DN258 - High Efficiency I/O Power Generation for Mobile Pentium III Microprocessors

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High Efficiency I/O Power Generation for Mobile
Pentimum III Microprocessors – Design Note 258
Wei Chen
The demand for higher performance notebook computers has fueled the development of faster and more
power-hogging microprocessors. These microprocessors also require a faster input/output (I/O) bus and a
faster clock. From a power management perspective,
this means that core, I/O and clock power supplies
should be able to handle more power. This requires that
core and I/O DC/DC converters operate more efficiently
and be as small as possible. Linear Technology recommends the LTC ®1778 to provide I/O power for the next
generation Pentium III microprocessors. The I/O input
voltage requirement is 1.25V; transient (AC) tolerance
is ±9% and static (DC) tolerance is ±5%. Load current
requirements are as follows:
Processor V TT: 2.7A
830M chipset V TT: 0.7A
1. Internal graphics using the 830M engine: 3.6A
2. External AGP graphics: 1.6A
Total maximum I/O current: 7.0A
The LTC1778 is a synchronous step-down switching
regulator controller that provides synchronous drive for
two external N-channel MOSFET switches. The true current mode control architecture has an adjustable current
limit, can be easily compensated, is stable with ceramic
output capacitors and does not require a power-wasting
sense resistor. An optional discontinuous mode of operation increases efficiency at light loads. The LTC1778
operates over a wide range of input voltages from 4V
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.
CSS
0.1μF
1
RPG
100k 2
POWER
GOOD
3
4
CC1
470pF RC
20k
5
CC2
100pF
6
7
R1
20k
R2
11.3k
The 830M chipset core has the following two possibilities:
8
LTC1778
RUN/SS BOOST
PGOOD
VRNG
FCB
TG
SW
PGND
BG
ITH
SGND
INTVCC
ION
VIN
VFB
EXTVCC
16
15
DB
CMDSH-3
CB
0.22μF
14
M1
1/2 FDS6982S
L1
1.8μH
13
M2
1/2 FDS6982S
12
11
CVCC
4.7μF
10
RF
1Ω
9
CF
0.1μF
RON
576k
DN258 F01
CIN: TAIYO YUDEN TMK432BJ106MM
COUT: CORNELL DUBILIER ESRD181M02B
L1: TOKO 919AS-1R8N
Figure 1. LTC1778 Mobile Pentium III I/O Supply
05/01/258_conv
CIN
10μF
25V
w2
+
COUT
180μF
4V
VIN
5V TO 25V
VOUT
1.25V
7A
1μF
to 36V and provides output voltages from 0.8 to 0.9 •
VIN. Switching frequencies up to nearly 2MHz can be
chosen, allowing wide latitude in trading off efficiency
for component size. Fault protection features include
a power-good output, current limit foldback, optional
short-circuit shutdown timer and an overvoltage soft
latch. The LTC1778 is available in a 16-Lead narrow
SSOP package.
Figure 1 shows a typical LTC1778 application schematic
for a mobile Pentium III I/O supply. This circuit is optimized for small size and high efficiency from an input
supply that varies from 5V to 24V. In order to reduce
board space, it uses a single, dual N-channel FDS6982S
MOSFET and only one 180μF (Panasonic SP) output
capacitor. The superfast internal gate drivers with a typical rise time of 20ns help to minimize switching losses
and the strong gate drivers help minimize conductive
losses. Figure 2 shows the transient response for a 0A
to 5.5A load step. It can be seen that the LTC1778 can
easily meet the I/O transient and static specification
with only one output capacitor. The LTC1778 allows the
use of many different kinds of output capacitors such
as aluminum electrolytic, tantalum, POSCAP, NEOCAP,
SPs and ceramic, because of OPTI-LOOP® compensation that allows the feedback loop to be compensated
externally. Figure 3 shows the same output voltage
transient with two 150μF POSCAP output capacitors.
Note that the equivalent series resistance (ESR) of a
POSCAP is 40mΩ, which is approximately twice that of
an SP capacitor. Therefore, two POSCAPs are required
to achieve the same output voltage transient response
as one SP capacitor. Figure 4 shows typical efficiency
curves for VIN = 5V and VIN = 15V, VOUT = 1.25V, ILOAD
= 10mA to 7A. It can be seen that the efficiency is better than 85% for load currents up to 5A. The measured
MOSFET case temperature is only 70°C for VIN = 12V
at ILOAD = 7A. This circuit can be implemented on a
0.5" × 1" board space.
VOUT VOLTAGE
TRANSIENT
RESPONSE
50mV/DIV
VOUT VOLTAGE
TRANSIENT
RESPONSE
50mV/DIV
5.5A
5.5A
2A/DIV
2A/DIV
0A
0A
DN258 F02
10μs/DIV
10μs/DIV
Figure 2. Output Voltage Transient Response for
IL = 0A to 5.5A with One SP Output Capacitor
DN258 F03
Figure 3. Output Voltage Transient Response for
IL = 0A to 5.5A with Two POSCAP Output Capacitors
100
VIN = 5V
90
EFFICIENCY (%)
80
VIN = 15V
70
60
50
40
30
20
10
VOUT = 1.25V
IL = 10mA TO 7A
0
0.01
0.1
1
OUTPUT CURRENT (A)
10
DN258 F04
Figure 4. Efficiency Curves for Figure 1
Data Sheet Download
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dn258f_conv LT/TP 0501 375K • PRINTED IN THE USA
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