DN443 - Step-Down Synchronous Controller Operates from Inputs Down to 2.2V

Step-Down Synchronous Controller Operates from Inputs Down
to 2.2V
Design Note 443
Wei Gu
Introduction
Many telecommunications and computing applications
need high efficiency step-down DC/DC converters that
can operate from a very low input voltage. The high
output power synchronous controller LT3740 is ideal
for these applications, converting input supplies ranging
from 2.2V to 22V to outputs as low as 0.8V with load
currents from 2A to 20A. Applications include distributed
power systems, point-of-load regulation and conversion
of logic supplies.
A major challenge in designing a step-down converter
with low VIN is that the gate voltage for the N-channel
MOSFETs is not readily available. The LT3740 solves
this problem by integrating a DC/DC step-up converter
for generating its own MOSFET gate drive voltage with
a small inexpensive external inductor. This function
permits the use of inexpensive off-the-shelf 5V gatedrive MOSFETs, offering up to 3% higher efficiency than
sub-logic gate-drive MOSFETs and eliminating the need
for a secondary supply.
The LT3740 operates at a fixed 300kHz frequency and
employs valley current mode control to deliver excellent
transient response and very low on-times. Furthermore,
a power good signal is available to monitor the output
voltage. The tracking capability built into XFER can be
used to implement control of the output voltage when
ramping up and ramping down. If less than 0.8V is applied
to XFER, the LT3740 uses this voltage as its reference
for regulation.
“Dying Gasp” Applications
The LT3740’s low VIN feature makes it a good fit in “dying gasp” applications for high reliability computers. In
these systems, when the power gets cut, a large bank of
electrolytic capacitors at the input charged to 12V or so,
serve as input to the power controller. The LT3740 can
operate down to 2.2V, which allows it to run longer from
the input capacitors’ charged voltage than a controller
requiring a higher input voltage. This effectively reduces
the input capacitance requirements, which can save
significant board space.
Figure 1 shows a typical application circuit. The LT3740
circuit, after its input power gets cut, is able to supply the
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VIN
12V
L2
22μH
LT3740
SWB
BGDP
VIN
CIN
3300μF
CB
1μF
BIAS
RANGE
TGATE
SHDN
M1
PH8230E
L1 0.9μH
SW
VOUT
1.5V
10A
100k
XREF
BGATE
1μF
M2
PH3830L
D1
B320A
100μF
×3
SN+
RS
7mΩ
PGOOD
SN–
VC
69.8k
12k
GND
100pF
FB
2200pF
78.7k
DN443 F01
Figure 1. “Dying Gasp” Application Circuit
06/08/443
load for 40ms as shown in Figure 2. In comparison, the
output voltage would drop out after 25ms with a controller
that shuts down when the input reaches 7V.
Generate a Negative Voltage from a Low Positive
VIN
Figure 3 shows a 2.4V to 14V input to –3.3V at 3A output
converter. The LT3740 works particularly well in this application due to its wide input voltage range and ability
to operate down to 2.2V. The LT3740 also operates with
synchronous rectification, which allows the use of high
efficiency MOSFETs, instead of less efficient switching
diodes.
Wide Input Voltage Range
The LT3740 offers high efficiency over a wide input voltage range (2.2V to 22V) and produces output voltages
as low as 0.8V. The LT3740 employs valley current mode
control to deliver excellent transient response and very
low on-times. Figure 4 illustrates low duty cycle waveforms for a 22V input, 0.8V output application at a fixed
frequency of 300kHz.
Conclusion
The LT3740 can operate from low input voltages, providing a space- and cost-saving solution over a wide
input voltage range. The LT3740 is a versatile platform
on which to build DC/DC converter solutions that use
few external components and maintain high efficiencies
over wide load ranges. The integrated step-up regulator
facilitates true single-supply operation with an input
voltage as low as 2.2V.
VSW
5V/DIV
IOUT = 3A
VIN
10V/DIV
VOUT
1V/DIV
1μs/DIV
10ms/DIV
DN443 F02
DN443 F04
Figure 4. LT3740 Low Duty Cycle Waveforms
VIN: 12V
VOUT: 1.5V
IOUT: 4A
Figure 2. Input and Output Voltages Waveforms When Input
Power is Removed
VIN
2.4V - 14V
L2
22μH
LT3740
SWB
BGDP
VIN
CIN
100μF
CB
1μF
VOUT
BIAS
RANGE
TGATE
SHDN
M1
PH8230E
L1 0.9μH
SW
100k
XREF
BGATE
1μF
SN
M2
PH3830L
D1
B320A
100μF
×3
+
RS
7mΩ
PGOOD
–
SN
PGND
VC
267k
12k
GND
100pF
FB
2200pF
80.6k
VOUT
–3.3V
3A
DN443 F03
Figure 3. A Positive to Negative Converter
Data Sheet Download
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call (408) 432-1900, Ext. 3565
www.linear.com
Linear Technology Corporation
dn443 LT/TP 0608 392K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
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