Download DN520 - Versatile Industrial Power Supply Takes High Voltage Input

Versatile Industrial Power Supply Takes High Voltage Input
and Yields from Eight 1A to Two 4A Outputs – Design Note 520
Martin Merchant
Introduction
Today’s industrial electronic systems contain many
of the same components as consumer electronics—
microcontrollers, FPGAs, system-on-chip ASICs and
other electronics—requiring multiple low voltage rails
at widely varied load currents. Industrial applications
can also demand a pushbutton interface, an alwayson supply for a real-time clock (RTC) or memory and
the ability to take input power from a high voltage
supply. Other required features may be a watchdog
timer (WDT), a kill or reset button, software adjustable
voltage levels and error reporting of low input/output
voltages and high die temperature.
The LTC ®3375 is a highly configurable multioutput
step-down power converter that offers the features
often required by industrial electronics while providing the flexibility to configure various outputs with
maximum currents ranging from 1A to 4A.
Configurable Maximum Output Current
The LTC3375’s eight 1A channels can be combined to
produce various combinations of 1A, 2A, 3A and 4A
buck regulators, as shown by the 15 different output
current configurations in Table 1.
Connecting the feedback pin of a given channel to
its VIN pin configures that channel as a slave to the
adjacent channel. The switch pins of the two channels
are connected together to share a single inductor and
output capacitor. Master/slave channels are enabled via
the master’s enable pin and regulate to the master’s
feedback network.
Output current can be increased to 3A or 4A by connecting additional adjacent channels. The circuit in Figure
1 shows the LTC3375 configured with a 3A output, a
1A output, two 2A outputs and an always-on LDO. It
also illustrates how the LTC3375 can be connected
to control the start-up of an upstream external buck
controller via the on-chip pushbutton interface to
supply input power to the LTC3375 buck regulators.
11/13/520
Table 1. LTC3375 Maximum Current Configurations
NUMBER OF BUCKS OUTPUT CONFIGURATION
8
1A, 1A, 1A, 1A, 1A, 1A, 1A, 1A
7
1A, 1A, 1A, 1A, 1A, 1A, 2A
6
1A, 1A, 1A, 1A, 1A, 3A
6
1A, 1A, 1A, 1A, 2A, 2A
5
1A, 1A, 1A, 1A, 4A
5
1A, 1A, 1A, 2A, 3A
5
1A, 1A, 2A, 2A, 2A
4
1A, 1A, 2A, 4A
4
1A, 1A, 3A, 3A
4
1A, 2A, 2A, 3A
4
2A, 2A, 2A, 2A
3
1A, 3A, 4A
3
2A, 2A, 4A
3
2A, 3A, 3A
2
4A, 4A
External VCC LDO and External Input Power
Supply Start-Up Control
The LTC3375 can control an external LDO pass device
to supply its VCC power and any other low current
electronics such as an RTC. The VCC powers the
internal pushbutton circuitry, WDT, internal registers
and open-drain pull-ups. The external LDO in Figure
1 creates a 3.3V supply from the 24V rail.
When the pushbutton is pressed, the ON pin is released
and the RUN pin is pulled high on the LTC3891, supplying input power to the buck regulators of the LTC3375.
When the LTC3891 achieves regulation, the PGOOD pin
is released, enabling EN1 of the LTC3375 and turning
on the 2A regulator. The remaining regulators can be
enabled with the precision threshold enable pins or
via software-controlled I2C commands. Pressing the
pushbutton again for 10 seconds or more, or pulling
KILL low for 50ms or more, causes the ON pin to be
pulled low, disabling all of the buck regulators.
L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are
registered trademarks of Linear Technology Corporation. All other trademarks
are the property of their respective owners.
