LINEAR Dual DC-DC Controller for DDR Power with Differential VDDQ Sensing and ±50mA VTT Reference Design Note 503

Dual DC/DC Controller for DDR Power with Differential VDDQ Sensing and ±50m A VTT Reference Design Note 503 Ding Li Introduction High Effi ciency, 4.5V to 14V Input, Dual Output

The LTC®3876 is a complete DDR power solution, DDR Power Supply

compatible with DDR1, DDR2, DDR3 and DDR4 lower Figure 1 shows a DDR3 power supply that operates voltage standards. The IC includes VDDQ and VTT DC/ from a 4.5V to 14V input. Figure 2 shows effi ciency DC controllers and a precision linear VTT reference. curves for discontinuous and forced continuous modes A differential output sense amplifi er and precision of operation. internal reference combine to offer an accurate VDDQ

Load-Release Transient Detection

supply. The VTT controller tracks the precision VTTR

As output voltages drop, a major challenge for switch-

linear reference with less than 20m V total error. The

ing regulators is to limit the overshoot in VOUT during

precision VTTR reference maintains 1.2% regulation

a load-release transient. The LTC3876 uses the DTR

accuracy, tracking one-half VDDQ over temperature for

pin to monitor the fi rst derivative of the ITH voltage to

a ±50m A reference load.

detect load release transients. Figure 3 shows how this

EF FI CI EN CY

The LTC3876 features controlled on-time, valley cur- pin is used for transient detection. rent mode control, allowing it to accept a wide 4.5V to

The two RITH resistors establish a voltage divider from

38V input range, while supporting VDDQ outputs from

INTVCC to SGND, and bias the DC voltage on the DTR

1.0V to 2.5V, and VTT and VTTR outputs from 0.5V to

pin (at steady-state load or ITH voltage) slightly above

1.25V. Its phase-locked loop (PLL) can be synchronized

half of INTVCC. For a given CITH1, this divider does not

to an external clock between 200k Hz and 2MHz. It also

L, LT, LTC, LTM, Linear Technology, the Linear logo and OPTI-LOOP are registered

features voltage-tracking soft-start, PGOOD and fault

trademarks of Linear Technology Corporation. All other trademarks are the property protection. of their respective owners.

4.5V TO VIN LTC3876

SENSE1– SENSE2–

CIN1 0.1μF 0.1μF

180μF SENSE1+ SENSE2+

MT13.57k BOOST1 BOOST2 MT2

TG1 TG2

1.5V, L1, 0.47μH L2, 0.47μH VTT

20A SW1 SW2 0.75V

VDDQ DB1 ±10ADB2

DRVCC1 DRVCC2

COUT2 330μF INTVCC

MB1 1μF 4.7μF MB2 COUT3

COUT1

30.1k COUT4 100μF

100μF BG1 BG2

VIN = 12V, DCM

PGND VIN = 12V, CCM

VIN = 5V, DCM

VOUTSENSE1 1μF

VTTRVCC VIN = 5V, CCM

VOUTSENSE1

100k VTTSNS

PGOOD

PGOOD OUTPUT CURRENT (A)

0.1μF VTTR dn503 F02

TRACK/SS1 2.2μF

15k Figure 2. Effi ciency of Circuit

ITH1 15k

100k ITH2 in Figure 1 (VDDQ = 1.5V,

dn503 F01 f SW = 400k Hz, L = 470n H)

SGND RUN

Figure 1. 1.5V VDDQ/20A 0.75V VTT/10A DDR3 Power Supply