LTC3882_15 Datasheet, PDF(20/104 Page) Linear Technology – Dual Output PolyPhase Step-Down DC/DC Voltage Mode Controller with Digital Power System Management

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LTC3882_15 Datasheet, PDF (20/104 Pages) Linear Technology – Dual Output PolyPhase Step-Down DC/DC Voltage Mode Controller with Digital Power System Management

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LTC3882

OPERATION

at the selected frequency (FREQUENCY_SWITCH) using

its own internal oscillator, if an external clock signal is

subsequently lost.

The MFR_PWM_CONFIG_LTC3882 command can be used

to configure the phase of each channel. Desired phase

can also be set from EEPROM or external configuration

resistors as outlined in Table 11. Phase designates the

relationship between the falling edge of SYNC and the

internal clock edge that resets the PWM latch. That reset

turns off the top power switch, producing a TG/PWM fall-

ing edge. Additional small propagation delays to the PWM

control pins will apply.

The phase relationships and frequency are independent

of each other, providing numerous application options.

Multiple LTC3882 ICs can be synchronized to realize a

PolyPhase array. In this case the phases should be sepa-

rated by 360/n degrees, where n is the number of phases

driving the output voltage rail.

PolyPhase Load Sharing

Multiple LTC3882 ICs can be combined to provide a bal-

anced load-share solution by configuring the necessary

pins. The SHARE_CLK and SYNC pins of all load-sharing

channels should be bussed together. Connecting the

SYNC pins synchronizes the PWM controllers with each

other. Bussing the SHARE_CLK pins together allows the

phases to start synchronously. Refer to the discussion in

the previous Power-Up and Initialization section. The last

device to see all start-up conditions satisfied controls the

initiation of power sequencing for all phases.

Due to the low output impedance of the LTC3882 error

amplifiers, PolyPhase applications should use the error

amplifier of only one phase as the master. The FB pins of

each slave channel must be wired to VDD33, and the COMP

pins of each slave phase must be connected to the master

error amplifier COMP output. This disables the slave error

amplifiers and provides a single point of voltage control

and loop stabilization for the PolyPhase output rail.

For PolyPhase load sharing the LTC3882 also incorporates

an auxiliary current sharing loop. Referring back to Figure 1,

the instantaneous current of each slave phase is sensed

by current amplifier CA and compared to the IAVG pin. The

IAVG and IAVG_GND pins of each phase are wired together,

and a small capacitor (50pF to 200pF) between IAVG and

IAVG_GND stores a voltage corresponding to the average

master phase output current. The difference in this aver-

age and the instantaneous phase current is integrated.

The output of integrator S of each slave phase is then

proportionally summed with the master error amplifier

COMP output to adjust the duty cycle and balance the

current contribution of that phase. Additional hardware

configuration and digital programming requirements apply

in PolyPhase systems. Refer to the Applications Informa-

tion section for complete details on building PolyPhase

rails with the LTC3882.

Active Voltage Positioning

Load slope is programmable in the LTC3882 via the

MFR_VOUT_AVP PMBus command. The inductor cur-

rent measured at the ISENSE pins is converted to a voltage

which is then subtracted from the voltage reference at the

positive input of the error amplifier. The final load slope

is defined by the inductor current sense element and the

bits set in the MFR_VOUT_AVP PMBus command. Setting

MFR_VOUT_AVP to a value greater than 0.0% automatically

disables output servo mode for that channel.

Input Supply Monitoring

The input supply voltage is sensed by the LTC3882 at the

VINSNS pin. Undervoltage, overvoltage, valid on and off

levels can be programmed for VIN. Refer to the following

PMBus Command Details section for more information on

programming the input supply thresholds. In addition, the

telemetry ADC monitors the VINSNS voltage relative to

GND. Conversion results are returned by the READ_VIN

PMBus command.

Output Voltage Sensing and Monitoring

Both PWM channels allow remote, differential sensing of the

load voltage with VSENSE pins. The channel 1 output sense

pin VSENSE1â is internally shorted to GND (the exposed

pad). The telemetry ADC is fully differential and makes its

measurements of the output voltages of channels 0 and 1

at VSENSE0Â± and VSENSE1Â±, respectively. Conversion results

are returned by the READ_VOUT PMBus command.

3882f

For more information www.linear.com/LTC3882

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