LTM4677 datasheet(66/110 Pages) AD

66 / 110 page

LTC3888-1

Rev. 0

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APPLICATIONS INFORMATION

DC1613 or DC2086 will power the LTC3888-1 VDD33

supply, allowing in-circuit configuration or manufactur-

ing customization. With VDD33 applied in this fashion,

the device(s) can be initialized when the system is not

of 0x2B to command 0xBD. If this is followed with a value

of 0xC4 to 0xBD at address 0x5B, the LTC3888-1 will

begin to communicate and allow PMBus command values

to be updated at its normal PMBus device address. Any

changes will become permanent (stored to EEPROM) by

a subsequent STORE_USER_ALL command. When VIN

is reapplied, MFR_RESET must be excuted to establish

normal operation if command 0xBD has been used for

communication in the absence of system power.

The DC1613 I2C connections are opto-isolated from

the host PC USB. The DC1613 3.3V current limit is only

100mA, so it should be used to power only one or two

LTC3888-1 devices in-system. Because of this limited

current sourcing capability, only the LTC3888-1 devices,

their associated pull-up resistors and the I2C pull-up

resistors should be powered from the isolated 3.3V sup-

ply provided by the DC1613. Using the DC2086 will enable

in-system programming of several tens of LTC3888-1

devices without normal system power applied. Some

small current, normally less than 1mA, may be driven

back into the internal INTVCC LDO output or VIN supply

by each LTC3888-1 when power is applied only to VDD33.

Any other device sharing the I2C bus with the LTC3888-1

should not have internal body diodes between SDA/SCL

pins and their respective logic supply, because this will

interfere with bus communication in the absence of sys-

tem power. Configure both masters to be off, for example

by driving both RUN pins low, to avoid providing power

to the load until the part is fully configured.

DESIGN EXAMPLE

As a design example, consider a 280W dual output appli-

cation such as the one shown in Figure 46, where VIN =

12V, VOUT0 = 1V, and VOUT1 = 1.8V. Both rails support

a rated load of 100A. The TDA21470 is chosen as the

power stage for both rails based on its output current

capability, feature set, and small package size. Assume

that an auxillary 5V supply VDR will be available in the

system for TDA21470 power requirements. Details are

shown in Figure 46 for biasing and bypassing one of the

power stages. This detail is not shown for the remaining

seven TDA21470 devices but should be repeated for each

(including VIN bypass and output filter capacitance), with

components located close to each device using the PCB

layout guidelines given in previous discussion.

The necessary local bypassing is then provided for the

LTC3888-1 INTVCC (4.7µF), VDD33 (2.2µF) and VDD25

(1µF) LDO outputs. These LDO outputs should not be

shared with outputs of other ICs that might have the same

name, because they have independent internal control

loops.

Next, the regulated output for each rail is established by

selecting suitable values for R284, R285 (VOUT1), R277

and R278 (VOUT0). Maintaining a total load on DAOUT of

50kΩ to 100kΩ is recommended.

The PWM_CONFIG pin is left open to select the factory

default master/slave configuration (4+4) and a PWM oper-

ating frequency (FREQUENCY_SWITCH) of 500kHz, both

of which are desired targets for this appliation. In this

configuration the four phases of each rail are separateed

by 90°, and none of the eight phases overlap, producing

the lowest input and output ripple possible.

The design will plan on a nominal output ripple of about

55% of IOUT to minimize the magnetics volume, and the

inductance value is chosen based on this assumption.

Each phase supplies an average of 25A to the output at

full load, resulting in a ripple of 14AP-P in each choke. A

215nH inductor would create this peak-to-peak ripple at

500kHz on the 1.8V rail. (Ripple on the 1V rail will be less.)

A Cooper FP1007R3-R22-R 220nH inductor is selected

with a 50A saturation current limit at room temperature.

For input filtering, a bank of six 270μF Panasonic OS_CON

capacitors are selected to provide acceptable AC imped-

ance against the designed converter ripple current. High

frequency bypassing (1Ω into 2.2μF) is provided for the

LTC3888-1 itself, and additional ceramic bulk bypass local

to each power stage will also lower ESR on this input at

higher frequencies.

LTM4677 Datasheet(PDF) 66 Page - Analog Devices

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