DN233 - Unique High Efficiency 12V Converter Operates with Inputs from 6V to 28V

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Unique High Efficiency 12V Converter Operates
with Inputs from 6V to 28V – Design Note 233
Christopher B. Umminger
Generating an output DC voltage from an input voltage
that can vary both above and below the regulation point
is a challenging power supply problem with a variety
of solutions. Flyback or forward converters work well,
but are difficult to use if a small and efficient solution
is desired at high output currents. A cascade of two
converters, such as a boost followed by a buck or linear
regulator, is another possibility. Alternately, one can
use a switching regulator in a SEPIC configuration that
requires two inductors and an intermediate capacitor.
These solutions are complex and have a high cost in
components, board area and efficiency. However, one
can also use the LTC ®1625 No RSENSE ™ controller in a
circuit that is capable of both up and down conversion
and requires only a single inductor and no sense resistor.
converter stage. During the first portion of each cycle,
switches M1 and M3 are on while M2 is off. The input
voltage is applied across the inductor and its current
increases. After the LTC1625 current comparator trips,
M1 and M3 are turned off and M2 and D2 conduct for
the remainder of the cycle. During this time, current is
delivered to the output while –VOUT is applied across
the inductor and its current decreases.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
and No RSENSE is a trademark of Linear Technology Corporation. All other
trademarks are the property of their respective owners.
100
EFFICIENCY (%)
90
12V Output, Single Inductor, Buck/Boost
Converter
An example of such a circuit is shown in Figure 1 to
provide a 12V output with inputs that can range from
6V to 18V. All of the circuitry to the left of the inductor
is identical to a typical buck converter implemented with
the LTC1625. However, now the output (right) side of the
inductor is also switched using an additional MOSFET
M3 and a diode D2. These two devices act like a boost
VOUT = 12V
VIN = 6V
VIN = 18V
80
VIN = 12V
70
60
50
0.01
0.1
1
LOAD CURRENT (A)
Figure 2. Efficiency of the 12V Output
Single Inductor Buck/Boost Converter
RF
1Ω
1
2
RC 20k
CC1
4.7nF
CC2
220pF
TK
16
M1
Si4412DY
L1 60μH
15
3
14
RUN/SS
SW
LTC1625
4
13
FCB
TG
5
12
ITH
BOOST
6
7
R1
11k
SYNC
VIN
+
CF
0.1μF
8
SGND
INTVCC
BG
VOSENSE
VPROG
PGND
DB
CMDSH-3
11
10
9
VIN
6V TO
18V
CIN
22μF
50V
D2
MBRS340T3
VOUT
12V
M3
Si4412DY
+
CSS 0.1μF
EXTVCC
CVCC
4.7μF
+
CB
0.33μF
M2
Si4412DY
D1
MBRS140T3
R2
100k
DN233 F01
CIN: UNITED CHEMICON THCR70E1H226ZT
COUT: AVX TPSV107020R0085
L1: 60μH/2.5A
Figure 1. 12V Output Single Inductor Buck/Boost Converter
06/00/233_conv
10
DN133 F02
COUT
100μF
20V
w2
VIN
IOUT
6
12
18
0.7
1.1
1.3
The duty cycle for the Figure 1 circuit is equal to VOUT/
(VIN + VOUT ). When VIN is equal to VOUT, a fifty percent
duty cycle is required to balance the volt-seconds across
the inductor. Both the input and output capacitors must
filter a square pulse current in this topology. The average value of the inductor current is equal to the sum of
the input and output currents. Since the LTC1625 uses
MOSFET VDS sensing to control the inductor current
peaks, the output current limit depends upon the duty
cycle and will vary with the input voltage. At VIN = 12V,
the maximum output current is about 1.1A. Efficiency
of the circuit is shown in Figure 2. Note that diode D2
prevents current reversal which causes cycle skipping at
low load currents and improves the light load efficiency.
converter efficiency. Gate drive for this switch is derived
from the LTC1625 BG pin, buffered by the LTC1693-2
and then level shifted to the output with the network
formed by C4, R4 and D4. Another change increases
the allowed input voltage, which is limited in the Figure 1 circuit by the breakdown voltage of the M3 gate.
This impediment is overcome using a clamp network
formed by R5, C3 and Z1 to derive the turn-on signal
for switch M3. The signal is buffered by the other half
of the LTC1693-2 to drive M3. The efficiency of this
circuit is shown in Figure 4.
100
EFFICIENCY (%)
Synchronous Circuit for Higher Power, Higher VIN
Several modifications can be made to the Figure 1
circuit to improve its operation as shown in Figure 3.
In order to process more power, lower on-resistance
MOSFET switches are used along with a higher current
inductor. The number of input and output capacitors is
also increased due to the higher RMS currents flowing
through them.
1
2
CSS 0.1μF
3
4
RC
20k
CC1
2.2nF
5
CC2
220pF
6
7
R1
11k
8
EXTVCC
VIN
SYNC
TK
RUN/SS
SW
FCB
TG
LTC1625
ITH
SGND
VOSENSE
VPROG
16
M1
FDS6670A
15
14
VIN = 6V
VIN = 12V
VIN = 18V
80
0
1
2
3
4
LOAD CURRENT (A)
CIN
22μF
50V
w3
VIN
6V TO 28V
VIN
IOUT
6
12
24
3.0
4.0
5.5
INTVCC
BG
PGND
6
DN233 F04
D2
MBRS835L
L1
15μH
13
VOUT
12V
CB 0.33μF
BOOST
5
Figure 4. Efficiency of the Synchronous
Buck/Boost Circuit
RF
1Ω
+
90
70
At higher power levels, it is desirable to use a synchronous switch M4 across the output diode D2, allowing
one to reduce the current rating of D2 and improve the
CF
0.1μF
VOUT = 12V
12
DB
CMDSH-3
11
10
+
M2
FDS6670A
R5
100k
9
D1
MBRS835L
CIN: UNITED CHEMICON THCR70E1H226ZT
COUT: AVXTPSV107020R0085
L1: 15μH/10A, 77120-A7, 13 TURNS, 16 GAUGE
M4
FDS6675
R3
1k INTVCC
1
Z1
MMBZ
5240B
CVCC
4.7μF
R2
100k
D3
CMDSH-3
C3
500pF
8
R4
10k
D4
CMDSH-3
M3
FDS6670A
7
+
COUT
100μF
20V
w6
2 LTC1693-2
3
4
C4
0.1μF
5
6 LTC1693-2
INTVCC
DN233 F03
Figure 3. Synchronous 12V Output Buck/Boost Converter
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