Unique Power Control and Features
The I2C interface allows extensive control of regulator
operation. Each regulator may be set to a high efficiency
Burst Mode® operation to save power at light loads or
set to forced continuous mode for lower output ripple
voltage. Each regulator can also have the switching
cycle phase shifted by 0°, 90°, 180°, or 270° with
respect to the reference clock to allow a lower input
ripple current when multiple outputs are supplying large
loads. Another feature is the ability to manipulate each
output voltage up or down by adjusting the feedback
reference voltage from the default 725mV setting in
25mV steps (ranging from 425mV to 800mV). The I2C
interface is also used for reporting error conditions
for each regulator.
The LTC3375 has a reset (RST) pin and an interrupt
request (IRQ) pin, which can be programmed to report
when any regulator’s output voltage has dropped below
92.5% of the regulation point. The IRQ pin can also be
programmed to report when the input voltage drops
below the undervoltage lockout (UVLO) threshold or
when the die temperature has reached a set temperature
threshold. The regulator’s PGOOD and UVLO status,
24V
TG
LTC3891
RUN
BOOST
SENSE+
SGND
0.1µF
SENSE–
1µF
MTOP
L1
8µH
RSENSE
7mΩ
VCC
COUT
3.3V
330µF 15mA
SW3
SW4
VSHNT
22µF
1.02M
SGND
19.1k
LTC3891 PGOOD
MTOP, MBOT: Si7850DP
L1 COILCRAFT SER1360-802KL
COUT: SANYO 10TPE330M
D1: DFLS1100
2.5V
2A
2.2µH
1.02M
100µF
3.3V
2A
287k
LTC3375
SW5
FBVCC
2.2µH
22µF
649k
576k
1.8V
1A
FB5
100k
EXTVCC
VFB
100µF
FB3
VCC
1nF
1.02M
412k
107Ω
1Ω
2.2µH
FB1
47k
FZT629B
5V
6A
MBOT
BG
ITH
TRACK/SS
D1
SW1
SW2
FB2
FB4
FB7
FB8
24V
SW
FREQ
0.1µF
VIN1-VIN8
2.2µF
PGND
ILIM
34.8k
Conclusion
The LTC3375 can be configured with multiple regulated
1A to 4A outputs totaling up to 8A, and includes many
features required by today’s industrial electronics.
INTVCC
PLLIN/MODE
470pF
One problem with microprocessors is that a software
bug can cause the program to hang. The LTC3375
includes a watchdog timer input (WDI) pin to monitor the SCL pin or some other pin to determine if the
software is still running. If the software has stopped
running, the watchdog timer output (WDO) pin can be
used to reset the microprocessor or power down the
HV buck and the LTC3375 buck regulators. Connecting the WDO pin to the RST pin of a microprocessor
causes the microprocessor to reset when the WDT is
not satisfied. Connecting the WDO pin to the KILL pin
causes the ON pin to go low, disabling the HV buck and
all LTC3375 regulators. The KILL pin can be pulled low
by a pushbutton “paper clip” switch to power down
all the regulators as a last resort.
5V
1.5A
CIN
22µF
VIN
LT3375 EN1
PGOOD
LTC3375 ON
the die temperature warning and the measured die
temperature can be monitored by the microprocessor
via the I2C interface.
432k
1µF
I2C
CONTROL
SW6
SW7
SW8
SCL
SDA
WDI
VCC
0.01µF
PB
PUSHBUTTON
1.2V
3A
649k
RT
CT
402k
68µF
422k
FB6
EN1
SYNC
EN3
EN5
EN6
EN2
EN4
EN7
EN8
MICROPROCESSOR
CONTROL
2.2µH
EXPOSED PAD
IRQ
RST
WDO
TEMP
ON
KILL
MICROPROCESSOR
CONTROL
LTC3891 RUN
PUSHBUTTON
DN520 F01
Figure 1. Low Voltage Power Supply with Pushbutton Control of Upstream HV Buck and Always-On LDO
Data Sheet Download
www.linear.com/LTC3375
Linear Technology Corporation
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dn520f LT/AP 1113 111K • PRINTED IN THE USA
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