LT8685SRVPBF datasheet(15/24 Pages) AD

15 / 24 page

LT8685S

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

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:

V(CHAN_UVLO) =

RUV1+RUV2

RUV2

• 0.81V

RUV2

RUV1

ENx

VINx

LT8685S

8685S F02

Figure 2. Individual Channel UVLO Resistor Divider Connection

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)

RT (kΩ)

0.35

154

0.5

107

0.75

49.9

1.25

1.5

1.75

26.4

22.6

2.25

19.6

2.5

17.2

2.75

15.4

13.7

The following equation approximates the values shown

in Table 1.

RT =

55.4

fSW – 0.002

– 5

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

sitive to noise so the resistor should be placed close to

the LT8685S and away from noise sources.

The oscillator generates four clock phases. When operat-

ing independently, the relative phases of the channels are

0° for CH1, 180° for CH2, 270° for CH3 and 90° for CH4.

When combining channels, the master channel phase

determines the combined channel’s clock phase.

COMBINING CHANNELS

The LT8685S provides the ability to combine multiple

regulators to create a single regulator capable of greater

output current using a single inductor.

The allowed channel combinations are given in Table 2.

Table 2. Allowed Channel Combinations

CONFIGURATION

NUMBER OF INDEPENDENT REGULATORS

1, 2, 3, 4

1, 2, 3 + 4

1 + 2, 3, 4

1 + 2, 3 + 4

When combining, the lowest numbered channel assumes

control of the combined regulators. For example, with

channel 1 and channel 2 combined, channel 1 assumes

control (master) and channel 2 becomes dependent

(slave). A feedback network is connected only to the

master channel to program the combined regulator out-

put voltage. The slave channel’s feedback pin must be

connected to INTVCC.

Combined channels must have a low impedance connec-

tion between their respective VIN pins, SW pins, and BST

pins. Only a single inductor is connected to the com-

bined SW pins. Though combined channels’ BST pins

are connected together, each channel should retain their

individual boost capacitors.

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