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LT8611

8611p

APPLICATIONS INFORMATION

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:

=R2

VOUT

0.970V

–1

⎛

⎝⎜

⎞

⎠⎟

(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 efficiency

are desired, use large resistor values 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 = 1.7µA +

VOUT

+R2

⎛

⎝⎜

⎞

⎠⎟

VOUT

VIN

⎛

⎝⎜

⎞

⎠⎟

⎛

⎝⎜

⎞

⎠⎟

(2)

where 1.7μA is the quiescent current of the LT8611 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 3.3V application with R1 = 1M and

R2 = 412k, the feedback divider draws 2.3μA. With VIN =

12V and n = 80%, this adds 0.8μA to the 1.7μA quiescent

current resulting in 2.5μA no-load 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 should be connected from VOUT to FB.

Setting the Switching Frequency

The LT8611 uses a constant frequency PWM architecture

that can be programmed to switch from 200kHz to 2.2MHz

by using a resistor tied from the RT pin to ground. A table

showing the necessary RT value for a desired switching

frequency is in Table 1.

The RT resistor required for a desired switching frequency

can be calculated using:

RT =

46.5

fSW

– 5.2

(3)

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

quency in MHz.

Table 1. SW Frequency vs RT Value

fSW (MHz)

RT (kΩ)

0.2

232

0.3

150

0.4

110

0.5

88.7

0.6

71.5

0.7

60.4

0.8

52.3

1.0

41.2

1.2

33.2

28.0

1.6

23.7

1.8

20.5

2.0

18.2

2.2

15.8

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 induc-

tor and capacitor values may be used. The disadvantages

are lower efficiency and a smaller input voltage range.

The highest switching frequency (fSW(MAX)) for a given

application can be calculated as follows:

fSW(MAX) =

VOUT + VSW(BOT)

tON(MIN) VIN –VSW(TOP) + VSW(BOT)

() (4)

where VIN is the typical input voltage, VOUT is the output

voltage, VSW(TOP) and VSW(BOT) are the internal switch

drops (~0.3V, ~0.15V, respectively at maximum load)

and tON(MIN) is the minimum top switch on-time (see the

Electrical Characteristics). This equation shows that a

slower switching frequency is necessary to accommodate

a high VIN/VOUT ratio.

For transient operation, VIN may go as high as the abso-

lute maximum rating of 42V regardless of the RT value,

however the LT8611 will reduce switching frequency as

necessary to maintain control of inductor current to as-

sure safe operation.

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