Aug 2002 VRM8.5 Design with the LTC3720 Achieves Small Size and Fast Transient Response

DESIGN IDEAS
VRM8.5 Design with the LTC3720
Achieves Small Size and Fast Transient
Response
by David Chen
Several different brands of CPUs
fall into Intel® VRM8.5 category. Depending upon clock frequency and
computation power, these CPUs consume different levels of supply current
ranging from several amperes to 30A.
The newly released LTC3720 single-
3.3V
POWER GOOD
BAT54C
3
CSS
0.1µF
1
0.01µF
INT VCC
2
4
CC 220pF R
VP2
105k
RVP1 21.0k
5
6
20k
220pF
7
David Chen
Lower the Output Voltage Ripple of
Positive-to-Negative DC/DC Converters
with Optimum Capacitor Hook-Up
................................................... 22
Keith Szolusha
Use a Single Input to Acquire Two
Similar Signals Simultaneously and
Other AC Techniques for the LTC1864
................................................... 24
Derek Redmayne and Mark Thoren
2.5A, 4MHz Monolithic Synchronous
Regulator Offers a High Efficiency,
Compact Solution by Reducing
External Component Count and Size
................................................... 27
Joey M. Esteves
White LED Driver in Tiny SC70
Package Achieves 84% Efficiency
................................................... 29
Pit-Leong Wong
Boost DC/DC Converter Synchronizes
to any Frequency ......................... 30
Gary Shockey
Monolithic Synchronous Step-Down
Regulators Pack 600mA Current
Rating in a ThinSOT™ Package
................................................... 31
Jaime Tseng
Inductorless, Low Noise Step-Down
DC/DC Converter Saves Space and
Provides Efficient 1.5V Output .... 33
Bill Walter
Low Voltage, High Current DC/DC
Power Supply with Load Sharing and
Redundancy ................................ 34
Henry J. Zhang and Wei Chen
10
1k
High Performance Op Amps Deliver
Precision Waveform Synthesis .... 21
Jon Munson
VIN
4.5V TO 21V
2k
8
CION
1000pF
9
510k
VIN
100pF
11
220pF
12
VID25mV
VID0
13
14
PGOOD
BOOST
RUN/SS
TG
CIN
10µF
25V
×4
27
1µF
25V
M1
26
VON
SW
VRNG
SENSE+
FCB
SENSE–
DB
CMDSH-3
25
L1
24
23
ITH
PGND
LTC3720
SGND
22
M2
BG
21
SGND
D1
INTVCC
20
ION
4.7µF 6.3V
VIN
19
VFB
0.1µF
EXTVCC
18
VFB
VOSENSE
VID4
VID0
VID3
VID1
VID2
5V
3.3V
VCC
17
VID3
PGND SGND
0.1µF
16
VID2
15
VID1
+
M1: IRF7811A ×2
M2: IRF7822 ×2
D1: UPS840
L1: 0.8µH CEP125U-0R8
COUT
680µF
2.5V
×2
VOUT
1.075V TO 1.800V
20A
1µF
6.3V
Figure 1. A 20A VRM8.5 design using the LTC3720
phase PWM controller is designed for
CPUs that consume up to 20A. It
features a valley current control architecture that speeds up the VRM
response to step load changes, two
on-chip high current gate drivers for
N-channel power MOSFETs, a current sensing mechanism that does
not require an additional sense resistor and a 5-bit VID table that is
compatible with Intel VRM 8.5. The
resulting VRM 8.5 design has a small
size and a fast transient response.
The LTC3720 also achieves a minimum on-time below 100ns and a
wide input range from 4V to 36V.
These are important characteristics
for notebook CPU applications where
the input-to-output ratio is usually
Intel is a registered trademark of Intel Corporation
20
10Ω
CB 0.22µF
28
high. Other LTC3720 features include
a programmable current limit, an
output overvoltage soft latch, a capacitor-programmable soft start, an
continued on page 32
90
88
86
EFFICIENCY (%)
DESIGN IDEAS
VRM8.5 Design with the LTC3720
Achieves Small Size and Fast
Transient Response ..................... 20
VRON
84
82
80
78
76
74
VIN = 12V
VOUT = 1.475V
72
70
0
2
4
6 8 10 12 14 16 18 20
LOAD CURRENT (A)
Figure 2. Better than 80% efficiency
is achieved over a 1A–20A load range.
