LT8608 datasheet(12/24 Pages) LINER

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LT8608/LT8608B

Rev C

For more information www.analog.com

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:

R1= R2

OUT

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.

WhenusinglargeFBresistors,a10pFphaseleadcapacitor

should be connected from VOUT to FB.

Setting the Switching Frequency

The LT8608 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. When in spread spectrum modu-

lation mode, the frequency is modulated upwards of the

frequency set by RT.

Table 1. SW Frequency vs RT Value

fSW (MHz)

RT (kΩ)

0.2

221

0.300

143

0.400

110

0.500

86.6

0.600

71.5

0.700

60.4

0.800

52.3

0.900

46.4

1.000

40.2

1.200

33.2

1.400

27.4

1.600

23.7

1.800

20.5

2.000

18.2

2.200

16.2

Operating Frequency Selection and Trade-Offs

Selection of the operating frequency is a trade-off between

efficiency, component size, and input voltage range. The

advantageofhighfrequencyoperationisthatsmallerinduc-

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:

SW (MAX ) =

OUT + VSW (BOT )

ON(MIN) VIN – VSW ( TOP ) + VSW (BOT )

(

)

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

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

drops (~0.55V, ~0.35V, respectively at max load) and

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

Characteristics). This equation shows that slower switch-

ing frequency is necessary to accommodate a high VIN/

VOUT ratio.

For transient operation VIN may go as high as the Abs Max

rating regardless of the RT value, however the LT8608

will reduce switching frequency as necessary to maintain

control of inductor current to assure safe operation.

The LT8608 is capable of maximum duty cycle approach-

ing 100%, and the VIN to VOUT dropout is limited by the

RDS(ON) of the top switch. In this mode the LT8608 skips

switch cycles, resulting in a lower switching frequency

than programmed by RT.

For applications that cannot allow deviation from the pro-

grammed switching frequency at low VIN/VOUT ratios use

the following formula to set switching frequency:

IN(MIN) =

OUT + VSW (BOT )

1– f

SW • tOFF(MIN)

– V

SW (BOT ) + VSW ( TOP )

where VIN(MIN) is the minimum input voltage without

skipped cycles, VOUT is the output voltage, VSW(TOP) and

VSW(BOT) are the internal switch drops (~0.55V, ~0.35V,

respectively at max load), fSW is the switching frequency

(set by RT), and tOFF(MIN) is the minimum switch off-time.

Note that higher switching frequency will increase the

minimuminputvoltagebelowwhichcycleswillbedropped

to achieve higher duty cycle.

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