DN409 - Triple Output 3-Phase Controller Saves Space and Improves Performance in High Density Power Converters

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Triple Output 3-Phase Controller Saves Space and Improves
Performance in High Density Power Converters
Design Note 409
Mike Shriver
Today’s telecommunications, server and network applications require power from a multitude of voltage
rails. Having more than ten rails ranging from 5V to 1V
or less is common. These boards are typically crowded
with heat-producing FPGAs or microprocessors, thus
demanding power converters that are both compact and
highly efficient. Furthermore, the converters may need
to meet other requirements such as a fast load step
response and rail tracking.
220kHz, 400kHz or 560kHz, or it can be synchronized
to an external clock between 160kHz and 700kHz. The
controller can step down from input voltages as high as
36V and the output voltage can be programmed from
0.6V to 5V.
Figure 1 shows a high density triple output DC/DC converter with each output delivering up to 5A using the
LTC3773 controller. Figure 2 shows the efficiency of each
output versus load current; where up to 93% efficiency is
achieved. Reductions in space are realized by the use of
dual channel FETs and a switching frequency of 400kHz
which permits the use of 7mm × 7mm ferrite inductors.
The LTC® 3773 switching regulator meets and even goes
beyond the above requirements. This device is a 3-phase,
triple output synchronous buck controller with built-in
gate drivers packaged in either a 5mm × 7mm QFN or
a 36-pin SSOP. Its switching frequency can be set to
68.1k
POWER DOWN VOUT1
POWER DOWN VOUT2
10Ω
POWER DOWN VOUT3
1000pF
, LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technology
Corporation. All other trademarks are the property of their respective owners.
10Ω
VCC
4.5V TO 7V
PGOOD
150pF
6
10k
7
10k
8
0.01µF
9
0.01µF
10
11
12
20k
SW2
ITH2
TG2
LTC3773
ITH3
BOOST2
VFB2
BOOST3
VFB3
TG3
TRACK2
SW3
TRACK3
BG1
SENSE2–
SENSE2+
14
15
16
17
18
31.6k
10Ω
1000pF
L2, L3:
TDK RLF7030T-1R5M5R4
10Ω
PGOOD
SENSE1+
PHASEMD
SENSE1–
SDB1
SGND
13
L1:
TDK RLF7030T-2R2M5R4
BOOST1
10Ω
19
VDR
31
30
1
0.1µF
29
5
2.2µF
10Ω
6, 7
L1
2.2µH
28
27
26
CMDSH-3
CMDSH-3
24
0.1µF
22µF
X5R
8 Si4816BDY
1
23
5
22
4
6, 7
VIN
4.7µF
25V
L2
1.5µH
7mΩ
21
VOUT2
2.5V/5A
+
COUT2
220µF
4V
22µF
X5R
2, 3
20
4.7µF
8 Si4816BDY
1
6, 7
0.1µF
VIN
4.7µF
25V
L3
1.5µH
7mΩ
VOUT3
1.8V/5A
COUT3
220µF
4V
4
2, 3
COUT1 COUT2 COUT3,:
SANYO POSCAP 4TPE220MF
Figure 1. High Density 5A Converter. Total Circuit Size = 1.5in2, with Components on Both Sides
02/15/409
VOUT1
3.3V/5A
+
2, 3
0.1µF
25
7mΩ
COUT1
220µF
4V
4
5
1000pF
10Ω
BG2
BG3
5.9k
5
8 Si4816BDY
CMDSH-3
SW1
CLKOUT
1nF
4
150pF
4.7µF
25V
32
ITH1
PLLIN/FC
150pF
8.2k
1nF
33
TG1
PLLFLTR
15k
3
34
VFB1
VCC
20k
1nF
35
TRACK1
SENSE3+
2
36
SDB2
PGND
1
SENSE3–
0.01µF
37
SDB3
38
39
VIN
4.5V TO 20V
47µF
25V
+
10k
+
22µF
X5R
DN409 F01
Switching the three rails out of phase results in improved
performance and reduced cost. The use of triple phase
operation instead of single phase can result in a reduction
of the input capacitor ripple current by over 50% as shown
in Figure 3, allowing the use of less input capacitance.
The outputs of two or more phases can be tied together
Compensation of each rail is achieved with an RC network
on the ITH pin (error amplifier output). The external ITH
compensation and the current mode topology allow the
designer to easily stabilize a converter with the minimal
amount of output capacitance using a variety of capacitor types including conductive polymer, tantalum and
ceramic while still achieving a fast load step response
(see Figure 4).
94
93
3.3V
EFFICIENCY (%)
92
91
90
2.5V
89
which results in output ripple current reduction as well
and a faster load step response. Up to six phases can
be synchronized using the CLKOUT pin (on the QFN
part only). Fast and accurate current sharing among the
parallel phases is a result of the LTC3773’s peak current
mode architecture.
1.8V
88
87
86
85
84
1
0
2
3
LOAD CURRENT (A)
4
5
DN409 F02
INPUT CAPACITOR RIPPLE CURRENT (AMPS RMS)
Figure 2. Efficiency of the LTC3773 Converter at VIN = 12V,
fSW = 400kHz. One Rail Enabled at a Time
7
Other features of the LTC3773 include rail tracking and
sequencing, a PGOOD signal, and three selectable light
load operating modes (continuous conduction mode,
Burst Mode® operation and pulse skip mode).
Conclusion
Now designers have a clear and practical solution when
they need a compact and cost effective triple supply
rail requirement in their telecom, server or network
systems.
6
5
SINGLE PHASE
1.8VOUT(AC)
50mV/DIV
4
3
TRIPLE PHASE
2
5A
LOAD STEP
2A/DIV
1
2.5A
0
4
6
8
10 12 14 16
INPUT VOLTAGE (V)
18
20
DN409 F03
Figure 3. Input Capacitor Ripple Current Comparison for
Single Phase and Triple Phase Operation
VOUT1 = 3.3V/5A, VOUT2 = 2.5V/5A, VOUT3 = 1.8V/5A
Single Phase: φ1,2,3 = 0º
Triple Phase: φ1,2,3 = 0º,120º, 240º
50µs/DIV
DN409 F04
VIN = 12V
Figure 4. 1.8V Load Step Response
Data Sheet Download
For applications help,
call (408) 432-1900, Ext. 2134
www.linear.com
Linear Technology Corporation
dn409f LT/TP 0207 409K • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2006
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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