LTM4677 datasheet(54/110 Pages) AD

54 / 110 page

LTC3888-1

Rev. 0

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USING OUTPUT VOLTAGE SERVO

For best output voltage accuracy, enable VOUT

servo mode on the master phase by setting bit 6 of

MFR_PWM_MODE_LTC3888-1 once PMBus output

control is fully programmed. In VOUT servo mode, the

LTC3888-1 will adjust the regulated output voltage based

on its related monitor ADC reading. Every time that output

voltage ADC channel is converted, the VOUT servo will

step the internal EA reference by 297.5μV until the output

produces the correct ADC reading.

In order for the LTC3888-1 to servo VOUT, VOUT_SCALE_

LOOP must be used. This will allow the final VOUT cre-

ated by the servo at any condition to have ±0.2% typical

accuracy (±0.5% absolute maximum). The raw output

created by the application must be within ±5.5% of

VOUT_COMMAND for the VOUT servo to function to full

effectiveness, so external feedback resistors with ±0.5%

tolerance are recommended.

When the master channel is turned on, VOUT servo is

enabled after all of the following conditions are satisfied.

• Bit 6 of MFR_PWM_MODE_LTC3888-1 Is Set

• The Soft-Start Sequence Is Complete

• A VOUT_UV_FAULT Is Not Present

• An IOUT_OC_FAULT Is Not Present

VOUT servo mode then engages after TON_MAX_ FAULT_

LIMIT has expired as shown in Figure 30, unless that limit

is set to 0s (infinite). In that case, the mode is engaged as

soon as the above conditions are satisfied.

SOFT-START AND STOP

The LTC3888-1 uses digital ramp control to create both

soft-start and soft-stop.

The LTC3888-1 must enter the run state prior to soft-start.

The RUN pins are released after the part initializes and VIN

is determined to be greater than the VIN_ON threshold.

Once in the run state, soft-start is performed after any

additional prescribed delay (see next section) by actively

regulating the load voltage while digitally ramping the

target voltage from 0V to the final expected value. Rise

time of the voltage ramp can be programmed using

VOUT_TRANSITION_RATE to minimize inrush currents

associated with the start-up voltage ramp. The maximum

rate at which the LTC3888-1 can move the output in this

fashion is 100µs/step. Soft-start is disabled by setting

VOUT_TRANSITION_RATE to 4V/ms. The LTC3888-1

will internally perform the math necessary to assure the

voltage ramp is controlled to the desired slope. However,

the voltage slope cannot be any faster than fundamental

limits imposed by the power stage. The larger VOUT_

TRANSITION_RATE becomes, the more noticeable an

output voltage stair-step may become.

The LTC3888-1 also supports soft turn off in the same

manner it controls turn on. Soft-off is performed when

the RUN pin goes low or if the part is commanded off.

The LTC3888-1 can produce a controlled ramp off as long

as VOUT_TRANSITION_RATE is sufficiently slow to allow

the control loop and power stage to achieve the desired

slope. If VOUT_TRANSITION_RATE is too high to dis-

charge the load capacitance, the output will not reach 0V.

In this case, the power stage will still be commanded off

at the end of soft-off and VOUT will decay at a rate deter-

mined by the load. The larger VOUT_TRANSITION_RATE

becomes, the more noticeable an output voltage stair-step

may become. If a rail faults off for any reason, all related

PWM phases are instantly commanded off. The output

will then decay as a function of load current.

TIME-BASED OUTPUT SEQUENCING AND RAMPING

The LTC3888-1 TON_DELAY and TOFF_DELAY commands

can be used in combination with VOUT_TRANSITION_

RATE to implement a wide range of versatile sequencing

and ramping schemes. The key to time-based sequencing

and ramping is the ability of LTC3888-1 master phases

to move their outputs up and down according to PMBus

command values as shown in Figure 30 and Figure 31.

There is a fixed delay and other timing uncertainty associ-

ated with all changes in output voltage controlled by the

LTC3888-1. A nominal fixed timing delay of 270µs exists to

process any change in output voltage including soft-start/

stop and margining. The start of all time-based output

operations occur with an uncertainty of ±50µs and have

a nominal step resolution of 100µs. This means the mini-

mum controlled time delay the LTC3888-1 can produce

APPLICATIONS INFORMATION

LTM4677 Datasheet(PDF) 54 Page - Analog Devices

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