Design Solutions 41 - Dual Output DC/DC Converter Solutions for Xilinx FPGA Based Systems

Design Solutions 41
February 2004
Dual Output DC/DC Converter Solutions
for Xilinx FPGA Based Systems
Charlie Zhao
INTRODUCTION
Xilinx FPGAs require at least two power supplies: VCCINT
for core circuitry and VCCO for I/O interface. For the latest
Xilinx FPGAs, including Virtex-II Pro, Virtex-II and Spartan-3, a third auxiliary supply, VCCAUX may be needed. In
most cases, VCCAUX can share a power supply with VCCO.
The core voltages, VCCINT, for most Xilinx FPGAs, range
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 (VCCO) vary from 1.2V to 3.3V. The auxiliary
voltage VCCAUX 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 (tCCPO) for the
core voltage VCCINT is less than 50ms during power-on.
Some Xilinx FPGA families also have minimum VCCINT
ramp time requirements.
New dual output DC/DC regulators from Linear Technology, 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.
VIN = 2.5V
TO 5.5V
C1
10µF
6.3V
RUN2
VIN
RUN1
MODE/SYNC
POR
LTC3407
L2
2.2µH
VOUT2 = 2.5V
AT 600mA
SW2
R4
887k R3
280k
RESET
L1
2.2µH
SW1
C5 22pF
C3
10µF
6.3V
R5
100k
C4 22pF
VOUT1 = 1.8V
AT 600mA
VFB1
VFB2
R2
GND
C1, C2, C3: TAIYO YUDEN JMK316BJ106ML
R1 887k
443.5k
C2
10µF
6.3V
L1, L2: MURATA LQH32CN2R2M33
AN95 F01
Figure 1. High Efficiency 2.5V/600mA
and 1.8V/600mA Regulators
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a registered trademark of Linear Technology Corporation.
Table 1. Core Voltage Requirement for Xilinx FPGA Families
VCCINT
Virtex-II Pro
Virtex-II
Virtex-E
Extended
Memory
1.5V
1.5V
1.8V
Virtex-E
Virtex
Spartan-3
Spartan-IIE
Spartan-II
Spartan-XL
Spartan
1.8V
2.5V
1.2V
1.8V
2.5V
3.3V
5V
1
Design Solutions 41
100
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.
95
2.5V
EFFICIENCY (%)
90
1.8V
85
80
75
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.
70
VIN = 3.3V
Burst Mode OPERATION
NO LOAD ON OTHER CHANNEL
65
60
10
100
LOAD CURRENT (mA)
1
1000
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.
100
VO = 2.5V
EFFICIENCY (%)
90
LTC3736: 2-Phase, Dual Synchronous, DC/DC
Controller for 5A Loads
80
70
60
50
40
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
10V
Q1B
Si7540DP
LTC3736
20
Q1A
Si7540DP
VOUT1
2.5V
(5A AT 5VIN)
+
C6
150µF
4V
21
L1
1.5µH
IHLP2525CZ-01
17
22
19
18
R3
118k
1%
14
6
23
24
R4
187k
1%
R6
59k
1%
1
C11
220pF
R7
59k
1%
R8
15k
VO = 1.8V
VIN = 3.3V
10
100
1000
LOAD CURRENT (mA)
1
AN95 F03
Figure 3. Efficiency vs Load Current
for the LTC3736 Converter
R1
10Ω
5
VIN
PGND1
SENSE1+
TG1
SW1
BG1
SYNC/FCB
RUN/SS
TRACK
IPRG1
VFB1
ITH1
4
VIN
2.7V TO 9.8V
C1
22µF
10V
R2
1M
Q2B
Si7540DP
25
PGND 12
PGND2
11
SENSE2+
15
TG2
10
SW2
13
BG2
16
PGND
9
PGOOD
3
PLLLPF
2
IPRG2
7
VFB2
8
ITH2
L2
1.5µH
IHLP2525CZ-01
Q2A
Si7540DP
C9
150µF
4V
+
R5
118k
1%
C14
220pF
R10
59k
1%
R9
15k
AN95 F04
Figure 4. High Efficiency 2.5V/5A and 1.8V/5A Dual Output Converter with Output Tracking
2
10000
VOUT2
1.8V
(5A AT 5VIN)
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 VCCINT and the Spartan-3 family needs only 1.2V
VCCINT. 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
2-Phase
Dual Controller
SW1 (V)
SW2 (V)
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.
2.0
INPUT CAPACITOR RMS CURRENT
IL1
IL2
IIN
1.8
SINGLE PHASE
DUAL CONTROLLER
1.6
1.4
2- PHASE
DUAL CONTROLLER
1.2
1.0
0.8
0.6
0.4
VOUT1 = 2.5V/2A
VOUT2 = 1.8V/2A
0.2
AN95 F05
0
2
Figure 5. Example Waveforms for a Single-Phase
Dual Converter vs the 2-Phase LTC3736
3
4
8
6
5
7
INPUT VOLTAGE (V)
9
10
AN95 F06
Figure 6. RMS Input Current Comparision
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.
3
Design Solutions 41
VIN
3.3V TO 28V
CIN
10µF
35V
×4
+
4.7µF
CMDSH-3
17
VCC
M1
27
28
L1
1.4µH
+
COUT1
330µF
4V
×2
12.1k
0.22µF
26
25
M3
D1
B340A
22
19.1k
24
23
3
6
1.5M
VIN
29
2
10
33k
6.04k
CMDSH-3
10Ω
1µF
VOUT1
2.5V
15A
M1, M2: RENESAS HAT2168
M3, M4: RENESAS HAT2165
COUT1: SANYO POSCAP 4TPD330M
COUT2: SANYO POSCAP 2R5TPD470M
L1, L2: PANASONIC ETQP3H
5V
1
10k
0.01µF
6.04k
180pF
21
30
DRVCC PWRGD
TG2
TG1
BOOST2
BOOST1
SW1
SENSE1+
LTC3708
SENSE2+
BG2
BG1
SENSE1–
SW2
SENSE2–
PGND2
PGND1
VFB1
VFB2
TRACK2
FCB
ION1
ION2
ITH1
ITH2
INTLPF
EXTLPF
RUN/SS
TRACK1
SGND
5
VRNG1 VRNG2
32
11
0.1µF
M2
14
13
L2
1.2µH
0.22µF
15
16
VOUT2
1.8V
15A
D2
B340A
M4
20
+
12.1k
18
COUT2
470µF
2.5V
×2
19
7
31
12 1.07M
fIN
VIN
8
9
4
10k
6.04k
0.01µF
100k
25k
33k
5V
180pF
AN95 F07
Figure 7. High Efficiency 2.5V/15A and 1.8V/15A Dual Output Converter with Output Tracking
100
EFFICIENCY (%)
95
90
85
80
75
70
0.01
20VIN TO 2.5VOUT
5VIN TO 2.5VOUT
20VIN TO 1.8VOUT
5VIN TO 1.8VOUT
0.1
1
LOAD CURRENT (A)
10 15
AN95 F08
Figure 8. Efficiency vs Load Current for the LTC3708 Converter
500mV/DIV
500mV/DIV
2ms/DIV
AN95 F09
Figure 9. Up/Down Coincident Tracking
4
Linear Technology Corporation
2ms/DIV
AN95 F10
Figure 10. Up/Down Ratiometric Tracking
dsol41 LT/TP 0204 1K • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
 LINEAR TECHNOLOGY CORPORATION 2004
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