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LTM4639 Folha de dados(PDF) 10 Page - Linear Technology

Nome de Peças LTM4639
Descrição Electrónicos  Ultrathin, Triple Output, Step-Down 關Module Regulator for DDR-QDR4 Memory
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Fabricante Electrônico  LINER [Linear Technology]
Página de início  http://www.linear.com
Logo LINER - Linear Technology

LTM4639 Folha de dados(HTML) 10 Page - Linear Technology

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LTM4632
10
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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