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LT8642-1

Rev. 0

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APPLICATIONS INFORMATION

Setting the Switching Frequency

The LT8642-1 uses a constant frequency PWM architec-

ture that can be programmed to switch from 200kHz to

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

table showing the necessary RT value for a desired switch-

ing frequency is in Table 1.

The RT resistor required for a desired switching frequency

can be calculated using Equation 2.

RT =

46.5

fSW

−5.2

(2)

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

1.4

28.0

1.6

23.7

1.8

20.5

2.0

17.8

2.2

15.8

3.0

10.7

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 with Equation 3.

fSW(MAX) =

VOUT + VSW(BOT)

tON(MIN) VIN − VSW(TOP) + VSW(BOT)

(

)

(3)

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

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

drops (~0.2V, ~0.1V, respectively at maximum load) and

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

Electrical Characteristics). Equation 3 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 18V regardless of the RT value,

however the LT8642-1 will reduce switching frequency

as necessary to maintain control of inductor current to

assure safe operation.

The LT8642-1 is capable of a maximum duty cycle of

approximately 99%, and the VIN-to-VOUT dropout is lim-

ited by the RDS(ON) of the top switch. In this mode the

LT8642-1 skips switch cycles, resulting in a lower switch-

ing frequency than programmed by RT.

For applications that cannot allow deviation from the pro-

grammed switching frequency at low VIN/VOUT ratios use

Equation 4 to set switching frequency.

VIN(MIN) =

VOUT + VSW(BOT)

− fSW • tOFF(MIN)

− VSW(BOT) + VSW(TOP) (4)

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 (~0.2V, ~0.1V,

respectively at maximum 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.

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