LTC2977 datasheet(65/138 Pages) AD

65 / 138 page

LTM4664A

Rev. 0

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phasing with respect to the SYNC pin. Phase relationship

values are indicated with 0° corresponding to the falling

edge of SYNC being coincident with the turn-on of the

top MOSFETs, MTn.

The MFR_PWM_CONFIG command can be altered via

I2C commands, but only when switching action is dis-

engaged, i.e., the module’s outputs are turned off. The

MFR_PWM_CONFIG command takes on the value stored

in NVM at SVIN power-up, but is overridden according

to a resistor pin-strap applied between the FSWPH_CFG

pin and SGND only if the module is configured to respect

resistor pin-strap settings (MFR_CONFIG_ALL[6] = 0b).

Table 3 highlights available resistor pin-strap and corre-

sponding MFR_PWM_CONFIG[2:0] settings.

Some combinations of FREQUENCY_SWITCH and

MFR_PWM_CONFIG[2:0] are not available by resistor

pin-strapping the FSWPH_CFG pin. All combinations of

supported values for FREQUENCY_SWITCH and MFR_

PWM_CONFIG[2:0] can be configured by NVM program-

ming—or, I2C transactions, provided switching action is

disengaged, i.e., the module’s outputs are turned off.

Care must be taken to minimize capacitance on SYNC

to assure that the pull-up resistor versus the capacitor

load has a low enough time constant for the application

to form a “clean” clock. (See “Open-Drain Pins”, later in

this section.)

When an LTM4664A PSM is configured as a sync slave,

it is permissible for external circuitry to drive the SYNC

pin from a current-limited source (less than 10mA), rather

than using a pull-up resistor. Any external circuitry must

not drive high with arbitrarily low impedance at SVIN pow-

er-up, because the SYNC output can be low impedance

until NVM contents have been downloaded to RAM.

Recommended LTM4664A PSM switching frequencies of

operation for many common VIN-to-VOUT applications are

indicated below. When the two channels of an LTM4664A

PSM are stepping input voltage(s) down to output volt-

ages whose recommended switching frequencies below

are significantly different, operation at the higher of the

two recommended switching frequencies is preferable,

but minimum on-time must be considered. (See Minimum

On-Time Considerations section.)

Table 9. Recommended PSM Switching Frequency for Various

VIN-to-VOUT Step-Down Scenarios

7.5VIN

10VIN

12VIN

14.5VIN

0.5

250kHz

0.7

0.8

0.9

1.0

1.2

350kHz

1.5

OUTPUT CURRENT LIMIT PROGRAMMING

The cycle-by-cycle current limit (= VISENSE/DCR) is pro-

portional to COMP_Cn, which can be programmed from

1.45V to 2.2V using the PMBus command IOUT_OC_

FAULT_LIMIT. The LTM4664A PSM uses only the sub-mil-

liohm sensing to detect current levels. See page103. The

LTM4664A PSM has two ranges of current limit program-

ming. The value of MFR_PWM_MODE[2] is reserved and

the MFR_PWM_MODE[7], and IOUT_OC_FAULT_LIMIT

are used to set the current limit level, see the section of

the PMBus commands, the device can regulate output

voltage with the peak current under the value of IOUT_OC_

FAULT_LIMIT in normal operation. In case of output cur-

rent exceeding that current limit, a OC fault will be issued.

Each of the IOUT_OC_FAULT_LIMIT ranges will effects the

loop gain, and subsequently effects the loop stability, so

setting the range of current limiting is a part of loop design.

The LTpowerCAD Design Tool can be used to look at the loop

stability changes if current limit is adjusted. The LTM4664A

PSM will automatically update the current limit as the induc-

tor temperature changes. Keep in mind this operation is on

a cycle-by-cycle basis and is only a function of the peak

inductor current. The average inductor current is monitored

by the ADC converter and can provide a warning if too much

average output current is detected. The overcurrent fault is

detected when the COMP_Cn voltage hits the maximum

value. The digital processor within the LTM4664A PSM

provides the ability to either ignore the fault, shut down

and latch off or shut down and retry indefinitely (hiccup).

Refer to the overcurrent portion of the Dual 25A/30A PSM

Operation section for more detail. The READ_POUT can be

used to readback calculated output power.

DUAL 25A/30A PSM APPLICATIONS INFORMATION

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