current of the LT8615 and the second term is the current in the feedback divider

reflected to the input of the buck operating at its light load efficiency ‘n’. For a 5V application with R1 = 1M and R2 =

187k, the feedback divider draws 4.2μA. With VIN = 12V and n = 85%, this adds 2μA to the 1.7μA quiescent current,

12VIN TO 3.3VOUT AT 50mA

BURST MODE OPERATION

2µs/DIV

5V/DIV

500mA/DIV