How to Produce Negative Output Voltages from Positive Inputs
Using a µModule Step-Down Regulator
Design Note 1021
Jaino Parasseril
Introduction
Linear Technology’s DC/DC step-down µModule®
regulators are complete switchmode power supplies in a
surface-mount package. They include the DC /DC controller, inductor, power switches and supporting circuitry.
These highly integrated regulators also provide an easy
solution for applications that require negative output
voltages. In other words, these products can operate as
inverting buck-boost regulators. As a result, the lowest
potential in the circuit is not the standard 0V, but –V
OUT
,
which must be tied to the µModule regulator’s GND. All
signals are now referred to –V
For this discussion, the LTM
.
OUT
®
8025 (36V, 3A) is used to
demonstrate how a buck µMo dule regulator c an be altered
to produce a negative output voltage with level-shifting
circuitry for synchronization. This approach can be applied to other µModule regulators, such as the LTM8022
(36V, 1A), LTM8023 (36V, 2A) and LTM8027 (60V, 4A).
Design Guide
A conventional buck (step-down) µModule regulator
can be easily confi gured to generate negative output
voltages by confi guring it as an inverting buck-boost
c on v er t e r, as i ll u st ra te d i n Fi gu r e 1 . T h e n eg at iv e t e rm in a l
of the input supply is connected to the V
µModule regulator and the GND pin is tied to the –V
rail. The actual input voltage (V
’) seen by the µModule
IN
pin of the
OUT
OUT
regulator is the difference between the input supply (V
and the output voltage (–V
). This voltage must be
OUT
within the allowable input range of the part. Additionally,
the absolute value of the output voltage must not exceed
the maximum output voltage rating of the µModule
regulator. Since the part is now operating as an inverting
buck-boost, the switch current is larger than in its buck
counterpar t. Hence, parameters such as output current,
switching frequency, thermal performance, etc. must be
considered to stay within the part’s limits. Refer to Appendix for detailed discussions and calculations. Refer
to Table 1 for a selection guide of example buck µModule
regulators confi gured as inverters.
Table 1. Example of Buck (Step-Down) DC/DC μModule
Regulators Confi gured as Inverters
I
OUT(MAX)
μModule Regulator
LTM8020 0.165A
LTM8021 0.475A
LTM8022 1A
LTM8023 1.6A
LTM8025 2.95A 2.2A
LTM8027 4A 3.65A
L, LT, LTC, LTM, Linear Technology, the Linear logo and µModule are registered
trademarks of Linear Technology Corporation. All other trademarks are the property of
their respective owners.
12V
→ –5V
IN
OUT
24VIN → –12V
See LTM8025
and LTM8027
OUT
IN
)
12/11/1021
V
IN
V
IN
+
–
µModule
REGULATOR
GND
V
OUT
V
OUT
R
LOAD
dn1021 F01
(a) Buck µModule Regulator Confi gured
for Positive Output Voltages
Figure 1. How to Confi gure a Buck Module for Negative Output Voltages
VIN’ = VIN – (–V
VIN’: ACTUAL INPUT
VOLTAGE SEEN BY
µModule REGULATOR
OUT
(b) Buck µModule Regulator Confi gured
for Negative Output Voltages
V
IN
V
)
IN
+
REGULATOR
–
µModule
GND
V
OUT
R
LOAD
–V
OUT
5V
0V
750kHz
0.01µF
V
–7V
–12V
100k
750kHz
20V TO 24V
CMDSH2-3
IN
4.7µF
63.4k
V
IN
RUN/SS
LTM8025
SHARE
SYNC
RT ADJ
PGOOD
GND
Figure 2. LTM8025 Schematic for –12V Output
V
OUT
AUX
BIAS
34.8k
dn1021 F02
22µF
–V
OUT
–12V AT 2A
–12V Output Application
The LTM8025 is a 36VIN, 3A step-down µModule
converter that can support output voltages up to 24V.
With minimal design effort, it can be easily confi gu r e d t o g e n e r a t e n e g a t i v e o u t p u t v o l t a g e s . F i g u r e 2 s h o w s
an LTM8025 schematic generating –12V at 2A from an
input range of 20V to 24V. The actual input volt age seen by
the LTM8025 is V
= 20V, V
’ = 20V – (–12V) = 32V. Because the maximum
IN
IN
’ = V
IN
– (–V
). For instance, if VIN
OUT
input rating of the LTM8025 is 36V, the input supply in
this specifi c application is limited to 24V.
Additionally, the internal oscillator of the LTM8025 can
b e s y n c h r o n i z e d b y a p p l y i n g a n e x t e r n a l 2 5 0 k H z t o 2 M H z
clock signal to the SYNC pin. For negative output voltages, the clock must be level-shifted to account for the
lower potential. This example has a 0V to 5V, 750kHz
input clock signal. By adding a few passive components,
the input clock is level-shifted to produce a –12V to
–7V signal, which is then applied to the SYNC pin of
V
IN
10V/DIV
the LTM8025. Figure 3 shows the start-up waveforms
for the –12V output application.
Run/Shutdown
The LTM8025 has a RUN/SS pin that provides shutdown
along with soft-start functions. In order to shut down
the part, the RUN/SS pin must be pulled below 0.2V. For
negative output applications, the LTM8025 GND is tied
to –V
above –V
to 2.5V above –V
. So, the RUN/SS voltage must be below 0.2V
OUT
to turn off the part, whereas it must be tied
OUT
for normal operation.
OUT
Conclusion
Step-down µModule regulators, such as the LTM8025,
can be easily confi gured for negative output voltages. For
negative outputs, the LTM8025 operates as an inverting
buck-boost, so the maximum allowable output current is
lower than typical buck topologies. If synchronization is
desired, proper level-shifting circuitry is required. For a
complete description of the LTM8025, including operation and applications information, refer to the data sheet.
