DN281 - Wide Input Range, High Efficiency DDR Termination Power Supply Achieves Fast Transient Response

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Wide Input Range, High Efficiency DDR Termination Power
Supply Achieves Fast Transient Response – Design Note 281
Wei Chen
Introduction
Today’s complicated computing and communication
systems demand high system memory bandwidths. The
emerging standard is Double Data Rate (DDR) memory
because of its higher data rate and relatively low cost.
A typical DDR memory system needs at least two main
power supplies: VDD for the I/O power and V TT for the
termination power. To ensure good signal quality and
fast data rate, the termination power supply (V TT ) must
always track the VDD with V TT = VDD/2. V TT can be as
low as 0.6V. Since the termination resistors can carry
current in either direction, the V TT power supply must
be able to both source and sink current while tracking
the VDD supply. A family of new termination/tracking
controllers, including the LTC ®3717, LTC3718 and
LTC3831, satisfy these DDR requirements.
on-time programmed by the input voltage and output
voltage. This scheme allows a fairly constant switching
frequency while achieving an extremely fast load transient response. In addition, the controller’s minimum
on-time is less than 100ns, allowing for a very small
duty cycle at a very high switching frequency, thus
minimizing the size of the inductor and capacitors. This
circuit is suitable for high step-down applications such
as 20V input and 1.25V output.
The LTC3717 also includes a 5V internal LDO that can
be used to drive an efficient logic-level power MOSFET.
If an external 5V bias is available in the system, it can
be applied to the EXTVCC pin to disable the internal
5V LDO and reduce the power loss of the controller
at high input voltage. The reference tracking input of
the LTC3717 is attenuated 50% internally, achieving a
0.65% regulation accuracy and eliminating the need for
an external 1:1 resistor divider in the DDR termination
power supply design.
Overview of the LTC3717
The LTC3717 is a No RSENSE ™ current mode synchronous buck tracking controller that senses inductor current via the RDS(ON) of the bottom FET, eliminating the
sense resistor and associated power loss. The LTC3717
implements a unique constant on-time architecture with
L, LT, LTC, LTM, Linear Technology, the Linear logo and OPTI-LOOP are registered
trademarks of Linear Technology Corporation. All other trademarks are the property
of their respective owners.
INTVCC
CMDSH-3
1
2
PGOOD
R5
10k
C13
1000pF
VIN
VDD
2.5V
R6
499k
1%
VIN
4.5V TO 24V
0.33μF
0.1μF
RUN/SS BOOST
PGOOD
TG
LTC3717
3
INTVCC
VRNG
SW
4
ITH
PGND
C14 100pF 5
SGND
BG
6
ION
INTVCC
0.1μF
7
VFB
VCC
8
VREF
EXTVCC
16
Q1
Si7840DP
15
CIN
10μF
35V
w4
D1
B340A
L1
1.5μH
14
13
4.7μF
Q2
Si7440DP
12
11
D3
B340A
+
+
COUT
270μF
2V
w2
47μF
25V
10μF
6.3V
VTT
1.25V
±10A
INTVCC
1Ω
VIN
10
9
0.1μF
1μF
5V
OPTIONAL
COUT: PANASONIC EEFUEOD271R
L1: CEP125-1R5MC
DN282 F01
Figure 1. High Efficiency ±10A LTC3717 VTT Supply from 4.5V to 24V Input
04/02/281_conv
Design Example
Figure 1 shows a ±10A design using LTC3717. The
input voltage can vary from 5V to 24V. The input voltage can be below 5V if an external 5V bias is available
for powering the VCC pin of LTC3717. This design uses
only two SO-8 PowerPak MOSFETs from Siliconix to
deliver ±10A current. To achieve a higher output current,
use an inductor with a higher current rating and lower
RDS(ON) MOSFETs. This circuit achieves 84% efficiency
at 250kHz switching frequency, 1.25V/10A output and
12V input, as shown in Figure 2.
The combination of the unique constant on-time current
mode architecture of LTC3717 and the OPTI-LOOP®
compensation design produces an excellent load
transient response. Figure 3 shows a typical load
transient waveform. With only two SP output caps
(270μF/2V), the output voltage variation is less than
100mV for a 10A load step.
Conclusion
The LTC3717 DDR termination power supply achieves
high efficiency and fast load transient response for high
input applications. If the available power sources are
less than 5V, the LTC3718 can be used. The LTC3718
integrates an LTC3717 controller with a 1.2MHz boost
regulator for the 5V MOSFET gate power. If the input
voltage is between 3.3V and 8V, the LTC3831, a voltage mode synchronous buck tracking controller, can
also be used.
100
EFFICIENCY (%)
90
80
LOAD
CURRENT
5A/DIV
70
60
OUTPUT
VOLTAGE VTT
100mV/DIV
50
40
2
0
4
8
6
LOAD CURRENT (A)
10
12
DN281 F03
TIME 20μs/DIV
DN282 F02
Figure 2. Measured Efficiency at 1.25V Output, 12V Input
Figure 3. Load Transient Waveforms
5V
CMDSH-3
1
2
PGOOD
R5
10k
C13
1000pF
INTVCC
3
4
C14 100pF 5
6
VIN
R6
499k
1%
VDD
2.5V
0.1μF
VDD
2.5V
0.33μF
0.1μF
7
8
RUN/SS BOOST
PGOOD
TG
LTC3717
VRNG
SW
ITH
SGND
PGND
BG
ION
INTVCC
VFB
VCC
VREF
EXTVCC
16
Q1
Si7840DP
15
CIN
10μF
35V
w4
D1
B340A
L1
1.5μH
14
13
4.7μF
Q2
Si7440DP
12
D3
B340A
+
11
+
COUT
270μF
2V
w2
47μF
25V
10μF
6.3V
VTT
1.25V
±10A
10
9
5V
0.1μF
COUT: PANASONIC EEFUEOD271R
L1: CEP125-1R5MC
DN282 F04
Figure 4. High Efficiency ±10A LTC3717 VTT Supply from VDD Input
Data Sheet Download
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