LT8604 datasheet(11/20 Pages) AD

11 / 20 page

LT8604

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

Figure 1. SW Burst Mode Frequency vs Load

to the switching frequency programmed by the resistor

at the RT pin as shown in Figure 1. The output load at

which the LT8604 reaches the programmed frequency

varies based on input voltage, output voltage, and induc-

tor choice.

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:

= R1

VOUT

0.778V

– 1

⎛

⎝⎜

⎞

⎠⎟

1% resistors are recommended to maintain output volt-

age accuracy.

The total resistance of the FB resistor divider should be

selected to be as large as possible when good low load

efficiency is desired: The resistor divider generates a

small load on the output, which should be minimized to

optimize the quiescent current at low loads.

Setting the Switching Frequency

The LT8604 uses a constant frequency PWM architec-

ture that can be programmed to switch from 200kHz to

2.2MHz by using a resistor tied from the RT pin to ground.

Table 1 shows the necessary RT value for a desired

switching frequency.

Achieving Ultralow Quiescent Current

To enhance efficiency at light loads, the LT8604 enters

into 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

LT8604 delivers single small pulses of current to the out-

put capacitor followed by sleep periods where the output

power is supplied by the output capacitor. While in sleep

mode the LT8604 consumes 1.7μA.

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

rent pulses decreases (see Figure 1) and the percentage

of time the LT8604 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 2.5µA 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.

Figure 2. Burst Mode Operation

While in Burst Mode operation, the current limit of the top

switch is approximately 40mA resulting in output voltage

ripple shown in Figure 2. As the load ramps upward from

zero the switching frequency will increase but only up

L = 33µH

VIN = 12V

VOUT = 3.3V

LOAD CURRENT (mA)

250

500

750

1000

1250

1500

1750

2000

2250

2500

8604 F01

5µs/DIV

VSW

5V/DIV

VOUT

5mV/DIV

20mA/DIV

8604 F02

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