DN551 - Ultrathin Triple Output μModule Regulator for DDR, QDR and QDR-IV SRAM Fits 0.5cm2 Area and Backside of PCB

Ultrathin Triple Output µModule Regulator for DDR, QDR
and QDR-IV SRAM Fits 0.5cm2 Area and Backside of PCB
Design Note 551
Sam Young and Afshin Odabaee
Introduction
Delivering the highest RTR (random transaction rate)
of QDR (quad data rate) SRAMs, QDR-IV provides up to
400Gbps data transfer for high bandwidth networking,
high performance computing and intensive data
processing applications. A key challenge at these
faster data rates is maintaining the integrity of the
data transferred between the SRAM and devices such
as high speed FPGAs and processors.
A good solution is to place the SRAM—QDR-IV, QDR
or DDR, for example—very close to the interfaced
devices on the PCB’s topside. To conserve PCB area
and minimize induced PCB parasitic noise on data
bus lines, the DC/DC regulator circuit powering the
QDR-IV SRAM data bus drivers should be placed
nearby. The challenge is finding space for regulators
on a densely populated PCB.
Using a complete DC/DC regulator with onboard
inductor and MOSFETs housed in a compact package
is one solution. But the scarcity of area on the top
of the PCB can render even compact solutions
insufficient. If the footprint, height and weight of the
DC/DC regulator solution can be reduced enough,
it can be placed on the backside of the PCB where
space is available.
VTT, VDDQ, VREF from 12VIN in a Tiny Ultrathin
Package
The LTM®4632 is a complete triple output step-down
µModule ® regulator specifically designed to support
all three voltage rails required by the new QDR-IV and
older DDR RAMs, housed in a 0.21g miniature ultrathin
profile LGA package (6.25mm × 6.25mm × 1.82mm).
Included in the package are the switching controllers,
divide-by-2 circuit, power FETs, inductors and support
components. Its tiny footprint and low external
component count (as low as one resistor and three
capacitors) occupies only 0.5cm2 (dual-sided) or
06/16/551
1cm2 (single-sided) while its thin profile enables
mounting on the PCB bottom side to free up space
on the topside for super-compact board designs.
The LTM4632 operates from an input voltage between
3.3V and 15V, providing precision output rail voltages
between 0.6V and 2.5V. Its two switching regulator
outputs, VOUT1 and VOUT2, provide up to 3A for VDDQ
and ±3A for VTT bus termination rails, respectively.
Its third output provides a low noise buffered 10mA
output for the termination reference (VTTR) tracking
voltage. Figure 1 shows the LTM4632 circuit in
a typical DDR3 application, illustrating its simple
solution and small component count.
VIN
3.6V TO
15V
22µF
4V
PGOOD1 PGOOD2
10µF
16V
VDDQ
VIN
VOUT1
RUN1 LTM4632
VOUT2
RUN2
INTVCC
VTTR
SYNC/MODE
FB1
TRACK/SS1
COMP1
VDDQIN
22µF
4V
VDDQ
1.5V, 3A
VTT
0.75V, ±3A
VTTR
0.75V, 10mA
COMP2
GND
60.4k
DN551 F01
Figure 1. Typical LTM4632 DDR3 Application
Powering More SRAM Modules
The LTM4632’s design flexibility enables it to support
a broad range of application requirements. For
example, its VDDQIN input allows the VTT and VREF
rail voltages to be set as either a typical 1/2 × VDDQ
voltage or programmed by an external reference
voltage for other values. The LTM4632 can be
configured as a two phase single output rail for VTT
in applications needing more than a ±3A termination
L, LT, LTC, LTM, Linear Technology, the Linear logo, µModule and
PolyPhase are registered trademarks of Linear Technology Corporation. All
other trademarks are the property of their respective owners.
rail current. These features allow the LTM4632 to
support voltage requirements for many different
SRAMs and increase load current requirements for
larger memory arrays.
Figure 2 illustrates the flexibility of the LTM4632.
The two switching regulator outputs of the LTM4632
are connected in a PolyPhase ® current sharing
configuration to provide up ±6A VTT for larger memory
banks. For more than 6A VDDQ, the LTM4632 can
be combined with other µModule regulators, such as
the LTM4630, to provide up to 36A for large SRAM
arrays. Efficiency and power loss are shown in
Figure 3, with thermal performance for the LTM4632
shown in Figure 4.
VIN
4.5V TO 15V
22µF
25V
VDDQ
VDDQ
1.8V, 36A
VOUT1
VIN
LTM4630
22µF
25V
×2
VOUT2
100µF
16V
×6
GND
PGOOD1 PGOOD2
VOUT1
VIN
RUN1 LTM4632 VOUT2
RUN2
VTTR
INTVCC
FB1
SYNC/MODE
TRACK/SS1
VDDQIN
47µF
4V
×3
INTVCC
VTT
0.9V, ±6A
VTTR
0.9V, 10mA
COMP1
COMP2
Figure 4. LTM4632 Thermal Performance. 12V Input,
3A. (Figure 2 Design)
over a wide range of operating conditions and output
capacitance. Voltage regulation for its switching
regulator outputs is precise, with guaranteed low
±1.5% maximum total DC output voltage error over
line, load and temperature.
Figures 5 and 6 show the fast transient performance
and tight load regulation of the LTM4632 VTT rail of
the Figure 2 circuit.
VOUT
100mV/DIV
AC-COUPLED
LOAD STEP
10A/DIV
DN551 F05
10µs/DIV
GND
DN551 F02
Figure 5. VTT Load Step, −3A to 3A (Figure 2 Design)
Figure 2. LTM4632 Two Phase Single Output ±6A VTT
with 36A LTM4630 VDDQ Supply
0.4
0.3
3.0
90
2.7
2.4
EFFICIENCY
70
2.1
60
1.8
50
1.5
40
1.2
30
0.9
POWER LOSS
20
0.6
10
0
POWER LOSS (W)
EFFICIENCY (%)
80
0.2
0.3
0
1
2
4
3
LOAD CURRENT (A)
5
6
0
VTT ERROR (%)
100
0.1
0
–0.1
–0.2
–0.3
–0.4
–6
–4
0
–2
2
VTT LOAD CURRENT (A)
4
6
DN551 F06
Figure 6. VTT Load Regulation (Figure 2 Design)
DN551 F03
Figure 3. LTM4632 Efficiency and Power Loss. 12V
Input. (Figure 2 Design)
Tight Regulation with Fast Transient Response
The LTM4632’s unique controlled on-time current
mode architecture and internal loop compensation
allow for a fast transient with good loop stability
Data Sheet Download
www.linear.com/LTM4632
Linear Technology Corporation
Conclusion
The ultrathin LTM4632 provides a complete high
performance regulator solution for all three rails
required in DDR/QDR RAM applications. Its wide
operating range, features and compact solution size
make it highly flexible and robust, and capable of
fitting into the tightest spaces on the topside and
backside of a PCB.
For applications help,
call (408) 432-1900, Ext. 2463
dn551f LT/AP 0616 111K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
 LINEAR TECHNOLOGY CORPORATION 2016
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