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SC1480A

DDR and DDR Memory VTT power Supply Controller

Semtech Corporation 

SC1480A

POWER MANAGEMENT

Applications Information (Cont.)

( ) ( ) L = VBAT(MIN)

V − V BAT(MIN)

OUT

• t H ON _ VBAT(MIN)

0.5 • IOUT

the DC error. The DC error will be 1% plus the tolerance

of the feedback resistors, thus 2% total for 1%

feedback resistors.

and

.or our DDR2 VTT example:

( ) ( ) L = VBAT(MAX)

V − V BAT(MAX)

OUT

• t H ON _ VBAT(MAX)

0.5 • IOUT

ERRST = 36mV and ERRDC = 18mV, therefore

RESR_ST(MAX) = 26mΩ

.or our DDR2 VTT example:

LVBAT(MIN) = 1.6µH and LVBAT(MAX) = 2.1µH

We will select an inductor value of 2.2µH to reduce the

ripple current, which can be calculated as follows:

I = (V − V )• t L A RIPPLE _ VBAT(MIN)

BAT ( MIN )

OUT

ON _ VBAT(MIN)

P−P

and

I = (V − V )• t L A RIPPLE _ VBAT(MAX)

BAT ( MAX )

OUT

ON _ VBAT(MAX )

P−P

( ) RESR _ TR(MAX)

=

ERRTR − ERRDC

IOUT

+

IRIPPLE _ VBAT(MAX )

2



Ohms

Where ERRTR is the transient output tolerance. Note that

this calculation assumes that the worst case load

transient is full load. .or half of full load, divide the IOUT

term by 2.

.or our DDR2 VTT example:

ERRTR = 72mV and ERRDC = 18mV, therefore

.or our DDR2 VTT example:

RESR_TR(MAX) = 14.6mΩ for a 3A load transient

IRIPPLE_VBAT(MIN) = 1.06AP-P and IRIPPLE_VBAT(MAX) = 1.4AP-P

.rom this we can calculate the minimum inductor

current rating for normal operation:

I = I + I 2 A INDUCTOR(MIN)

OUT(MAX )

RIPPLE _ VBAT(MAX )

(MIN)

.or our DDR2 VTT example:

We will select a value of 15mΩ maximum for our

design.

Note that for constant-on converters there is a minimum

ESR requirement for stability which can be calculated as

follows:

RESR(MIN)

=

2•

3

π • COUT

•

fSW

IINDUCTOR(MIN) = 3.7A(MIN)

This criteria should be checked once the output

capacitance has been determined.

Next we will calculate the maximum output capacitor

equivalent series resistance (ESR). This is determined by

calculating the remaining static and transient tolerance

allowances. Then the maximum ESR is the smaller of the

calculated static ESR (RESR_ST(MAX)) and transient ESR

(RESR_TR(MAX)):

Now that we know the output ESR we can calculate the

output ripple voltage:

V = R • I V RIPPLE _ VBAT(MAX)

ESR RIPPLE _ VBAT(MAX) P−P

and

( ) R = ESR _ ST(MAX)

ERRST − ERRDC • 2 Ohms

IRIPPLE _ VBAT(MAX )

Where ERRST is the static output tolerance and ERRDC is

V = R • I V RIPPLE _ VBAT(MIN)

ESR RIPPLE _ VBAT(MIN) P−P

.or our DDR2 VTT example:

 2003 Semtech Corp.

11

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