LT8613 datasheet(14/26 Pages) LINER

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LT8619/LT8619-5

8619f

For more information www.linear.com/LT8619

APPLICATIONS INFORMATION

FB Resistor Network

The output voltage is programmed with a resistor divider

between VOUT and the FB pin. Choose the resistor values

according to:

R1= R2

VOUT

0.8V

– 1

⎛

⎝⎜

⎞

⎠⎟

Reference designators refer to the Block Diagram. 1%

resistors are recommended to maintain output voltage

accuracy.

If low input quiescent current and good light-load effi-

ciency are desired, use a large resistor value for the FB

resistor divider. The current flowing in the divider acts as

a load current, and will increase the no-load input current

to the converter, which is approximately:

IQ = 5.2µA +

VOUT

R1+R2

⎛

⎝

⎜

⎞

⎠

⎟

VOUT

VIN

⎛

⎝

⎜

⎜⎜

⎞

⎠

⎟

⎟⎟

⎛

⎝

⎜

⎞

⎠

⎟

where 5.2μA is the quiescent current of the LT8619 and

the second term is the current in the feedback divider

reflected to the input of the buck operating at its light load

efficiency, η. For a 3.3V application with R1 = 1M and

R2 = 316k, the feedback divider draws 2.5μA from VOUT.

With VIN = 12V and η = 85%, this adds 0.8μA to the 5.2μA

quiescent current resulting in 6μA quiescent current from

the 12V supply. Note that this equation implies that the

no-load current is a function of VIN; this is plotted in the

Typical Performance Characteristics section.

When using large FB resistors, a 4.7pF to 10pF phase

lead capacitor, C1, should be connected from VOUT to FB.

Setting the Switching Frequency

The LT8619 uses a constant frequency PWM architec-

ture that can be programmed to switch from 300kHz to

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

The RT resistor required for a desired oscillator frequency

can be roughly obtain using:

RT =

50.07

fOSC

– 5

where RT is in kΩ and fOSC is the desired switching fre-

quency in MHz.

Table 1 and Figure 4 show the typical RT value for a

desired oscillator frequency.

Table 1. Oscillator Frequency vs RT Value (1% Standard Value)

fOSC (MHz)

RT (kΩ)

fOSC (MHz)

RT (kΩ)

0.3

162

1.4

30.9

0.4

121

1.6

26.1

0.5

95.3

1.8

22.6

0.6

78.7

2.0

20.0

0.7

66.5

2.2

17.8

0.8

57.6

0.9

51.1

1.0

45.3

1.2

36.5

Figure 4. Oscillator Frequency vs RT Value

Operating Frequency Selection and Trade-Offs

Selection of the operating frequency is a trade-off between

efficiency, component size, and input voltage range. The

advantage of high frequency operation is that smaller

inductor and capacitor values may be used. The disadvan-

tages are lower efficiency and a smaller input voltage range.

For force continuous mode operation, the highest oscil-

lator frequency (fOSC(MAX)) for a given application can be

approximately given by the 1st order equation:

fOSC(MAX) =

ILOADRSW(BOT) + VOUT

tON(MIN) VIN –ILOADRSW(TOP) +ILOADRSW(BOT)

(

)

Where VIN is the input voltage, VOUT is the output volt-

age, RSW(TOP) and RSW(BOT) are the internal switch on

resistance (~0.45Ω, ~0.22Ω, respectively) and tON(MIN)

RT (kΩ)

80 100 120 140 160

0.2

0.6

1.0

1.4

1.8

2.2

8619 F04

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