LINEAR TECHNOLOGY LTC3773 Technical data

advertisement

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 effi cient. Furthermore, the converters may need
to meet other requirements such as a fast load step
response and rail tracking.

®

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

15k

0.01µF

L1:
TDK RLF7030T-2R2M5R4

L2, L3:
TDK RLF7030T-1R5M5R4

C

OUT1 COUT2 COUT3

SANYO POSCAP 4TPE220MF

1nF

8.2k

1nF

5.9k

1nF

10k

10k

0.01µF

20k

,:

POWER DOWN V
POWER DOWN V
POWER DOWN V

0.01µF

20k

150pF

150pF

150pF

31.6k

10Ω

Figure 1. High Density 5A Converter. Total Circuit Size = 1.5in2, with Components on Both Sides

1000pF

OUT1

OUT2

OUT3

37

39

SDB3

PGND

1

TRACK1

2

V

FB1

3

I

TH1

4

SGND

5

I

TH2

6

I

TH3

7

V

FB2

8

V

FB3

9

TRACK2

10

TRACK3

11

12

10Ω

–

–

SENSE2

+

SENSE2

SENSE3

13 14 15 16 17 18 19

1000pF

10Ω

10Ω

1000pF

36

35

–

SDB238SDB1

SENSE1

LTC3773

SENSE3+VCCPLLFLTR

2.2µF

10Ω

34

10Ω

33 32

+

SENSE1

PLLIN/FC

10Ω

0.1µF

PHASEMD

CLKOUT

PGOOD

BOOST1

TG1

SW1

SW2

TG2

BOOST2

BOOST3

TG3

SW3

BG1

BG2

V

BG3

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 effi ciency of each
ou tp ut ve rsus lo ad cu rr ent ; w here up to 93 % e f fi ci en cy i s
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.

, LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technology
Corporation. All other trademarks are the property of their respective owners.

V

CC

CMDSH-3

0.1µF

0.1µF

CMDSH-3

CMDSH-3

0.1µF

4.7µF

4.5V TO 7V

4.7µF

V

IN

+

4.5V TO 20V
47µF

25V

8 Si4816BDY

1

5

4

8 Si4816BDY

1

5

4

8 Si4816BDY

1

5

4

25V

L1

2.2µH

L2

1.5µH

L3

1.5µH

V

IN

4.7µF
25V

V

IN

4.7µF
25V

7mΩ

7mΩ

7mΩ

C

OUT1

220µF

4V

C

OUT2

220µF

4V

C

OUT3

220µF

4V

+

+

+

22µF

X5R

6, 7

2, 3

6, 7

2, 3

6, 7

2, 3

PGOOD

DR

10k

31

30

29

28

27

26

25

24

23

22

21

20

V

OUT1

3.3V/5A

22µF
X5R

V

OUT2

2.5V/5A

22µF
X5R

V

OUT3

1.8V/5A

DN409 F01

02/15/409

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 reduc tion
of the input cap acitor 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

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.

94

93

92

91

90

89

88

EFFICIENCY (%)

87

86

85

84

0

123 5

3.3V

2.5V

1.8V

LOAD CURRENT (A)

4

DN409 F02

Figure 2. Effi ciency of the LTC3773 Converter at VIN = 12V,
fSW = 400kHz. One Rail Enabled at a Time

7

6

5

4

3

2

1

0

INPUT CAPACITOR RIPPLE CURRENT (AMPS RMS)

4

81261810 14 20

INPUT VOLTAGE (V)

SINGLE PHASE

TRIPLE PHASE

16

DN409 F03

C o m p e n s a t i o n o f e a c h r a i l i s a c h i e v e d w i t h a n R C n e t w o r k
on the I

pin (error amplifi er output). The external ITH

TH

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 conduct i v e p o l y m e r , t a n t a l u m a n d
ceramic while still achieving a fast load step response
(see Figure 4).

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.

1.8V

OUT(AC)

50mV/DIV

LOAD STEP

2A/DIV

VIN = 12V

2.5A

5A

50µs/DIV

DN409 F04

Figure 3. Input Capacitor Ripple Current Comparison for
Single Phase and Triple Phase Operation
V

= 3.3V/5A, V

OUT1

Single Phase: φ
Triple Phase: φ

Data Sheet Download

1,2,3

1,2,3

= 2.5V/5A, V

OUT2

= 0º

= 0º,120º, 240º

= 1.8V/5A

OUT3

www.linear.com

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

●

[phone]

FAX: [phone] ● www.linear.com

Figure 4. 1.8V Load Step Response

For applications help,

call [phone], Ext. 2134

dn409f LT/TP 0207 409K • PRINTED IN THE USA

© LINEAR TECHNOLOGY CORPORATION 2006