LINEAR TECHNOLOGY LTC3407 Technical data

Dual Output DC/DC Converter Solutions
for Xilinx FPGA Based Systems

Charlie Zhao

Design Solutions 41

February 2004

INTRODUCTION

Xilinx FPGAs require at least two power supplies: V
for core circuitry and V

for I/O interface. For the latest

CCO

CCINT

Xilinx FPGAs, including Virtex-II Pro, Virtex-II and Spar­tan-3, a third auxiliary supply, V
most cases, V
The core voltages, V

can share a power supply with V

CCAUX

, for most Xilinx FPGAs, range

CCINT

may be needed. In

CCAUX

CCO

.

from 1.2V to 2.5V. Some mature products have 3V, 3.3V
or 5V core voltages. Table 1 shows the core voltage
requirement for most of the FPGA device families. Typical
I/O voltages (V
voltage V

CCAUX

) vary from 1.2V to 3.3V. The auxiliary

CCO

is 2.5V for Virtex-II Pro and Spartan-3, and

is 3.3V for Virtex-II.
Each FPGA family has a specific quiescent supply current,

ranging from under 100mA to about 2A. For applications
with multiple FPGAs, the core supply current can be higher
than 10A.

With multiple voltage rails in today’s systems (FPGA, DDR
memory, data converter ICs, etc.), supply sequencing and
tracking are quite important for proper start-up and shut­down. Ramp time requirement should also be satisfied.
For example, the recommended ramp time (t
core voltage V

is less than 50ms during power-on.

CCINT

Some Xilinx FPGA families also have minimum V

CCPO

) for the

CCINT

ramp time requirements.
New dual output DC/DC regulators from Linear Technol-

ogy, the LTC®3407, LTC3736 and LTC3708, greatly sim­plify the design of an optimal power supply solution for
systems using Xilinx FPGAs.

LTC3407: Dual Synchronous, 600mA, DC/DC
Regulator

The LTC3407 is a dual synchronous step-down DC/DC
converter with integrated power switches. It provides a
compact and high efficiency power solution for FPGAs
with supply currents up to 600mA. The switching regula­tor operates from a 2.5V to 5.5V input voltage range and
has an adjustable output range from 0.6V to 5V. Its internal
1A switches provide up to 96% efficiency, eliminating the
need for external MOSFETs and Schottky diodes. Figure 1
is an application example for 2.5V/600mA and 1.8V/
600mA supplies. Figure 2 shows the efficiency curves of
the circuit vs load current.

V

= 2.5V

IN

TO 5.5V

V

= 2.5V

OUT2

AT 600mA

C3

10µF

6.3V

C1, C2, C3: TAIYO YUDEN JMK316BJ106ML L1, L2: MURATA LQH32CN2R2M33

, LTC and LT are registered trademarks of Linear Technology Corporation.

Burst Mode is a registered trademark of Linear Technology Corporation.

R4

887k

C1
10µF

6.3V

L2

2.2µH

280k

R3

RUN2 V

MODE/SYNC

LTC3407

SW2

V

FB2

GND

RUN1

IN

POR

SW1

V

R5
100k

RESET

L1

2.2µH

C4 22pFC5 22pF

FB1

R1

443.5k

R2

887k

Figure 1. High Efficiency 2.5V/600mA
and 1.8V/600mA Regulators

V

OUT1

AT 600mA

C2
10µF

6.3V

= 1.8V

AN95 F01

Table 1. Core Voltage Requirement for Xilinx FPGA Families

Virtex-E

Extended

Virtex-II Pro Virtex-II Memory Virtex-E Virtex Spartan-3 Spartan-IIE Spartan-II Spartan-XL Spartan

V

CCINT

1.5V 1.5V 1.8V 1.8V 2.5V 1.2V 1.8V 2.5V 3.3V 5V

1

Design Solutions 41

100

95

90

85

80

75

EFFICIENCY (%)

70

VIN = 3.3V

65

Burst Mode OPERATION
NO LOAD ON OTHER CHANNEL

60

1

LOAD CURRENT (mA)

10 100 1000

2.5V

1.8V

AN95 F02

Figure 2. LTC3407 Efficiency Curve

The LTC3407 has a constant 1.5MHz switching frequency,
allowing the use of tiny inductors and capacitors. Select­able Burst Mode® operation provides high efficiency at
light loads. The IC has short-circuit protection and a
power-on reset (power good) output. It is available in
small thermally enhanced 10-lead MSOP and 3 × 3 DFN
packages.

load current requirements for most FPGA applications.
The LTC3736 receives input from 2.7V to 9.8V and pro­duces output voltages ranging from 0.6V to 9.5V. Figure␣ 3
shows that up to 95% efficiency is achieved. An applica­tion example is shown in Figure 4.

