LTC6902 datasheet(10/28 Pages) LINER

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LTM4632

4632fc

For more information www.linear.com/LTM4632

APPLICATIONS INFORMATION

Channel 1 Output Voltage Programming (Configured

as VDDQ)

The PWM controller for the VOUT1 has an internal 0.6V

reference voltage. As shown in the Block Diagram, a 60.4k

internal feedback resistor connects VOUT1 and FB1 pins

together. Adding a resistor RFB from FB1 pin to GND pro-

grams the output voltage:

RFB =

0.6V

VOUT – 0.6V

• 60.4k

Table 1. VFB Resistor Table (1%) vs Various Output Voltages

VOUT(V)

0.6

1.0

1.2

1.3

1.5

1.8

2.5

RFB(k)

OPEN

90.9

60.4

52.3

40.2

30.1

19.1

Channel 2 Output Voltage Programming (Configured

as VTT)

The PWM controller for the VOUT2 uses VTTR voltage as

a reference voltage. VOUT2 is directly connected to the

negative side of the error compiler to internally program

VOUT2 to equal to VTTR voltage, which equals to one half

of VDDQIN voltage.

VOUT2 = VTTR = VDDQIN/2

In a complete DDR memory power application which

require both VDDQ supply and VTT terminal outputs,

configure LTM4632 Channel 1 as VDDQ output by add-

ing a feed-back resistor from FB1 pin to GND. Feed VOUT1

(VDDQ output) voltage to VDDQIN pin to program Channel

2 as VTT output which equals half of the Channel 1 (VDDQ

output) voltage.

Input Decoupling Capacitors

The LTM4632 module should be connected to a low

AC-impedance DC source. For each regulator channel,

one piece 4.7µF input ceramic capacitor is required for

RMS ripple current decoupling. Bulk input capacitor is

only needed when the input source impedance is com-

promised by long inductive leads, traces or not enough

source capacitance. The bulk capacitor can be an electro-

lytic aluminum capacitor and polymer capacitor.

Without considering the inductor current ripple, for each

output, the RMS current of the input capacitor can be

estimated as:

ICIN(RMS) =

IOUT(MAX)

η%

• D •(1– D)

where η% is the estimated efficiency of the power module.

Output Decoupling Capacitors

With an optimized high frequency, high bandwidth design,

only single piece of 22µF low ESR output ceramic capaci-

tor is required for each LTM4632 output to achieve low

output voltage ripple and very good transient response.

Additional output filtering may be required by the sys-

tem designer, if further reduction of output ripples or

dynamic transient spikes is required. Table 5 shows a

matrix of different output voltages and output capacitors

to minimize the voltage droop and overshoot during a

0.75A (25%) load step transient. Multiphase operation

will reduce effective output ripple as a function of the

number of phases. Application Note 77 discusses this

noise reduction versus output ripple current cancella-

tion, but the output capacitance will be more a function

of stability and transient response. The Linear Technology

LTpowerCAD Design Tool is available to download online

for output ripple, stability and transient response analysis

and calculating the output ripple reduction as the number

of phases implemented increases by N times.

Burst Mode Operation

In applications where high efficiency at intermediate

current are more important than output voltage ripple,

burst mode operation could be used on Channel 1 by

connecting SYNC/MODE pin to INTVCC to improve light

load efficiency. In Burst Mode operation, a current rever-

sal comparator (IREV) detects the negative inductor cur-

rent and shuts off the bottom power MOSFET, resulting

in discontinuous operation and increased efficiency. Both

power MOSFETs will remain off and the output capacitor

will supply the load current until the COMP voltage rises

above the zero current level to initiate another cycle.

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