DN433 - A Positive-to-Negative Voltage Converter Can Be Used for Stable Outputs Even with a Widely Varying Input

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A Positive-to-Negative Voltage Converter Can Be Used for
Stable Outputs Even with a Widely Varying Input – Design Note 433
Victor Khasiev
+
–
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
01/08/433
Q
PWM
D
VO–
+
C
L
RLOAD
DN433 F01
Figure 1. Simplifed Block Diagram of
Positive-to-Negative Converter
VIN+
+
–
V1
D
VO–
+
–
V1
Q
D
VO–
+
L
C
RLOAD
L
+
C
RLOAD
DN433 F02
(2a) Transistor Q is On
(2b) Transistor Q is Off
Figure 2. Equivalent Circuits Show the Operation
of the Positive-to-Negative Converter
Q GATE
ON
OFF
ON
OFF
IL
A new generation of Linear Technology high voltage synchronous step-down converters, such as the LT®3845,
make it possible to implement positive-to-negative
conversions for a variety of applications.
Basic Operation
Figure 1 shows a simplified block diagram of a positive-tonegative converter. Figure 2 shows an equivalent circuit,
which helps in understanding the basic operation of the
circuit in Figure 1. When transistor Q is on (Figure 2a),
diode D is reverse biased and the current in inductor L
increases. When Q is off (Figure 2b), inductor L changes
polarity, diode D becomes forward biased, and current
flows from inductor L to the load and capacitor C. The
voltage across capacitor C and the load is negative, relative
to system ground. Figure 3 shows a timing diagram.
V1
+
This topology is particularly useful when the input varies
above or below the output. In such cases, a traditional
step-down regulator would not be able to regulate once
the battery voltage drops below the output, thus shortening the useful battery run time. Buck-boost solutions
and other topologies such as a SEPIC solve this problem,
but they tend to be more complicated and expensive. The
positive-to-negative converter topology presented here
combines the simplicity of a step-down converter and
the regulation range of a buck-boost topology.
VIN+
+
An obvious application of a positive-to-negative converter
is generating a negative voltage output from a positive
input. However, a not-so-obvious use is to produce a stable
output voltage in an application that has a widely varying input. For example, a converter in a battery-powered
device, which has an inherently variable input voltage, can
produce a stable output voltage even if input voltage falls
below the absolute value of the output voltage. However, an
obvious drawback is reverse polarity, which can be easily
overcome in this application. The supplied circuitry can use
the negative output as the system ground and the negative
battery terminal as the “positive” voltage source.
VIN
VL
VOUT
DN433 F03
Figure 3. Converter Timing Diagram
The duty cycle range can be found from following
expression:
D=
VO
VIN + VO
DMAX =
DMIN =
VO
VIN(MIN) + VO
VO
VIN(MAX ) + VO
Component Stress in a Positive-to-Negative
Topology
VMAX is the maximum voltage across transistor Q and
diode D (Figure 2), where:
VMAX = VIN(MAX) + |VO|
The maximum current, IMAX, through transistor Q, inductor L and diode D can be derived based on the following
equations, assuming continuous conduction mode:
IL =
VIN(MIN) • t •DMAX
IO
dI
, dI=
, IMAX =IL +
2
1–DMAX
L
where t is a switching period.
Circuit Description
Figure 4 shows a 9V to 15V input to –12V at 3A output
converter. The high voltage LT3845 is used for several
VIN
9V TO 15V
R2
51.1k
C5
0.47μF
1
2
3
4
C1
0.1μF
5
–12V (IC GND)
6
R3
143k
7
R4
16.2k
R6
49.9k
8
Conclusion
Very often electrical engineers have to design a negative
voltage source supplied from a positive voltage rail. The
positive-to-negative converter discussed in the article can
be a good alternative to a flyback or a SEPIC approach.
Q1
PH3075L
GND
VIN
BOOST
SHDN
SS
The entire converter power path contains the LT3845 high
voltage PWM controller, MOSFETs Q1 and Q2, inductor
L1, diode D1 and output filter capacitors COUT1–COUT3.
Diode D2 is a bootstrap diode and diode D3 provides bias
voltage for internal MOSFET drivers.
VIN
CIN1
22μF
25V
R1
249k
reasons, including the ability of its SW pin to withstand
65V, its integrated high side driver and differential current sense. The LT3845 can also provide synchronous
rectification, which allows the use of efficient MOSFETs
over less efficient switching diodes.
TG
LT3845
SW
VCC
BURST
FB
BG
VC
PGND
IS+
SYNC
IS–
fSET
SGND
17
16
15
14
13
C3
0.1μF
D2
BAS521
R8
10Ω
L1
13μH
PB2020.153
C6
OPT
RS1
6mΩ
R7
10Ω
COUT1
16ME470WF
D1
B160
12
11
Q2
PH1875L
D3 BAS521
10
9
C2
1μF
+
COUT3
10μF
25V
COUT2
10μF
25V
GND
VOUT
–12V
3A
–12V (IC GND)
DN433 F04
C4
2.2μF
R5
61.9k
Figure 4. Conversion of 9V-15V into –12V at 3A Based on the LT3845 High Voltage PWM Controller
91.5
14V
91.0
15V
EFFICIENCY (%)
90.5
13V
12V
90.0
9V
10V
89.5
89.0
88.5
88.0
1.0
1.5
2.0
2.5
3.0
LOAD CURRENT (A)
DN433 F05
Figure 6. Transient Response to
an Output Load Step of 1A to 2A
Figure 5. Efficiency for the Figure 4
Circuit with Varying Input Voltage to
a Fixed –12V Output
Figure 7. Start-Up Waveform
for the Circuit in Figure 4 with
VIN = 14V, VOUT = –12V, IOUT = 2A
Data Sheet Download
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call (408) 432-1900, Ext. 3161
www.linear.com
Linear Technology Corporation
dn433f LT/TP 0108 387K • PRINTED IN THE USA
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