Feb 1999 No Rsense Controller Delivers 12V and 100W at 97% Efficiency

DESIGN IDEAS
No RSENSE Controller Delivers 12V
and 100W at 97% Efficiency by Christopher B.
Umminger
maximum temperature of 75°C in a
25°C environment. L1 is a customwound inductor using fourteen turns
of 15 gauge wire on a Magnetics, Inc.
Kool Mµ® 77206-A7 core. The entire
converter takes up a volume of only
0.65in3 and processes an impressive
150W per cubic inch.
The circuit uses the LTC1625 No
RSENSE™ controller to deliver the high
output voltage with excellent efficiency. This controller provides true
current mode control without using a
sense resistor by monitoring the voltage drop across the power MOSFET
switches. Eliminating the sense
resistor saves board space and
improves efficiency. In this application, a 0.01Ω sense resistor would
dissipate about 0.7W at full load.
Many current mode controllers use
a sense resistor in series with the
inductor. Unfortunately, they must
restrict the maximum output voltage
RF 1Ω
CF
0.1µF
1
2
CSS 0.1µF
3
4
CC1
2200pF
RC1 20k
5
CC2 100pF
6
7
R1 3.92k
8
LTC1625CS
16
VIN
EXTVCC
M1
15
TK
SYNC
FDS6670A
14
SW
RUN/SS
13
TG
FCB
L1 15µH
12
BOOST
ITH
CB
11 DB CMDSH-3
0.22µF
INTVCC
SGND
10
D1
BG
VOSENSE
MBRS9
+
C
140T3
PGND
VCC
VPROG
M2
4.7µF FDS6670A
R2 35.7k
VIN
CIN 12V–28V
10µF
30V
×4
+
due to limits on the input range of the
current comparator. However, the
LTC1625 has no such constraint. The
circuit in Figure 1 uses the LTC1625
in its adjustable mode, with the VPROG
pin left open. The internal error
amplifier compares the voltage at the
VOSENSE pin to a 1.19V reference and
an external resistive divider sets the
output voltage.
Figure 2 shows that 97% efficiency
is achieved over a wide range of load
current. The application uses the FCB
pin to disable Burst Mode operation
and force continuous, synchronous
operation down to no load. Enabling
Burst Mode would keep the efficiency
above 90% down to a load of only
50mA. The current mode control of
the LTC1625 incorporates foldback
current limiting that reduces the output current to 6A when the output is
shorted.
Kool Mµ is a registered trademark of Magnetics, Inc.
100
95
VOUT
12V/8.5A
+
COUT
150µF
16V
×2
EFFICIENCY (%)
Heat removal presents a thorny
problem in many of today’s compact
systems. This is especially the case
when power converters deliver high
output voltages with several amperes
of current and are processing tens to
hundreds of watts. In this regime, a
converter with only moderate efficiency will have a significant amount
of waste heat and may require heat
sinks and additional air flow. A very
high efficiency converter can reduce
the wasted power, which saves space
and lowers costs.
The circuit shown in Figure 1 is a
power converter that produces a 12V
output at up to 8.5A from an input
that can range between 12V and 28V.
The 100W of output power is converted at 97% efficiency with only 3W
dissipated on the board. No special
heat sinks were used other than a
widened VIN trace connected to the
drain of M1. This point reached a
90
85
VIN = 24V
VOUT = 12V
80
0
CIN: SANYO OS-CON 30SC10M
COUT: SANYO OS-CON 16SA150M
4
6
LOAD CURRENT (A)
8
10
(619) 661-6835
Figure 1. 100W, 12V, 8.5A supply
26
2
Figure 2. Efficiency vs load current for
Figure 1’s circuit
Linear Technology Magazine • February 1999