LTC3882-1

54

Rev A

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APPLICATIONS INFORMATION

output filter capacitance for some allowed output voltage

variation over the anticipated load range. An example of

AVP is shown in Figure 37. Refer to LTC Design Solution

10 for additional examples of using AVP to advantage.

MFR_VOUT_AVP specifies the percent reduction in pro-

equal to IOUT_WARN_LIMIT. LTC3882-1 AVP supports

to a ±7.5% tolerance about a nominal output voltage at

roughly 50% load. In order to effectively use AVP, apply

the following steps.

1. Set IOUT_OC_WARN_LIMIT. This specifies the master

phase output current at which the programmed AVP

level will apply. Generally this is above the 100% load

point to avoid spurious warnings at full load.

siredwithnoloadontheoutput.VOUT_MARGIN_HIGH/

LOW also specify no-load values when AVP is enabled.

AVP on the LTC3882-1 can only reduce the output from

these levels.

downstream rails. Due to this, cascade sequencing should

not be implemented without an external fast supervisor

to monitor downstream rails and assert a system fault if

problems occur.

Using Output Voltage Servo

For best output voltage accuracy, enable digital

servo mode on the master phase by setting bit 6 of

MFR_PWM_MODE_LTC3882-1. In digital servo mode,

the LTC3882-1 will adjust the regulated output voltage

based on its related ADC voltage reading. Every 90ms the

digital servo loop will step the LSB of the DAC (nominally

1.375mV or 0.6875mV depending on the voltage range

bit) until the output is at the correct ADC reading.

When the master channel is turned on, digital servo is

enabled after all of the following conditions are satisfied.

• MFR_PWM_MODE_LTC3882-1 Bit 6 Is Set

• The TON_RISE Sequence Is Complete

• A VOUT_UV_FAULT Is Not Present

• An IOUT_OC_FAULT Is Not Present

• MFR_AVP = 0%

Digital servo mode then engages after TON_MAX_

FAULT_LIMIT has expired as shown in Figure 29, unless

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

engaged as soon as the above conditions are satisfied.

Using AVP

The LTC3882-1 features digitally programmable active

voltage positioning (AVP), where output voltage set point

is automatically adjusted as a function of output current at

the full bandwidth of the converter. AVP normally entails

specifyinganoutputloadlineforavoltagemodeswitcherto

allowcurrentsharingbetweenmasterphasesconnectedin

parallel. While AVP can be used to this effect in LTC3882-1

control loop is recommended instead. This will produce

more accurate sharing across a wider number of phases

without degrading supply output impedance.

However,AVPcanstillbeusedtogreatbenefitinLTC3882-1

applications. AVP can be applied to minimize the size of

Figure 37. Active Voltage Positioning

38821 F37

WITH AVP

AVP DISABLED

(10A/DIV)

(50mV/DIV)

(10A/DIV)

LOOP: BW = 118kHz, PM = 58°, GM = 7dB

(50mV/DIV)

108mV

173mV