SC1480AEVB datasheet(10/23 Pages) SEMTECH

10 / 23 page

 2003 Semtech Corp.

www.semtech.com

SC1480A

POWER MANAGEMENT

Applications Information (Cont.)

The on pulse in the SC1480A is calculated to give a

pseudo fixed frequency. Nevertheless, some frequency

variation with line and load can be expected. This variation

changes the output ripple voltage. Because constant on

regulators regulate to the valley of the output ripple, ½

of the output ripple appears as a DC regulation error.

.or example, if RE.OUT=0.9V, then the valley of the

output ripple will be 0.9V. If the ripple is 20mV with VBAT

= 6V, then the DC output voltage will be 0.91V. If the

ripple is 30mV with VBAT = 25V, then the DC output

voltage will be 0.915V.

The output inductor value may change with current. This

will change the output ripple and thus the DC output

voltage. It will not change the frequency.

Switching frequency variation with load can be minimized

by choosing MOS.ETs with lower R

DS(ON). High RDS(ON)

MOS.ETs will cause the switching frequency to increase

as the load current increases. This will reduce the ripple

and thus the DC output voltage.

Input (VBAT) Supply Selection

The SC1480A can be configured so that VTT is generated

directly from the battery. Alternatively, VTT can be

generated from VDDQ. Since the battery configuration

generally yields better overall efficiency and performance,

the recommended method is to generate VTT from the

battery.

Design Procedure

Prior to designing an output and making component

selections, it is necessary to determine the input voltage

range and the output voltage specifications. .or purposes

of demonstrating the procedure the VTT output for the

schematic in .igure 3 on Page 14 will be designed.

The maximum input voltage (V

BAT(MAX)) is determined by

the highest AC adaptor voltage. The minimum input

voltage (V

BAT(MIN)) is determined by the lowest battery

voltage after accounting for voltage drops due to

connectors, fuses and battery selector switches. .or the

purposes of this design example we will use a V

BAT range

of 8V to 20V.

.our parameters are needed for the output:

1) nominal output voltage, V

OUT (for DDR2 this is 0.9V)

2) static (or DC) tolerance, TOL

ST (for DDR2 this is

+/-40mV, or 4.44%, we will design for 4%)

3) transient tolerance, TOL

TR and size of transient (for

DDR2 this is undefined, so assume +/-8% for purposes

of this demonstration).

4) maximum output current, I

OUT (we will design for 3A)

Switching frequency determines the trade-off between

size and efficiency. Increased frequency increases the

switching losses in the MOS.ETs, since losses are a

function of VIN2. Knowing the maximum input voltage and

budget for MOS.ET switches usually dictates where the

design ends up. A default R

tON value of 715kΩ is

suggested as a starting point, but this is not set in stone.

The first thing to do is to calculate the on-time, t

ON, at

BAT(MIN) and VBAT(MAX), since this depends only upon VBAT,

OUT and RtON. .or VOUT < 3.3V:

()

)

MIN

(

BAT

OUT

tON

)

MIN

(

VBAT

−

•













•

.rom this value of t

ON we can calculate the nominal

switching frequency as follows:

()Hz

)

MIN

(

VBAT

)

MIN

(

BAT

OUT

)

MIN

(

VBAT

•

and

()Hz

)

MAX

(

VBAT

)

MAX

(

BAT

OUT

)

MAX

(

VBAT

•

ON is generated by a one-shot comparator that samples

BAT via RtON, converting this to a current. This current is

used to charge an internal 3.3p. capacitor to V

OUT. The

equations above reflect this along with any internal

components or delays that influence t

ON. .or our DDR2

VTT example we select R

tON = 715kΩ:

ON_VBAT(MIN) = 329ns and tON_VBAT(MAX) = 162ns

SW_VBAT(MIN) = 342kHz and fSW_VBAT(MAX) = 278kHz

Now that we know t

ON we can calculate suitable values

for the inductor. To do this we select an acceptable

inductor ripple current. The calculations below assume

50% of I

OUT which will give us a starting place.

SC1480AEVB Datasheet(PDF) 10 Page - Semtech Corporation

SC1480AEVB Datasheet(HTML) 10 Page - Semtech Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Datasheet Download

Link URL