LT8686SJVPBF datasheet(15/26 Pages) AD

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LT8686S

Rev. 0

For more information www.analog.com

The LT8686S combines four buck converters to provide a

flexible system supply that can be configured to generate

two to four regulated outputs while occupying minimal

board space.

PRECISION UNDERVOLTAGE LOCKOUT

The accurate 0.8V threshold (falling) of the EN/UVLO pin

enables a programmable LT8686S undervoltage lockout

feature realized by connecting an external resistor divider

between the VIN1 input supply pin and the EN/UVLO pin.

An EN/UVLO logic low input turns off all channels regard-

less of the state of EN2, EN3, and EN4 pins.

The LT8686S UVLO divider circuit is shown in Figure 1.

The UVLO threshold is given by Equation 1.

V(LT8686S_UVLO) =

RUV1+RUV2

RUV2

•0.8V

(1)

EN/UVLO

VIN

LT8686S

RUV1

RUV2

8686S F01

Figure 1. LT8686S UVLO Resistor Divider Connection

Similar to the EN/UVLO pin, the accurate 0.81V reference

of the EN2, EN3 and EN4 pins enables a programma-

ble undervoltage lockout feature for each correspond-

ing channel, realized by connecting an external resistor

divider between the respective channel’s input supply pin

and its EN pin.

The individual channel UVLO divider circuit is shown in

Figure 2. The threshold is given by Equation 2.

V(CHAN_UVLO) =

RUV1+RUV2

RUV2

•0.81V

(2)

ENx

VINx

LT8686S

RUV1

RUV2

8686S F02

Figure 2. Individual Channel UVLO Resistor Divider Connection

APPLICATIONS INFORMATION

SWITCHING FREQUENCY

Each channel operates from a clock provided by an

internal oscillator whose frequency is determined by an

external resistor connected from the RT pin to ground.

By selecting the appropriate external resistor value, the

switching frequency may be programmed from 350kHz

to 3MHz.

Table 1 shows the recommended value of the RT resistor

for some common switching frequencies.

Table 1. Switching Frequency (fSW) vs RT Value

SWITCHING FREQUENCY

(MHz)

(kΩ)

0.35

154

0.5

107

0.75

1.0

49.9

1.25

39.0

1.5

32.0

1.75

26.4

2.0

22.6

2.25

19.6

2.5

17.2

2.75

15.4

3.0

13.7

Equation 3 approximates the values shown in Table 1.

RT =

55.4

fSW 0.002

(3)

Where fSW is in MHz, and RT is kΩ. The RT pin is sensi-

tive to noise so the resistor should be placed close to the

LT8686S and away from noise sources.

The oscillator generates four-clock phases. When oper-

ating independently, the relative phases of the channels

are 0° for CH1, 180° for CH2, 270° for CH3 and 90° for

CH4. When combing channels, the master channel phase

determines the combined channel’s clock phase.

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