DN1022 - Negative Buck Converter with Short-Circuit Protection and Shutdown

Negative Buck Converter with Short-Circuit Protection and Shutdown
Design Note 1022
Victor Khasiev
Introduction
Negative buck converters are increasingly used to “step
down” (by absolute value) negative voltages. The main
reason behind the increasing demand is the standardization
of switching transformers, which are typically produced
with one or two secondary windings. For example, if a
system employs a transformer with two secondary windings to produce ±12V, and the design also requires –3.3V,
engineers tend to lean toward solutions, such as a negative
buck, that don’t require changing the main transformer.
Circuit Description and Performance
Figure 1 shows a negative buck converter that generates
–3.3V at 3A from a –12V rail. The power train includes
inductor L1, diode D1 and MOSFET Q1. The LTC3805-5
controller IC includes a complete set of essential functions
including short-circuit protection (the current level can
be precisely set), converter enable/disable, and programmable switching frequency.
An internal shunt regulator allows biasing the IC directly
from the input rail. Despite the simplicity of the topology,
which makes it an attractive choice for many designers,
there are two important design considerations in a negative
buck converter: sensing the output voltage and remote
shutdown. The controller is referenced to the negative
voltage, yet the output voltage and ON/OFF signal are
referenced to the system ground (see Figure 1).
To close the regulation loop, a current mirror based
on transistor Q3 is used. Resistor RPRG programs the
current flowing into resistor RFB, which sets the output
voltage. In this example, when the output voltage is equal
to –3.3V, the 3.31k RPRG resistor sets the current into
resistor RFB at 1mA. This current creates a 0.8V drop
across resistor RFB, which is equal to the reference voltage of the internal error amplifier.
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R8
4.99k
ON OFF 5V
0V
4
5
3
R12
511k
R9
10k
1
GND
2
6
GND
R11
1k
Q2
MBT3906DW1T1
C2
0.1μF
R10
511k
C1
0.1μF
1
2
C3
2.2nF
R5
15k
3
4
5
R7
100k
–12V INPUT
GND
CIN
10μF
RFB
800
LTC3805-5
SSFLT GATE
ITH
VCC
FB
OC
RUN
FS
ISENSE
SYNC
GND
R4
316
8
OUT
330μF
RPRG
3.31k
VOUT
–3.3V AT 3A
MBT39069W171
4
5
Q1
HAT2165H
10
9
D1
PDS1040
L1
6μH
+C
R3
1k
3
2
1
6
7
6
R2
3.12k
R1
0.015
31.1k
–12V INPUT
Figure 1. A Negative Buck Converter Based on the LTC3805 Produces –3.3V at 3A from a –10V to –14V Input
09/11/1022
The optional shut-down circuit is based on transistor Q2.
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, transfer
function and other parameters are similar to the wellknown buck converter.
The efficiency is about 90%, as shown Figure 2. The load
characteristic is shown in Figure 3. At loads exceeding
4.5A, the output voltage begins to drop and at 5.0A the
converter enters into a short-circuit protection state. In
this state, the input current does not exceed 20mA. The
output voltage recovers after the short circuit is removed.
Line and load regulation are better than ±1% over a wide
–40°C to 70°C temperature range. Waveforms of the
start-up and transient response for the circuit in Figure
1 are shown in Figures 4 and 5, respectively.
Conclusion
Negative buck converters are a popular way to produce
additional negative rails from a standard –12V rail. The
solution shown here produces –3.3V at 3A from a –12V
rail with a design that features high efficiency, overcurrent
protection, fast transient response and a smooth start-up.
References
Erickson, Robert, W, Fundamentals of Power Electronics,
2nd edition, ISBN 0-7923-7270-0
Efficiency vs Load
92
10V
12V
14V
91
10ms/DIV
EFFICIENCY (%)
90
VOUT
(1V/DIV)
89
88
87
86
85
84
1.5
2.0
2.5
OUTPUT CURRENT (A)
1
3.0
Figure 4. Start-Up into Full Load
Figure 2. Efficiency vs Input Voltage and Output Current
Overcurrent Protection
3.5
VIN = –12V
OUTPUT VOLTAGE (V)
1ms/DIV
3.0
VOUT
100mV/DIV
IOUT
1A/DIV
2.5
2.0
1.5
3
3.5
4.0
4.5
5.0
OUTPUT CURRENT (A)
5.5
Figure 3. Output Voltage vs Output
Current, with a –12V Input Voltage
Data Sheet Download
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Figure 5. Transient Response for a
Load Current Step from 1A to 2.5A
For applications help,
call (408) 432-1900, Ext. 2134
Linear Technology Corporation
dn1022 LT 0911 • PRINTED IN THE USA
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