In contrast to single-phase operation, the two channels of
a 2-phase switching converter are operated 180 degrees
out of phase. This technique interleaves the current pulses
coming from the topside MOSFET switches, greatly
reducing the total RMS input ripple current. This in
turn allows the use of smaller and lower cost input
capacitors, reduces the EMI attenuation requirement and
improves operating efficiency.

100

90

80

70

EFFICIENCY (%)

60

VO = 2.5V

VO = 1.8V

LTC3736: 2-Phase, Dual Synchronous, DC/DC
Controller for 5A Loads

The LTC3736 is a 2-phase dual synchronous step down
DC/DC controller. Power supplies using the LTC3736 can
provide 5A at both outputs with a 5V input, meeting the

C3

1µF

20
21
17
22
19
18
14

6
23
24

1

10V

LTC3736

PGND1
SENSE1
TG1
SW1
BG1
SYNC/FCB
RUN/SS
TRACK
I

PRG1

V

FB1

I

TH1

V

+

V

(5A AT 5V

OUT1

2.5V

IN

)

+

Si7540DP

Q1A

Si7540DP

C6
150µF
4V

Q1B

1.5µH

IHLP2525CZ-01

R3

118k

1%

R4

187k

1%

L1

C11

220pF

R7

59k

1%

R6

59k

1%

15k

R8

R1
10Ω

525

IN

PGND

12

PGND2

11

SENSE2+

15

TG2

10

SW2

13

BG2

16

PGND

9

PGOOD

3

PLLLPF

2

I

PRG2

7

V

FB2

8

I

TH2

4

50

40

10

1 100 1000 10000

LOAD CURRENT (mA)

Figure 3. Efficiency vs Load Current
for the LTC3736 Converter

C1

22µF

R2

10V

1M

R9
15k

C14
220pF

AN95 F04

IHLP2525CZ-01

R10
59k
1%

L2

1.5µH

R5

118k

1%

Q2B
Si7540DP

Q2A
Si7540DP

150µF

+

C9

4V

VIN = 3.3V

V

IN

2.7V TO 9.8V

V

OUT2

1.8V
(5A AT 5V

AN95 F03

)

IN

2

Figure 4. High Efficiency 2.5V/5A and 1.8V/5A Dual Output Converter with Output Tracking

Design Solutions 41

Figure 5 compares the input waveforms for a representa­tive single-phase dual switching converter to the 2-phase
dual switching converter. Figure 6 shows how the RMS
input current varies for single phase and 2-phase dual
controllers with 2.5V and 1.8V outputs over a wide input
voltage range. For most applications, 2-phase operation
will reduce the input capacitor RMS current requirement
to that of just one channel operating at maximum current
and 50% duty cycle.

The LTC3736 has a default switching frequency at 550kHz,
but can be externally synchronized from 300kHz to 750kHz.
The LTC3736 provides output tracking for controlled
ramp-up of two supply rails, programmable current limit,
output overvoltage protection, power good output and
selectable Burst Mode operation for high efficiency light
load operations. The IC is available in the tiny (4mm ×
4mm) thermally enhanced QFN package or the 24-lead
SSOP package.

LTC3708: 2-phase, Dual Synchronous, DC/DC
Controller for 15A Loads

The core supply voltages of the latest Xilinx FPGAs have
decreased towards 1V. The Virtex-II pro family requires

1.5V V
V

CCINT

and the Spartan-3 family needs only 1.2V

CCINT

. In the meantime, these FPGAs demand more
current from the power supplies. Some systems use more
than ten FPGAs per board so the resulting total supply
current can easily exceed 10A. The LTC3708-based dual
output supply is an ideal choice for such applications.

Single Phase

Dual Controller

SW1 (V)

SW2 (V)

2-Phase

Dual Controller

The LTC3708 is a 2-phase dual synchronous step-down
DC/DC controller with a wide input voltage range: from

3.3V up to 36V. Its output voltage can be programmed
down to 0.6V. Figure 7 shows the schematic of a dual
output 2.5V/15A and 1.8V/15A converter. As shown in
Figure 8, up to 95% efficiency can be achieved.

