PM6641 datasheet(29/47 Pages) STMICROELECTRONICS

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PM6641

Device description

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7.7

Outputs Soft-End

When the switching regulator enable pin (EN_1S8 for the VDDQ section, EN_1S5 and

EN_1S05 for chipset sections) goes down or when UV or thermal protections are detected,

the switching regulator output capacitor is actively discharged through a dedicated

discharge MOSFET of about 25

Ω typical resistance.

The PM6641 DDR supply allows choosing between two different output discharge

behaviors, involving the VDDQ (1S8) switching rail, VTT LDO termination and VTTREF

reference buffered voltage: the tracking discharge and the non-tracking discharge. This

selection is set by tying the Discharge pin (DSCG) to AVCC (tracking discharge enabled) or

to AGND (tracking discharge disabled).

When the 1.8 V rail is turned off (EN_1S8 goes low) and non-tracking discharge is active

(DSCG is low), or when UV or thermal protections are detected, the VDDQ and VTT rails

and the VTTREF buffer are discharged by internal discharge MOSFETs, through the

VSW_1S8, VTTFB and VTTREF pins respectively. VTT termination output capacitor is

discharged through 25

Ω dedicated MOSFET whereas VTTREF output capacitor is

discharged through 200

Ω dedicated MOSFET.

When the 1.8 V rail is turned off (EN_1S8 goes low) and tracking discharge is selected

(DSCG is high), tracking discharge takes place:

●

The 1.8 V rail regulator is discharged by internal MOSFET

●

The 0.9 V VTT LDO and VTTREF work in tracking with the half of 1.8 V rail

When the VTT LDO and VTTREF reach a voltage threshold of about 200-300 mV, the

device switches to non-tracking discharge mode and the internal discharge MOSFETs are

turned on.

7.8

Switching frequency selection

SET_SWF (pin 2) allows to vary the internal oscillator switching frequency, in the range of

500 kHz

1 MHz, by connecting this pin to AGND through a resistor between

70 k

Ω

140k

Ω.

The following table summarizes the output resistor – switching frequency correspondence:

When SET_SWF is tied to AVCC the internal reference is chosen and each regulator

performs a typical 750 kHz switching frequency.

Table 10. Typical values for switching frequency selection

RSET_SWF (kΩ)

Approx. switching frequency (kHz)

140

500

100

670

1000

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