LTM4677 datasheet(22/110 Pages) AD

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LTC3888-1

Rev. 0

For more information www.analog.com

If multiple LTC3888-1 ICs are used in an application,

shared RUN pins are held low until all units initialize and

VIN exceeds the VIN_ON threshold for all devices. A com-

mon SHARE_CLK signal can also ensure all connected

devices use the same time reference for initial start-up

even if RUN pins cannot be shared due to other design

requirements. SHARE_CLK is released by each IC once

the conditions for power sequencing have been fully satis-

fied, regardless of RUN pin state.

After a channel RUN pin rises above 2V and any specified

turn on delay (TON_DELAY) has expired, the LTC3888-1

performs an initial monotonic soft-start ramp on that

channel, allowing inrush current control. This is carried

out with a digitally controlled ramp of the internal EA

reference voltage from 0mV to the commanded value

(400mV factory default) at the VOUT_TRANSITION_RATE.

The soft-start feature is disabled by setting the value of

VOUT_TRANSITION_RATE to 4V/ms. The LTC3888-1 does

not initiate PWM operation until the commanded output

exceeds the actual rail voltage and ITH has risen to a level

that will avoid negative average choke current. This allows

the regulator to start up into a prebiased load.

TIME-BASED OUTPUT SEQUENCING

The LTC3888-1 supports time-based output on and off

sequencing using a shared time reference (SHARE_CLK).

Following a qualified command to turn on, each output is

enabled after waiting its programmed TON_DELAY. This

can be used to sequence outputs in a prescribed order

that can be reprogrammed as needed without hardware

modification. Channel off-sequencing is accomplished in

a similar way with the TOFF_DELAY command.

OUTPUT RAMPING CONTROL

The LTC3888-1 supports synchronized output on and

off ramping control using a shared time reference

(SHARE_CLK). Power rail on and off relationships

similar to those of conventional analog tracking func-

tions can be achieved by using programmed delays and

VOUT_TRANSITION_RATE. With LTC3888-1 digital con-

trol, ramping configurations can be reprogrammed as

needed without hardware modification.

Programmable fault responses and fault sharing can

ensure that any desired time-based output sequencing

and ramping control is properly accomplished each time

the system powers up or down. Refer to the Applications

Information section for various LTC3888-1 hardware and

PMBus command configurations needed to fully support

synchronization for time-based sequencing and output

ramping when using multiple ICs.

VOLTAGE-BASED OUTPUT SEQUENCING

It is also possible to sequence outputs on using cascaded

voltage events. To do this, the PGOOD status pin of one

PWM channel can be used to control the RUN pin of a

downstream channel. The controlling PGOOD pin holds

RUN low if VOUT is below the VOUT_UV_FAULT_LIMIT or

if power-good conditions are not being met. This keeps

the downstream channel off until acceptable output con-

ditions exist on the controlling channel. The LTC3888-1

does not readily support voltage-based off-sequencing.

Refer to the Applications Information section for more

details on voltage-based sequencing.

OUTPUT DISABLE

All PWMs on the IC are disabled any time VIN is below

the VIN_OFF threshold. The power stages are immediately

shut off to stop the transfer of energy to the load(s) as

quickly as possible.

PWM channels may also be disabled in response to cer-

tain internal fault conditions, an external fault propagated

through a FAULT pin, or loss of SHARE_CLK. In these

cases the power stages are immediately commanded off

to stop the transfer of energy to the load as quickly as pos-

sible. Refer to the following Fault Detection and Handling

section for additional details related to fault recovery.

Each rail can be disabled with an OPERATION command

at any time if enabled by ON_OFF_CONFIG. This will force

OPERATION

LTM4677 Datasheet(PDF) 22 Page - Analog Devices

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