DN210 - 2-Phase, Dual Switching Regulator Fits in Tight Places

advertisement
2-Phase Switching Regulator Fits in Tight Places
Design Note 210
Steve Hobrecht
The third generation switching regulator controllers
from Linear Technology address these new requirements. The LTC ®1628, a 2-phase, constant frequency,
dual synchronous buck controller, has been specifically
designed to minimize both the size and total cost of the
power system while delivering the highest efficiency
in the industry.
A 2-phase switching technique minimizes the peak
and resultant RMS input current while doubling the
effective frequency of the input current pulses. The
input capacitor ESR requirement is reduced by 40%
and the capacitance requirement is reduced by 75%
when compared to a single phase controller solution!
Ceramic input capacitors provide an ideal choice for
the resultant requirements. Figure 1 contrasts the
power loss due to the input RMS ripple current for a
single and 2-phase solution over the 5V to 25V input
voltage range for a 5V/3A and 3.3V/3A power supply.
The input path resistive losses include the ESR of the
input capacitor, internal battery resistance, PC trace/
connector losses, fuse resistance and diode or MOSFET
resistance that may be used in the power path for
switching power sources.
The third generation controllers have also been designed
to minimize the output capacitor requirements. External OPTI-LOOP® compensation allows optimal loop
compensation for the wide range of characteristics
associated with switching power supply output capaci08/99/210_conv
8
INPUT PATH POWER LOSS (W/Ω)
Almost every week manufacturers present yet another
component that has been reduced in size by 30% to 50%
while maintaining similar performance characteristics.
The size and weight requirements of the notebook
and sub-notebook computer market has resulted in
the development of increasingly smaller components.
These consumer products have placed very stringent
requirements on size, weight and run time. Because of
component density, heat dissipation has also become
one of the highest concerns for notebook computers
and other portable equipment.
7
SINGLE
PHASE
6
VOUT1 = 5V/3A
VOUT2 = 3.3V/3A
5
POWER
SAVED
4
3
2
2-PHASE
1
0
0
5
10
15
INPUT VOLTAGE (V)
20
25
DN210 F01
Figure 1. Input Path Power Loss
tors. A unique Burst Mode® architecture keeps the low
current output voltage ripple down to a value close to
the normal value for the continuous mode of operation,
even when using output capacitors in the 47μF range.
The output capacitor ESR requirement is determined
by the expected load step and the capacitance value is
determined by output current and speed of the feedback
loop. A higher capacitance value cannot make up for
high ESR and very low ESR cannot make up for insufficient capacitance.
A third key element in reducing solution size is an internal
current foldback circuit that operates when the output
voltage drops below 70% of the nominal value. This
feature enables the use of MOSFETs properly sized for
the output load current without additional power dissipation margin required for short-circuit conditions.
The application shown in Figure 2 occupies less than
two square inches of PC board area (components
on one side only) and delivers 5V/3A, 3.3V/4A, and
12V/150mA—the three main system power supplies
for a notebook computer. The application in this Design
Note addresses very common requirements of small
L, LT, LTC, LTM, Linear Technology, the Linear logo OPTI-LOOP and Burst Mode
are registered trademarks of Linear Technology Corporation. All other trademarks
are the property of their respective owners.
R20 66.5k 1%
C13
27pF
R5
20k
1%
+
R9
1M
1
2
R3
105k, 1%
C11
1000pF
3
4
C20 0.01μF
5
C21 0.01μF
6
3.3V
STANDBY
8
C9
27pF
9
C10
27pF
10
R7
6.8k
C8 470pF
11
12
R7
6.8k
C14
27pF
13
R4
63.4k
1%
C12
1000pF
14
SENSE1 +
TG1
SENSE1 –
SW1
VOSENSE1
BOOST1
FREQSET
VIN
STBYMD
BG1
FCB
EXTVCC
LTC1628
ITH1
INTVCC
SGND
PGND
3.3VOUT
BG2
BOOST2
ITH2
VOSENSE2
SW2
SENSE2 –
TG2
SENSE2 +
RUN/SS2
D5
10MQ100N
28
T1
1:1.8
C23
4.7μF
10V
R1
0.015Ω
27
VOUT1
5V
3A
26
C3
0.1μF
25
M1A
M1B
D1
MBRM
140T3
24
23
R11 10Ω
22
D3
C18Ω
0.1μF
21
20
C1 47μF 6.3V
SP CAP
22μF
50V CER
C15
1μF
D4
C16 4.7μF
C2 56μF 4V
SP CAP
19
17
16
ON/OFF
R7
221k 1%
R18
100k
1%
+
VOUT3
12V
150mA
C22
1μF
25V
M2A
M2B
L1
8μH
VIN
7V TO 28V
D2
MBRM
140T3
18
C4
0.1μF
LT1121
+
R7
15k
FLTCPL
+
C7
220pF
7
RUN/SS1
+
C5
0.1μF
R10
1M
R19 1M 1%
R2 0.015Ω
15
VOUT2
3.3V
3A
C6 0.1μF
DN210 F02
5V
STANDBY
VIN: 7V TO 28V
VOUT: 5V 3A, 3.3V 3A, 12V 150mA
SWITCHING FREQUENCY = 220kHz
D3, D4: CENTRAL CMDSH-3
M1, M2: FAIRCHILD FDS8936A
L1: SUMIDA CEP123-8R0MC
T1: 9μH 1:1.8 — BH ELECTRONICS 501-0657
Figure 2. Two Square Inch 5V/3.3V/12V Portable Power Supply
physical size and dynamic 1A step load changes. Single,
low ESR capacitors were used for the input and each
output. Dual-packaged MOSFETs are used to further
minimize PC board space. The inductors were selected
based upon the maximum inductance for a reasonable
case size while meeting peak current requirements. The
photo in Figure 3 shows normal output ripple voltage and
transient response to a 1.5A to 2.5A load step in continuous mode (5V output). The output ripple voltage is
25mVP-P and the 1A load steps result in 60mV output
voltage transients. The output ripple voltage in Burst
Mode operation is 70mVP-P. The efficiency of the power
supply with a 15V input voltage and 5V/3A, 3.3V/3A and
12V/120mA output voltages measures 91%.
Other features of the LTC1628 include:
• A 0.8V reference and 1% output voltage accuracy
over line, load and temperature variations
• 7.5% overvoltage “soft-latch”
• Defeatable overcurrent latch-off timer
• Adjustable soft-start
• 25μA shutdown current
2.5A
• 5V and 3.3V standby linear voltage regulators
1.5A
DN210 F03
Figure 3. Upper Trace: 5V Output, Transient Load Response
(50mV/DIV) Bottom Trace: Load Current (1A/DIV)
Data Sheet Download
www.linear.com
Linear Technology Corporation
• Loop bandwidth normally limited only by external
inductor, output capacitor, switching frequency and
the quality of the printed circuit board design
For applications help,
call (408) 432-1900
dn210f_conv LT/TP 0899 340K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 1999
Similar pages