DN458 - Buck Converter Eases the Task of Designing Auxiliary Low Voltage Negative Rails

Buck Converter Eases the Task of Designing Auxiliary
Low Voltage Negative Rails
Design Note 458
Victor Khasiev
Introduction
Many system designers need an easy way to produce
a negative 3.3V power supply. In systems that already
have a transformer, one option is to swap out the existing
transformer with one that has an additional secondary
winding. The problem with this solution is that many
systems now use transformers that are standard, offthe-shelf components, and most designers want to
avoid replacing a standard, qualified transformer with a
custom version. An easier alternative is to produce the
low negative voltage rail by stepping down an existing
negative rail. For example, if the system already employs
an off-the-shelf transformer with two secondary windings
to produce ±12V, and a –3.3V rail is needed, a negative
buck converter can produce the –3.3V output from the
–12V rail.
Leave the Transformer Alone: –3.3VOUT from –12VIN
Figure 1 shows a negative buck converter that generates
–3.3V at 3A from a –12V rail. The power train (indicated
4.99k
SHUTDOWN
INPUT
Despite the simplicity of this topology, there are some
design hurdles. The first is that the feedback loop must
control a negative output voltage via the controller’s
internal positive reference. The second is that the on/off
signal is referenced to the system ground.
To solve the output reference polarity problem, the regulation loop uses a current mirror based on transistors
Q2 and Q3. Resistor RPRG programs the current flowing
into resistor RFB which sets the output voltage. In this
example, when the output voltage is at the desired –3.3V,
the current through the 3.31k RPRG resistor is 1mA. This
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GND
Q4
511k
GND
CIN
10μF
D1
PDS1040
VIN = –12V
10k
511k
by bold lines in Figure 1) includes an inductor L1, a
diode D1 and a MOSFET Q1. The LTC3805-5 controller
includes short-circuit protection (the current level can
be precisely set), enable control and a programmable
switching frequency. An internal shunt regulator simplifies biasing this IC directly from the input rail.
Q5
1μF
1k
0.1μF
2.2nF
LTC3805-5
SSFLT
VCC
15k
100k
GATE
ITH
RUN
FS
OC
RPRG
3.31k
COUT
330μF
L1
6μH
316Ω
Q1
HAT2165H
1k
+
VOUT = –3.3V
AT 3A
Q2
Q3
3.12k
ISENSE
FB
SYNC
GND
0.015Ω
RFB
800Ω
51.1k
dn458 F01
–12V
Figure 1. A Negative Buck Converter Based on the LTC3805-5 Produces –3.3V at 3A from a –10V to –14V Input
02/09/458
current creates a 0.8V drop across resistor RFB, which
is equal to the reference voltage, VREF, of the internal
error amplifier:
VOUT =
VREF • RPRG
RFB
There is also an optional on/off circuit based on transistors
Q4 and Q5. If 5V is applied to resistor R8, the LTC3805-5
shuts down. Both circuits are referenced to the system
ground. The voltage stress on the power train components,
the transfer function and other parameters are similar to
positive input voltage buck converters.
This circuit operates at 90% efficiency, as shown in
Figure 2. Figure 3 shows the progressive overcurrent
VOUT = –3.3V
Conclusion
A negative buck converter is an easier way to generate
an additional negative rail in systems that already have
a larger negative voltage supply. This avoids undesirable replacement of standard transformers or modular
components.
3.5
91
VIN = –10V
90
VIN = –12V
89
VIN = –14V
VIN = –12V
3.0
–VOUT (V)
EFFICIENCY (%)
92
protection as the load current increases. The output
voltage drops at loads exceeding 4.5A, and at 5A the
converter enters into a short-circuit protection state
where the power is limited to 0.25W. The output voltage
recovers after the short-circuit is removed. In addition,
the line and load regulation has a maximum deviation
of less than 1%. Figures 4 and 5 show the start-up and
transient response waveforms, respectively.
88
87
86
2.5
2.0
85
84
1.5
1.5
1
2
LOAD (A)
2.5
3
3
dn458 F02
Figure 2. Efficiency vs Input Voltage and
Output Current for the Circuit in Figure 1
3.5
4
4.5
IOUT (A)
5
5.5
dn458 F03
Figure 3. Output Voltage vs Output Current, at
–12V Input Voltage for the Circuit in Figure 1
VOUT
100mV/DIV
AC COUPLED
–VOUT
1V/DIV
DC COUPLED
ILOAD
1A/DIV
10ms/DIV
dn458 F04
Figure 4. Start-Up into Full Load
Data Sheet Download
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1ms/DIV
dn458 F05
Figure 5. Transient Response for a
Load Current Step from 1A to 2.5A
For applications help,
call (408) 432-1900, Ext. 3161
Linear Technology Corporation
dn458 LT/TP 0209 155K • PRINTED IN THE USA
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