Linear Technology Magazine • August 2002
DESIGN IDEAS
100
95
VIN = 2.7V
90
VOUT
100mV/DIV
AC COUPLED
90
80
IL
500mA/DIV
ILOAD
500mA/DIV
EFFICIENCY (%)
EFFICIENCY (%)
85
VIN = 3.6V
80
VIN = 4.2V
75
70
60
VIN = 3.6V
50
VIN = 2.7V
40
70
30
VIN = 3.6V
20µs/DIV
VOUT = 1.8V
ILOAD = 50mA TO 600mA
Figure 3. LTC3406B-1.8 Transient
Response to a 50mA to 600mA
Load Step
occurs to maintain regulation. Efficiency in pulse skipping mode is lower
than Burst Mode operation at light
loads, but comparable to Burst Mode
operation when the output load exceeds 50mA.
1.8V/600mA Step-Down
Regulator Using All Ceramic
Capacitors
Figure 1 shows an application of the
LTC3406/LTC3406B-1.8 using all
ceramic capacitors. This particular
design supplies a 600mA load at 1.8V
with an input supply between 2.5V
and 5.5V. Ceramic capacitors have
the advantages of small size and low
equivalent series resistance (ESR),
making possible for very low ripple
65
VIN = 4.2V
20
60
0.1
1
10
100
OUTPUT CURRENT (mA)
1000
10
0.1
Figure 4. Efficiency vs Load
Current for LTC3406-1.8
voltages at both the input and output. For a given package size or
capacitance value, ceramic capacitors have lower ESR than other bulk,
low ESR capacitor types (including
tantalum capacitors, aluminum and
organic electrolytics). Because the
LTC3406/LTC3406B’s control loop
does not depend on the output
capacitor’s ESR for stable operation,
ceramic capacitors can be used to
achieve very low output ripple and
small circuit size. Figures 2 and 3
show the transient response to a 50mA
to 600mA load step for the LTC34061.8 and LTC3406B-1.8, respectively.
Authors can be contacted
at (408) 432-1900
1
100
10
OUTPUT CURRENT (mA)
1000
Figure 5. Efficiency vs Load
Current for LTC3406B-1.8
Efficiency Considerations
Figure 4 shows the efficiency curves
for the LTC3406-1.8 (Burst Mode
operation enabled) at various supply
voltages. Burst Mode operation significantly lowers the quiescent
current, resulting in high efficiencies
even with extremely light loads.
Figure 5 shows the efficiency curves
for the LTC3406B-1.8 (pulse skipping mode enabled) at various supply
voltages. Pulse skipping mode maintains constant-frequency operation
at lower load currents. This necessarily increases the gate charge losses
and switching losses, which impact
efficiency at light loads. Efficiency is
still comparable to Burst Mode operation at higher loads.
LTC3720, continued from page 20
optional short-circuit latch-off, a
Power Good indicator of output regulation and a current limit foldback for
overload protection. A selectable discontinuous conduction mode of
operation maintains high efficiency
at light loads, when the CPU is running at deep sleep mode, for example,
thereby improving battery life in portable applications.
Figure 1 shows the schematic diagram of a 20A VRM8.5 design for an
Intel processor operating at 1.2GHz.
Efficiency is greater than 80% over a
wide load range, as shown in Figure␣ 2.
With two 680µF Sanyo POSCAPs, the
output voltage deviation remains
within the VRM8.5 specification when
32
load current switches between CPU
leakage and full load, as shown in
Figure 3. The entire VRM design fits
into a 1.25"×1.5", double-sided PCB
area with an overall height below
0.35".
In summary, the LTC3720 is an
ideal device for low current CPU power
supplies. Its unique control architecture and its powerful gate drivers
facilitate the design of space-saving
VRMs that have a fast transient response. For CPUs that consume more
than 20A, the LTC1709-85 dualphase controller addresses the current
distribution and thermal management
issues associated with higher current
applications.
1.535V
VOUT
1.370V
20A
ILOAD
1A
25µs/DIV
Figure 3. With two POSCAPs at output, the
design in Figure 1 meets VRM 8.5 transient
requirements with significant margin.
Linear Technology Magazine • August 2002
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