LTM4677 datasheet(53/110 Pages) AD

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LTC3888-1

Rev. 0

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Table 9. Factory EEPROM Output-Related Settings

Quantity

PMBus Command

Value

UV Threshold

VOUT_UV_FAULT_LIMIT

−10%

Margin Low

VOUT_MARGIN_LOW

−5%

Margin High

VOUT_MARGIN_HIGH

+5%

OV Threshold

VOUT_OV_FAULT_LIMIT

+10%

Under and overvoltage warning limits are effectively dis-

abled by the default factory EEPROM settings. Several

other points should also be carefully noted when using

the LTC3888-1 default factory configuration.

1. In this mode VOUT_TRANSITION_RATE (in V/ms)

applies to the VFB point, not VOUT. Absolute slew rates

at VOUT will then be scaled by a factor of (R1+R2)/R2.

2. In this mode, bit 6 of MFR_PWM_MODE_LTC3888-1

must be left clear, which is the factory default. VOUT

servo should not be used with these EEPROM set-

tings. Refer to PWM Control Loops in Electrical

Characteristics for device tolerances that apply to

VOUT accuracy in this case. Also refer to command

details for MFR_PWM_MODE_LTC3888-1. Proper use

of VOUT servo in full PMBus control context is covered

in the next few paragraphs.

PMBUS OUTPUT VOLTAGE CONTROL

It is also possible to utilize the full range of digitally

programmable output voltage controls afforded by the

PMBUs interface once external components R1 and R2

have been chosen. The following steps should be taken

to properly enable this control mode on the LTC3888-1.

1. Disable the master (page) that will utilize full PMBus

control by using the RUN pin or OPERATION command.

2. Program VOUT_SCALE_LOOP to a value equal to

(R1+R2)/R2.

3. While internal calculations are performed to imple-

ment this change in output control mode, wait for

bits[6:5] of MFR_COMMON to both be clear. This may

take as much as 500ms.

4. Program VOUT_COMMAND to the value desired

(range of 300mV to 3.45V). See VOUT_MAX details

below for limitations on this value.

5. Program related values for VOUT margins, warning

levels and hard supervisor thresholds (UV/OV) using

good practices for the bracketing of nominal VOUT with

these quantities.

6. Re-enable the output using RUN pin and/or OPERATION

command. VOUT servo can also be enabled before the

rail is turned on, or at any time afterward. See Using

Output Voltage Servo below for details.

Once direct PMBus control of VOUT is established in this

way, VOUT_TRANSITION_RATE then applies to the output

voltage proper, not VFB. Also, when using VOUT_SCALE_

LOOP on the LTC3888-1, some restrictions apply.

1. VOUT_SCALE_LOOP cannot be changed on the fly.

The rail must be commanded off by RUN pin or

OPERATION command. Bits[6:5] of MFR_COMMON

should be polled whenever this command value is

written. A rail-off update time of up to 500ms may be

required. During this period, processing of any other

VOUT-related commands sent to the LTC3888-1 will

be delayed until new VOUT_SCALE_LOOP calcuations

have completed. It is strongly recommended to set

this value just once after the desired value of (R1+R2)/

R2 is determined.

2. Sending VOUT-related commands to the LTC3888-1

or re-enabling the output before the device indicates

it has completed processing of VOUT_SCALE_LOOP

for that channel may result in erroroneous faults or

undesirable controller operation.

3. VOUT_MAX is a read-only command that returns a

value based on VOUT_SCALE_LOOP, with a maximum

given by the read-only value of MFR_VOUT_MAX. This

calculated value sets the maximum output that can be

programmed with a particular R1/R2 combination for

nominal VOUT, margin or fault. See command details

for VOUT_MAX.

APPLICATIONS INFORMATION

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