LTC3815_15 Datasheet, PDF(30/42 Page) Linear Technology – 6A Monolithic Synchronous DC/DC Step-Down Converter with Digital Power System Management

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LTC3815_15 Datasheet, PDF (30/42 Pages) Linear Technology – 6A Monolithic Synchronous DC/DC Step-Down Converter with Digital Power System Management

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LTC3815

Applications Information

Therefore, the junction temperature of the regulator op-

erating at 70Â°C ambient temperature is approximately:

TJ = 1.26W â¢ 38Â°C/W + 70Â°C = 118Â°C

We can safely assume that the actual junction temperature

will not exceed the absolute maximum junction temperature

of 125Â°C. The actual die temperature can be verified with

the READ_TEMPERATURE_1 command after the power

supply is built and operating.

Note that for very low input voltage, the junction tempera-

ture will be higher due to increased switch resistance,

RDS(ON). It is not recommended to use full load current

for high ambient temperature and low input voltage.

To maximize the thermal performance of the LTC3815 the

exposed pad should be soldered to a ground plane. See

the PCB Layout Board Checklist.

2-Phase Operation

Using two LTC3815âs as a 2-phase regulator to supply

12A loads was discussed on Page 16. A few more details

need to be brought to the userâs attention to ensure cor-

rect operation:

1) PMBus connection and READ_IIN/IOUT: Although the

2-phase regulator will operate fine with the PMBus

interface connected to the master only, the READ_IOUT

and READ_IIN measured values will only be available

for the phase(s) that are connected. Since each phaseâs

LTC3815 supplies half the load in a 2-phase converter,

the value read (if from the master only) needs to be

doubled to obtain the total IOUT or IIN value. Because

of slight differences in tracking between phases due

to IC and inductor tolerances, a more accurate reading

will be obtained by connecting to both phases so that

total IOUT and IIN can be computed by summing each

phaseâs contribution.

2) Slave PGOOD/ALERT status and margining: When the

master and slave are monitoring the same output, it is

sufficient to monitor the masterâs PGOOD and ALERT

pins only. The PGOOD/ALERT pins from both master

and slave can also be wire ORâed if desired. However, if

the output voltage is margined with the PMBus interface,

the PGOOD/ALERT status of the slave will only be valid

if the slave knows what the new margined reference

is, i.e. the margin change needs to be sent to both the

master and the slave. This can be done easily by using

the MFR_RAIL_ADDRESS to assign the same rail ad-

dress to both the master and slave so that the margin

change can be done with a single write. Be aware that

PMBus reads from a common address need to be done

separately to avoid bus contentions.

3) Using the Masterâs CLKOUT: The CLKOUT pin provides

an 180Â° out-of-phase clock that can be connected to

the slaves MODE/SYNC pin as a simple way to run the

phases out-of-phase with each other. However, be

aware that the masterâs CLKOUT pin only provides this

out-of-phase clock when the master is using its internal

oscillator programmed from the RT pin. If the master

is externally clocked, the slaveâs anti-phase clock will

need to be obtained from another source.

3815p

For more information www.linear.com/LTC3815

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