PM6670AS datasheet(37/54 Pages) STMICROELECTRONICS

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PM6670AS

Device description

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7.2

VTTREF buffered reference and VTT LDO section

The PM6670AS provides the required DDR2/3 reference voltage on the VTTREF pin. The

internal buffer tracks half the voltage on the VSNS pin and has a sink and source capability

up to 15 mA.

Higher currents rapidly deteriorate the output accuracy. A 10 nF to 100 nF (33 nF typ.)

bypass capacitor to SGND is required for stability.

The VTT Low-Drop-Out linear regulator has been designed to sink and source up to 2 A

peak current and 1 A continuously. The VTT voltage tracks VTTREF within ±35 mV.

A remote voltage sensing pin (VTTSNS) is provided to recovery voltage drops due to

parasitic resistance. In DDR2/3 applications, the linear regulator input LDOIN is typically

connected to VDDQ output; connecting LDOIN pin to a lower voltage, if available in the

system, reduces the power dissipation of the LDO.

A minimum output capacitance of 20

µF (2x10 µF MLCC capacitors) is enough to assure

stability and fast load transient response.

7.2.1

VTT and VTTREF soft-start

Soft-start on VTT and VTTREF outputs is achieved by current clamping.

The LDO linear regulator is provided with a current foldback protection: when the output

voltage exits the internal ±10 % VTT-Good window, the output current is clamped at ±1 A.

Re-entering VTT-Good window releases the current limit clamping.

The foldback mechanism naturally implements a two steps soft-start charging the output

capacitors with a 1 A constant current.

Something similar occurs at VTTREF pin, where the output capacitor is smoothly charged at

a fixed 40 mA current limit.

7.2.2

VTTREF and VTT outputs discharge

The Tracking Discharge mechanism involves the VTT linear regulator. When the Suspend-

To-Disk state is entered, the switching regulator is turned OFF.

At the same time the LDO drains a 1A constant current from LDOIN and keeps VTT in track

with VTTREF that, in turn, is half the voltage at the VSNS pin. When the VDDQ output

reaches 400mV, the PM6670AS switches on the internal discharge MOSFETs to complete

the process (see Section 7.1.8: VDDQ output discharge on page 34).

In Soft Discharge (i.e. Non-Tracking Discharge) the PM6670AS disables the internal

regulators and suddenly turns on the discharge MOSFETs on each output.

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