LT8650SPJVPBF datasheet(17/34 Pages) AD

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LT8650SP

Rev. B

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APPLICATIONS INFORMATION

Table 1. SW Frequency vs RT Value

fSW (MHz)

RT (kΩ)

0.3

137

0.4

100

0.5

78.7

0.6

63.4

0.8

46.4

1.0

35.7

1.2

28.7

1.4

23.7

1.6

1.8

17.4

2.0

2.2

2.5

3.0

8.06

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:

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 (~0.3V, ~0.12V, respectively at maximum load) and

tON(MIN) is the minimum top switch on-time of 60ns (see

the Electrical Characteristics). This equation shows that a

slower switching frequency is necessary to accommodate

a high VIN/VOUT ratio. Choose the switching frequency

basedonwhichchannelhasthelowerfrequencyconstraint.

For transient operation, VINmaygoashighastheabsolute

maximumratingof42VregardlessoftheRTvalue,however

the LT8650SP will reduce switching frequency on each

channel independently as necessary to maintain control

of inductor current to assure safe operation.

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 = 3.7µA+

VOUT1

+R2

⎛

⎝⎜

⎞

⎠⎟

VOUT1

VIN1

⎛

⎝⎜

⎞

⎠⎟

⎛

⎝⎜

⎞

⎠⎟

where 3.7µA is the quiescent current of channel 1 and

common circuitries, the second term is the current in the

feedback divider reflected to the input of channel 1 oper-

ating at its light load efficiency n. For a 3.3V application

with R1 = 1M and R2 = 316k, the feedback divider draws

2.5µA. With VIN = 12V and n = 80%, this adds 0.9µA to the

3.7µAquiescentcurrentresultingin4.6µAno-loadcurrent

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.

A similar calculation can be done to determine the input

current contribution from the channel 2 feedback resis-

tors. For a 5V application with R3 = 1M, R4 = 191k, VIN

= 12V, and h = 80%, this adds 2.2µA to the input current

resulting in a total of 6.8µA with both channels on.

For a typical FB resistor of 1M, a 4.7pF to 10pF phase-lead

capacitor should be connected from VOUT to FB.

Setting the Switching Frequency

The LT8650SP uses a constant frequency PWM architec-

ture that can be programmed to switch from 300kHz to

3MHz by using a resistor tied from the RT pin to ground.

Table 1 shows the necessary RTvalueforadesiredswitch-

ing frequency.

The RT resistor required for a desired switching frequency

can be calculated using:

RT =

41.7

fSW

– 5.8

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

quency in MHz.

The two channels of the LT8650SP operate 180° out of

phase to avoid aligned switching edge noise and reduce

input current ripple.

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