DN523 - Inverting DC/DC Controller Converts a Positive Input to a Negative Output with a Single Inductor

Inverting DC/DC Controller Converts a Positive Input to a
Negative Output with a Single Inductor – Design Note 523
David Burgoon
There are several ways to produce a negative voltage
from a positive voltage source, including using a
transformer or two inductors and/or multiple switches.
However, none are as easy as using the LTC ®3863,
which is elegant in its simplicity, has superior efficiency
at light loads and reduces parts count compared to
alternative solutions.
Advanced Controller Capabilities
The LTC3863 can produce a –0.4V to –150V negative
output voltage from a positive input range of 3.5V to
60V. It uses a single-inductor topology with one active
P-channel MOSFET switch and one diode. The high
level of integration yields a simple, low parts-count
solution.
The LTC3863 offers excellent light load efficiency,
drawing only 70µA quiescent current in user-programmable Burst Mode ® operation. Its peak current
mode, constant frequency PWM architecture provides
positive control of inductor current, easy loop compensation and superior loop dynamics. The switching
frequency can be programmed from 50kHz to 850kHz
with an external resistor and can be synchronized to
an external clock from 75kHz to 750kHz. The LTC3863
offers programmable soft-start or output tracking.
Safety features include overvoltage, overcurrent and
short-circuit protection, including frequency foldback.
–5.2V, 1.7A Converter Operates from a 4.5V to
16V Source
The circuit shown in Figure 1 produces a –5.2V, 1.7A
output from a 4.5V–16V input. Operation is similar to a
flyback converter, storing energy in the inductor when
the switch is on and releasing it through the diode to
the output when the switch is off, except that with
the LTC3863, no transformer is required. To prevent
excessive current that can result from minimum ontime when the output is short-circuited, the controller
folds back the switching frequency when the output
is less than half of nominal.
The LTC3863 can be programmed to enter either high
efficiency Burst Mode operation or pulse-skipping at
light loads. In Burst Mode operation, the controller
directs fewer, higher current pulses and then enters
a low current quiescent state for a period of time
depending on load. In pulse-skipping mode, the
LTC3863 skips pulses at light loads. In this mode,
L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are
registered trademarks of Linear Technology Corporation. All other trademarks
are the property of their respective owners.
VIN
4.5V TO 16V
GND
CLKIN
TRACK
0.1µF
100µF
20V
10µF
25V
×2
0.016Ω
0.5W
1%
100k
45.3k
LTC3863EMSE
1
12
PLL/MODE GATE
61.9k 2
11
VIN
FREQ
3
10
GND
SENSE
4
9
SS
CAP
5
8
VFB
RUN
0.47µF
6
7
16V
VFBN
ITH
GND
14.7k
13 68pF
390pF
27nF
80.6k
Si7129DN
VOUT
–5.2V/1.7A
100pF
0.1µF
100V
10µH
919AS-100M
PDS540
40V
5A
33µF
16V
×2
+
+
150µF
16V
×2
1µF
16V
523k
Figure 1. Inverting Converter Produces –5.2V at 1.7A from a 4.5V to 16V Source
02/14/523
GND
DN523 F01
the modulation comparator may remain tripped for
several cycles and force the external MOSFET to stay
off, thereby skipping pulses. This mode offers the
benefits of smaller output ripple, lower audible noise
and reduced RF interference, at the expense of lower
efficiency compared to Burst Mode operation. This
circuit fits in about 0.5in2 (3.2cm2) with components
on both sides of the board.
Figure 2 shows the switch node voltage, inductor
current and ripple waveforms at 5V input and –5.2V
output at 1.7A. The inductor is charged (current rises)
when the PMOSFET is on, and discharges through the
diode to the output when the PMOS turns off. Figure 3
shows the same waveforms at 70mA out in pulseskipping mode. Notice how the switch node rings out
around 0V when the inductor current reaches zero. The
effective period stops when the current reaches zero.
Figure 4 shows the same load condition with Burst
Mode operation enabled. Power dissipation drops by
31% at this operating point, and efficiency increases
from 74% to 80.5%. At 12V input, the 45% reduction
in dissipation is even more dramatic.
High Efficiency
Figure 5 shows efficiency curves for both pulse-skipping and Burst Mode operation. Exceptional efficiency
of 85.2% is achieved at 1.7A load and 12V input. Note
how Burst Mode operation dramatically improves
efficiency at loads less than 0.2A. Pulse-skipping
efficiency at light loads is still much higher than that
obtained from continuous conduction.
Conclusion
The LTC3863 simplifies the design of converters
producing a negative output from a positive source.
It is elegant in its simplicity, high in efficiency, and
requires only a few inexpensive external components.
VSW
5V/DIV
VSW
5V/DIV
VO
50mV/DIV
IL
1A/DIV
IL
1A/DIV
VO
50mV/DIV
1µs/DIV
1µs/DIV
DN523 F02
Figure 2. Switch Node Voltage, Inductor Current and
Ripple Waveforms at 5V Input and –5.2V Output at 1.7A
Figure 4. Switch Node Voltage, Inductor Current and
Ripple Waveforms at 5V Input and –5.2V Output at 70mA
in Burst Mode Operation
90
85
80
EFFICIENCY (%)
VSW
5V/DIV
VO
50mV/DIV
75
70
65
VIN = 5V
VIN = 12V
VIN = 5V
VIN = 12V
60
Burst Mode
OPERATION
PULSE-SKIPPING
MODE
55
IL
1A/DIV
50
1µs/DIV
DN523 F03
Figure 3. Switch Node Voltage, Inductor Current and
Ripple Waveforms at 5V Input and –5.2V Output at 70mA
in Pulse-Skipping Mode
Data Sheet Download
www.linear.com/LTC3863
Linear Technology Corporation
DN523 F04
45
0.01
0.1
IOUT (A)
1
DN523 F05
Figure 5. Efficiency In Normal Mode and Burst Mode
Operation of the Circuit in Figure 1
For applications help,
call (408) 432-1900, Ext. 3420
dn523f LT/AP 0214 111K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
 LINEAR TECHNOLOGY CORPORATION 2014
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