LT8604 datasheet(12/20 Pages) AD

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LT8604

Rev. 0

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APPLICATIONS INFORMATION

Table 1. SW Frequency vs RT Value

fSW (MHz)

RT (kΩ)

0.2

221

0.3

143

0.4

110

0.5

86.6

0.6

71.5

0.7

60.4

0.8

52.3

0.9

46.4

1.0

40.2

1.2

33.2

1.4

27.4

1.6

23.7

1.8

20.5

2.0

18.2

2.2

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 advantage of high frequency operation is that

smaller inductor 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)

(

)

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

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

drops (~0.38V, ~0.14V, 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 LT8604

will reduce switching frequency as necessary to maintain

control of inductor current to assure safe operation.

The LT8604 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 LT8604 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:

VIN(MIN) =

VOUT + VSW(BOT)

1– fSW • tOFF(MIN)

– VSW(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 (~0.38V, ~0.14V,

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 minimum input voltage below which cycles will be

dropped to achieve higher duty cycle.

Inductor Selection and Maximum Output Current

The LT8604 is designed to minimize solution size by

allowing the inductor to be chosen based on the output

load requirements of the application. During overload or

short circuit conditions the LT8604 safely tolerates opera-

tion with a saturated inductor through the use of a high

speed peak-current mode architecture.

A good first choice for the inductor value is:

VOUT + VSW(BOT)

fSW

• 20

where fSW is the switching frequency in MHz, VOUT is

the output voltage, VSW(BOT) is the bottom switch drop

(~0.14V) and L is the inductor value in μH.

To avoid overheating and poor efficiency, an inductor

must be chosen with an RMS current rating that is greater

than the maximum expected output load of the applica-

tion. In addition, the saturation current (typically labeled

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