DN1021 - How to Produce Negative Output Voltages from Positive Inputs Using a uModule Step-Down Regulator

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 –VOUT,
which must be tied to the μModule regulator’s GND. All
signals are now referred to –VOUT.
For this discussion, the LTM®8025 (36V, 3A) is used to
demonstrate how a buck μModule regulator can 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 configured to generate negative output
voltages by configuring it as an inverting buck-boost
converter, as illustrated in Figure 1. The negative terminal
of the input supply is connected to the VOUT pin of the
μModule regulator and the GND pin is tied to the –VOUT
rail. The actual input voltage (VIN’) seen by the μModule
VIN
+
VIN
VOUT
μModule
REGULATOR
–
regulator is the difference between the input supply (VIN)
and the output voltage (–VOUT). This voltage must be
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
counterpart. 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 configured as inverters.
Table 1. Example of Buck (Step-Down) DC/DC μModule
Regulators Configured as Inverters
IOUT(MAX)
12VIN → –5VOUT
μModule Regulator
LTM8020
0.165A
LTM8021
0.475A
LTM8022
1A
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.
VIN
GND
See LTM8025
and LTM8027
LTM8023
VOUT
RLOAD
24VIN → –12VOUT
VIN ’ = VIN – (–VOUT)
+
VIN ’: ACTUAL INPUT
VOLTAGE SEEN BY
μModule REGULATOR
–
VIN
VOUT
μModule
REGULATOR
RLOAD
GND
dn1021 F01
(a) Buck μModule Regulator Configured
for Positive Output Voltages
–VOUT
(b) Buck μModule Regulator Configured
for Negative Output Voltages
Figure 1. How to Configure a Buck Module for Negative Output Voltages
12/11/1021
VIN
20V TO 24V
VIN
RUN/SS
4.7μF
750kHz
750kHz
5V
0V
0.01μF
–7V
–12V
CMDSH2-3
100k
VOUT
AUX
BIAS
LTM8025
SHARE
PGOOD
SYNC
RT
ADJ
GND
63.4k
22μF
34.8k
–VOUT
–12V AT 2A
dn1021 F02
Figure 2. LTM8025 Schematic for –12V Output
–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 configured to generate negative output voltages. Figure 2 shows
an LTM8025 schematic generating –12V at 2A from an
input range of 20V to 24V. The actual input voltage seen by
the LTM8025 is VIN’ = VIN – (–VOUT). For instance, if VIN
= 20V, VIN’ = 20V – (–12V) = 32V. Because the maximum
input rating of the LTM8025 is 36V, the input supply in
this specific application is limited to 24V.
Additionally, the internal oscillator of the LTM8025 can
be synchronized by applying an external 250kHz to 2MHz
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
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 –VOUT. So, the RUN/SS voltage must be below 0.2V
above –VOUT to turn off the part, whereas it must be tied
to 2.5V above –VOUT for normal operation.
Conclusion
Step-down μModule regulators, such as the LTM8025,
can be easily configured 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.
VIN
10V/DIV
RUN/SS
2V/DIV
VOUT
10V/DIV
200μs/DIV
dn1021 F03
Figure 3. LTM8025 Start-Up Waveforms for –12V Output
Data Sheet Download
www.linear.com
For applications help,
call (408) 432-1900, Ext. 3747
Linear Technology Corporation
dn1021 LT/TP 1211 REV A 305K • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2011
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
LOGIC
HIGH
VIN
VIN
VOUT
μModule
REGULATOR
CIN
0V
R2
R1
R3
Figure 6. Step-Down μModule Regulator with
Schottky Diode Protection for Negative Output
dn1021 F04
Figure 4. Run Level-Shift Circuit for Negative
Output Configuration
The shutdown threshold varies with each μModule 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 μModule 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.
VIN
20V TO 48V
LOGIC INPUT
–VOUT
dn1021 F06
TO RUN PIN
OF μMODULE
REGULATOR
–VOUT
5V
COUT
SCHOTTKY
DIODE
(OPTIONAL)
GND
Q1
LOGIC INPUT
–
Level-Shifting the Run Pin in a Negative Output
Application
Step-down μModule 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 off 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 μModule 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.
External Schottky Diode for Start-Up Protection
When configuring a μModule regulator for negative output
voltages, the combination of input and output capacitors
creates an AC voltage divider at the output. During startup, the output (–VOUT ) will initially go positive for a short
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
limit the amount of positive voltage, an external Schottky
diode between –VOUT and the input supply ground may be
required. Figure 6 shows a simplified μModule regulator
schematic with the Schottky diode protection.
+
APPENDIX
4.7μF
×2
2N3906
0V
20k
Design Considerations for Negative Output
Applications
For negative output applications, the input voltage seen by
the μModule regulator (VIN′) is the difference between the
input supply voltage (VIN) and the output voltage (–VOUT ):
VIN′ = VIN – (–VOUT )
As a result, the maximum input voltage (VIN(MAX)′) must
be below the μModule regulator’s abs max input voltage
(VIN_MODULE(MAX)).
VIN
VOUT
LTM8027
RUN
BIAS1
SS
BIAS2
RT
20k
48.7k
22μF
×4
AUX
SYNC
20k
(Equation 1)
SCHOTTKY
DIODE
(OPTIONAL)
ADJ
GND
56.2k
dn1021 F05
Figure 5. LTM8027 with Run Level-Shift Circuitry for –12V Output
VOUT
–12V
3A
Additionally, the switch current is higher for inverting applications compared to the positive output configuration.
Hence, the maximum output current (IOUT(NEG)) must
be derated from the μModule regulator’s typical rating
(IOUT(POS)) according to the following equation:
IOUT(NEG) ≤ (IOUT(POS)) • (1 – DCMAX)
(Equation 2)
where the max duty cycle,
VOUT
DCMAX =
VIN(MIN) + VOUT
VIN = 15V nominal (range: 12V to 18V)
VOUT = –5V
IOUT(NEG) = 2A
Selected μModule regulator: LTM8025
VIN_MODULE(MAX) = 36V
IOUT(POS) = 3A
Calculations:
Using Equations 1 to 3, the following values were
determined:
VIN(MAX)′ = VIN(MAX) – (VOUT ) = 18 – (–5) = 23V
(Equation 3)
Equation 2 is only an approximation. The following parameters need to be considered to get a more accurate value:
switching frequency, inductor current ripple, efficiency,
switch current limit derating at high duty cycle, etc.
Design Example:
Inverting power supply requirements:
LTM8025 data sheet ratings:
DCMAX =
VOUT
5
=
= 0.294
VIN(MIN) + VOUT 12+ 5
(IOUT(POS)) • (1 – DCMAX) = (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 μModule 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—sufficient for the 2A
requirement of this application.
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