RUN/SS
2V/DIV
V
OUT
10V/DIV
Figure 3. LTM8025 Start-Up Waveforms for –12V Output
Data Sheet Download
www.linear.com
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
200µs/DIV
dn1021 F03
For applications help,
call (408) 432-1900, Ext. 3747
dn1021 LT/TP 1211 REV A 305K • PRINTED IN THE USA
© LINEAR TECHNOLOGY CORPORATION 2011
APPENDIX
Level-Shifting the Run Pin in a Negative Output
Application
Step-down Module regulators are equipped with a Run
pin to enable and shut down the part. For negative output
applications, the Run voltage must be level-shifted to
properly turn of f the part. Using just a single PNP transistor and a few resistors, level-shifting can be achieved to
utilize the shutdown feature, as seen in Figure 4. When
the logic input is high, the Run voltage increases by an
amount determined by the voltage divider resistors R2
and R3. Once the Run voltage exceeds the shutdown
threshold, the Module regulator will turn on; as a result,
the output will drop to the programmed negative voltage.
To shut down the part, apply a logic low input to force the
Run voltage to the same potential as the negative output.
LOGIC
LOGIC INPUT
HIGH
0V
Q1
R2
R1
TO RUN PIN
OF µMODULE
REGULATOR
R3
External Schottky Diode for Start-Up Protection
When confi guring a Module regul ator for negative output
voltages, the combination of input and output capacitors
creates an AC voltage divider at the output. During startup, the output (–V
) will initially go positive for a short
OUT
period of time before dropping down to the intended negative potential. The positive voltage peak is dependent on
both the capacitance values and the input voltage step. To
li mi t t he am ou nt of po si ti ve vo lt a ge , a n e x te rn al Sc ho t tk y
diode between –V
and the input supply ground may be
OUT
required. Figure 6 shows a simplifi ed Module regulator
schematic with the Schottky diode protection.
V
IN
–
+
µModule
GND
V
OUT
dn1021 F06
SCHOTTKY
DIODE
C
OUT
(OPTIONAL)
–V
OUT
V
IN
C
IN
REGULATOR
Figure 6. Step-Down Module Regulator with
Schottky Diode Protection for Negative Output
–V
OUT
dn1021 F04
Figure 4. Run Level-Shift Circuit for Negative
Output Confi guration
The shutdown threshold varies with each Module regulator and is listed in their respective data sheet tables.
Scale the resistors R2 and R3 according to the logic high
input voltage and the Module regulator’s shutdown
threshold. Figure 5 shows an example of an LTM8027
–12V output application with the level-shifting circuitry.
In this example, the LTM8027 has a 5V logic input and a
Run pin resistor divider for about 2.5V, enough to exceed
the part’s 1.4V shutdown threshold.
V
IN
LOGIC INPUT
20V TO 48V
5V
2N3906
0V
20k
20k
20k
4.7µF
×2
48.7k
Design Considerations for Negative Output
Applications
For negative ou tput applications, the input volt age seen by
′
the Module regula tor (V
input supply volt age (V
′
V
= VIN – (–V
IN
OUT
As a result, the maximum input voltage (V
) is the dif ference between the
IN
) and the output volt age (–V
IN
OUT
) (Equation 1)
′
IN(MAX)
) must
be below the Module regulator’s abs max input voltage
(V
IN_MODULE(MAX)
V
IN
RUN
SS
SYNC
RT ADJ
LTM8027
GND
V
OUT
BIAS1
BIAS2
AUX
).
56.2k
dn1021 F05
22µF
×4
SCHOTTKY
DIODE
(OPTIONAL)
V
OUT
–12V
3A
):
Figure 5. LTM8027 with Run Level-Shift Circuitry for –12V Output
Additionally, the switch current is higher for inverting applications compared to the positive output confi guration.
Hence, the maximum output current (I
OUT(NEG)
) must
be derated from the Module regulator’s typical rating
(I
OUT(POS)
I
) according to the following equation:
OUT(NEG)
≤ (I
OUT(POS)
) • (1 – DC
) (Equation 2)
MAX
where the max duty cycle,
V
DC
MAX
=
V
IN(MIN)
OUT
+ V
(Equation 3)
OUT
Equation 2 is only an approx imation. The following paramete rs nee d to be c ons ider ed to g et a mor e ac cur at e va lue:
switching frequency, inductor current ripple, effi ciency,
switch current limit derating at high duty cycle, etc.
Design Example:
Inverting power supply requirements:
V
= 15V nominal (range: 12V to 18V)
IN
V
I
= –5V
OUT
OUT(NEG)
= 2A
Selected Module regulator: LTM8025
LTM8025 data sheet ratings:
V
IN_MODULE(MAX)
I
OUT(POS)
= 3A
= 36V
Calculations:
Using Equations 1 to 3, the following values were
determined:
V
IN(MAX)
DC
(I
OUT(POS)
MAX
= V
=
V
IN(MIN)
) • (1 – DC
IN(MAX)
V
OUT
– (V
+ V
MAX
) = 18 – (–5) = 23V
OUT
5
=
OUT
= 0.294
12+ 5
) = (3A) • (1 – 0.294) = 2.12A
′
The above calculations determined that the LTM8025
is a good candidate for this inverting application. The
maximum input voltage across the Module regulator
is 23V, well below the 36V maximum operating voltage.
With a max duty cycle of 29.4%, the maximum output
current is approximately 2.12A—suffi cient for the 2A
requirement of this application.
Loading...