DN546 - Ultrathin Dual 2.5A or Single 5A μModule Regulator Fits on the Backside of PCBs, Allowing Space on the Topside for Digital ICs

Ultrathin Dual 2.5A or Single 5A µModule Regulator Fits on the
Backside of PCBs, Allowing Space on the Topside for Digital ICs
Design Note 546
Sam Young, Afshin Odabaee
Introduction
The top side of a typical system board, such as a
PCIe card, is densely populated with FPGAs/ASICs/
microprocessors, transceivers, connectors, memory
ICs and DC/DC regulators, while the backside is often
unused. This is a common side effect of the significant
difference in top vs back side height restrictions,
where board specifications may allow topside devices
to reach a few centimeters, but restrict backside
packages to less than 2.3mm. What if functions
usually found on the topside, such as DC/DC regulator
circuits could be thinner and moved to the bottom?
Real estate on top would be available for expanding
memory or enhancing the board/given the additional
top side space.
Flexible Dual Supply in a Simple <0.5cm2
Footprint
The LTM4622 has a wide input voltage range of 3.6V
to 20V, and it can be configured to operate down to
3.1V for operation from a 3.3V input supply. It regulates
two voltages for a compact multi-rail solution, where
each output can supply up to 2.5A (3A peak) and is
capable of precisely regulating 0.6V to 5.5V within
±1.5% maximum total DC output voltage error over
line, load and temperature. For higher output current
up to 5A, simply tie the outputs for current sharing.
The LTM®4622 is a dual 2.5A, or single two-phase
5A, output step-down µModule® (power module)
regulator in a 6.25mm × 6.25mm × 1.82mm ultrathin
LGA package. At nearly the height of a soldered down
1206 case size capacitor, its ultralow height allows
mounting on the backside of a PCB, freeing space
on the topside of the board. The thin profile allows it
to meet demanding height restrictions such as those
required by PCIe and advanced mezzanine cards in
embedded computing systems.
Figure 1 shows the LTM4622 circuit in a typical dual
output application, illustrating the compact solution
size. Efficiency and power loss for the circuit operating
at 12V input are shown in Figure 2.
VIN
3.6V TO 20V
4.7µF
25V
The LTM4622 requires only three ceramic capacitors
and two resistors to complete a solution occupying
less than 1cm2 single-sided or 0.5cm2 on a doublesided PCB.
Reliable High Performance Regulation
The LTM4622 features a controlled on-time current
mode architecture for a fast transient response and
L, LT, LTC, LTM, Linear Technology, the Linear logo and µModule are
registered trademarks of Linear Technology Corporation. All other trademarks
are the property of their respective owners.
PGOOD1 PGOOD2
VIN
VOUT1
RUN1 LTM4622
VOUT2
RUN2
INTVCC
SYNC/MODE
ACTUAL SIZE
47µF
47µF
COMP1
COMP2
TRACK/SS1
FB1
TRACK/SS2
FREQ
GND
FB2
60.4k
VOUT1
1.5V, 2.5A
VOUT2
1.2V, 2.5A
40.2k
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Figure 1. Typical Application: 1.5V/2.5A, 1.2V/2.5A Dual
12/15/546
86
its two outputs to be combined in parallel for load
currents up to 5A.
1.0
84
0.8
EFFICIENCY
80
78
0.6
76
74
0.4
POWER LOSS
72
70
Figures 5 and 6 demonstrate the thermal and current
sharing performance of the LTM4622 when configured
for two-phase current sharing, generating a 3.3V
output at 5A (16.5W) from a 5V input.
0.2
1.2V, 1MHz
1.5V, 1MHz
68
66
POWER LOSS (W)
EFFICIENCY (%)
82
0
0.5
1.0
1.5
LOAD CURRENT (A)
2.0
0
2.5
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Figure 2. Efficiency, Power Loss at 12V Input
(Figure 1 Design)
Figures 3 and 4 demonstrate the fast transient and
prebias start-up performance for the 1.5V output rail
of the Figure 1 circuit.
VOUT
100mV/DIV
AC-COUPLED
Figure 5. VIN = 5V, VOUT = 3.3V/5A, TA = 25°C Thermals
3.0
CURRENT EACH CHANNEL (A)
loop stability over a wide voltage range. It provides
short-circuit, overvoltage and overtemperature
protection and ensures monotonic output voltage
ramping with tracking, soft-start and the ability to
start into a prebiased output. It places no limitation
on input supply slew rate.
2.5
2.0
1.5
CHANNEL 2
1.0
CHANNEL 1
0.5
0
0
3
1
2
4
TOTAL LOAD CURRENT (A)
5
dn546 F06
LOAD STEP
1A/DIV
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Figure 3. Load Step Response (Figure 1 Design)
[12V Input, 1.5V Output, 1.25A to 2.5A]
VOUT
1V/DIV
RUN
10V/DIV
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Figure 4. 12VIN, 1.8VOUT Start-Up into
Prebiased Output
Parallel Operation for Higher Current
Applications
The LTM4622's current mode architecture yields
reliable cycle-by-cycle current monitoring, allowing
Data Sheet Download
www.linear.com/LTM4622
Linear Technology Corporation
Figure 6. Accurate Current Sharing Over the Entire
Load Range [VIN = 5V, VOUT = 3.3V at 5A]
Mirrored Layout for Smaller PCB but
Higher Power
The LTM4622’s pin configuration is organized
symmetrically. So for higher current applications
where two LTM4622s can be paralleled for up to
10AOUT, one device can be on the topside of PCB and
the other mirrored on the bottom side, minimizing PCB
area while increasing output power and power density.
Conclusion
The ultrathin LTM4622 makes it possible to put a
high performance regulator for single and multi-rail
applications on the backside of a PCB, or into tight
spaces on the topside. Its wide operating range,
features and compact solution size make it a highly
flexible and robust solution.
For applications help,
call (408) 432-1900, Ext. 2463
DN546 LT/AP 1215 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 2015
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