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
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