The LTC3708 has output voltage up/down tracking capa­bility. The IC allows both coincident or ratiometric track­ing, as shown in Figures 9 and 10. The ramp rate can be
selected by a soft-start capacitor from RUN/SS pin to
ground. Multiple LTC3708s can easily be daisy-chained in
applications requiring more than two voltages to be tracked.

The 2-phase operation of the LTC3708 reduces power loss
and noise, and lowers the input-filtering requirement. The
constant on time, valley current mode control in the
LTC3708 allows fast transient response and thus mini­mizes the number of output capacitors. An internal phase­locked loop allows the IC to be synchronized to an external
clock for applications with more than two output rails. The
LTC3708 also features programmable current limit, out­put overvoltage protection and power good output. The IC
is available in the 5mm × 5mm QFN package.

Conclusion

An optimal power solution for multirail supply systems
incorporating the latest Xilinx FPGAs should provide mul­tiple outputs with supply tracking/sequencing. As board
real estate becomes more expensive, the power supply
must be more efficient and smaller while supplying higher
current in high-end applications. Linear Technology’s
latest dual output power management ICs: LTC3407,
LTC3736 and LTC3708, successfully address these chal­lenges. For data sheets and additional information on
other power solutions for Xilinx FPGAs, visit Linear
Technology’s web site at www.linear.com.

I

L1

I

L2

I

IN

AN95 F05

Figure 5. Example Waveforms for a Single-Phase
Dual Converter vs the 2-Phase LTC3736

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen­tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

INPUT CAPACITOR RMS CURRENT

V

OUT1

0.2
V

OUT2

0

2

SINGLE PHASE

DUAL CONTROLLER

DUAL CONTROLLER

= 2.5V/2A
= 1.8V/2A

35

6

4

INPUT VOLTAGE (V)

2- PHASE

7

9

8

10

AN95 F06

Figure 6. RMS Input Current Comparision

3

Design Solutions 41

V

IN

3.3V TO 28V

C

IN

10µF
35V
×4

CMDSH-3

5V

+

4.7µF

1µF

10Ω

CMDSH-3

M1, M2: RENESAS HAT2168
M3, M4: RENESAS HAT2165
COUT1: SANYO POSCAP 4TPD330M
COUT2: SANYO POSCAP 2R5TPD470M
L1, L2: PANASONIC ETQP3H

V

OUT1

2.5V
15A

M1

L1

1.4µH

19.1k

6.04k

B340A

D1

12.1k

C

OUT1

+

330µF
4V
×2

6.04k

0.22µF 0.22µF

M3

1.5M 1.07M

V

IN

33k 33k10k 10k

0.01µF

180pF

17 21 30

VCCDRV

27

TG1

28

BOOST1

26
25
22
24
23

3
6

29

2

10

1

0.1µF

LTC3708

SW1

+

SENSE1
BG1

–

SENSE1
PGND1
V

FB1

TRACK2
I

ON1

I

TH1

INTLPF
RUN/SS

SGND VRNG1 VRNG2

53211

CC

PWRGD

TG2

BOOST2

SW2

SENSE2+

BG2

SENSE2–

PGND2

V

FB2

FCB

I

ON2

I

TH2

EXTLPF

TRACK1

25k

14
13
15
16
20
18
19
7
31
12
8
9
4

100k

M2

M4

f

IN

V

IN

0.01µF

5V

Figure 7. High Efficiency 2.5V/15A and 1.8V/15A Dual Output Converter with Output Tracking

100

95

D2
B340A

180pF

L2

1.2µH

AN95 F07

12.1k

6.04k

V

OUT2

1.8V
15A

C

OUT2

+

470µF

2.5V
×2

Figure 8. Efficiency vs Load Current for the LTC3708 Converter

500mV/DIV

2ms/DIV

Figure 9. Up/Down Coincident Tracking

90

85

EFFICIENCY (%)

80

75

70

0.01

AN95 F09

20VIN TO 2.5V
5V

IN

20V
5V

IN

0.1 1 10 15
LOAD CURRENT (A)

TO 2.5V

TO 1.8V

IN

TO 1.8V

OUT

OUT

OUT

OUT

AN95 F08

500mV/DIV

2ms/DIV

AN95 F10

Figure 10. Up/Down Ratiometric Tracking

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4

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