LTC4418 datasheet(20/30 Pages) LINER

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LTC4418

Rev A

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APPLICATIONS INFORMATION

LTC4418 to connect its highest valid priority channel to

VOUT. Confirmation that all channels are off before the

slave is allowed to connect its channel to VOUT prevents

cross conduction from occurring.

Driving the master LTC4418’s EN low forces both master

and slave to disconnect all channels from the common

output and continue monitoring the input supplies. Driv-

ing the master LTC4418’s SHDN low places it in a reset

state where all of its channels are disconnected and CAS is

pulled high with a 20µA current source, allowing the slave

LTC4418 to become the master and connect its highest

valid priority channel to the common output.

DESIGN EXAMPLE

In this example, the LTC4418 prioritizes between 5V and

12V supplies for a 1.25A system as shown in Figure 7.

Power is only sourced from the 12V supply when the 5V

supply is invalid. The 12V supply has a 20mΩ source

resistance (RSRC) and the ambient conditions of the

system are between 25°C and 85°C. The design must

accommodate ±2.5% tolerance on the 5V supply and a

±10% tolerance on the 12V supply and limit the VOUT

droop to 500mV during switchover. The load capacitor

is assumed to be an electrolytic with a minimum ESR

(ESRCOUT) of 25mΩ at 25°C.

Determining OV/UV Windows

For the 5V V1 supply, ±2.5% tolerance sets an operational

windowof4.875Vto5.125V.Inordertoaccommodatethis

voltagerangetheOV/UVthresholdsmustallowfordesired

hysteresis, external resistive divider error and comparator

threshold error. For the 5V V1 supply, an additional ±2.5%

error is included for margin which means that the OV/UV

windowmustaccommodate4.75Vto5.25Vrange.For5%

external hysteresis or 250mV, set the UV1 = 4.5V and OV1

= 5.5V. For the 12V V2 supply, taking into account supply

tolerance and a margin for error sources the operational

window is 10.2V to 13.78V. Since external hysteresis is

used, channel 2 also has 250mV of hysteresis (±2.1%).

The OV/UV thresholds are UV2 = 9.95V and OV2 = 14.03V.

These thresholds determine the maximum possible dif-

ferential voltage between supplies at switchover.

LTC4418

MASTER

CAS TO EN CONNECTION

VS2

VOUT

CAS

SHDN

DISABLE ALL CHANNELS

SHDN MASTER

2 SUPPLIES

MASTER

2 SUPPLIES

SLAVE

1ST PRIORITY

SUPPLY

2ND PRIORITY

SUPPLY

4TH PRIORITY

SUPPLY

3RD PRIORITY

SUPPLY

VS1

LTC4418

SLAVE

VS5

VOUT

SHDN

VS7

4418 F06

VOUT

COUT

Figure 6. Cascaded Application

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