DN504 - 42V, 2.5A Synchronous Step-Down Regulator with 2.5μA Quiescent Current

42V, 2.5A Synchronous Step-Down Regulator with 2.5µA
Quiescent Current
Design Note 504
Hua (Walker) Bai
Introduction
The LT®8610 and LT8611 are 42V, 2.5A synchronous
step-down regulators that meet the stringent high
input voltage and low output voltage requirements of
automotive, industrial and communications applications. To minimize external components and solution
size, the top and bottom power switches are integrated
in a synchronous regulator topology, including internal
compensation. The regulator consumes only 2.5μA
quiescent current from the input source even while
regulating the output.
High Efficiency Synchonous Operation
Replacing an external Schottky diode with an internal
synchronous power switch not only minimizes the solution size, but also increases efficiency and reduces power
dissipation. The efficiency improvement is significant
in low output voltage applications where the voltage
drop of the Schottky diode represents a relatively large
portion of the output voltage. Figure 1 shows a 12V
to 3.3V circuit. Figure 2 shows the efficiency of this
circuit reaching 94%, which is 5% to 10% higher than
a comparable nonsynchronous circuit.
Short-Circuit Robustness Using Small Inductors
The LT8610 and LT8611 are specifically designed to
minimize solution size by allowing inductor size to be
selected based on the output load requirements of the
application, rather than the maximum current limits of
the IC. During overload or short-circuit conditions, the
LT8610 and LT8611 safely tolerate operation with saturated inductors through the use of a high speed peakcurrent mode architecture and a robust switch design.
For example, an application that requires a maximum
of 1.5A should use an inductor that has an RMS rating
of >1.5A and a saturation current rating of >1.9A. This
flexibility allows the user to avoid oversize inductors for
applications requiring less than maximum output current.
Current Sense and Monitoring with the LT8611
The LT8611 includes a flexible current control and
monitor loop using the ISN, ISP, IMON and ICTRL pins.
The ISP and ISN pins connect to an external sense
resistor that may be in series with the input or output
of the LT8611 or in series with other system currents.
The current limit loop functions by limiting the LT8611
output current such that the voltage between the ISP
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100
VIN
4.7μF
BST
0.1μF
EN/UV
SW
PG
SYNC
10nF
VO = 5V
L1
4.7μH
95
C1
47μF
LT8610
BIAS
VOUT
3.3V
2.5A
EFFICIENCY (%)
VIN
3.8V TO 42V
1M
TR/SS
1μF
FB
4.7pF
85
INTVCC
RT PGND GND
60.4k
412k
L1: HCM0703-4R7, COILTRONICS
C1: GRM32ER71A476K, MURATA
fSW = 700kHz
Figure 1. LT8610 12V to 3.3V Application
Achieves High Efficiency
07/12/504
VO = 3.3V
90
dn504 F01
80
0
0.5
1.0
1.5
LOAD CURRENT (A)
2.0
2.5
dn504 F02
Figure 2. Efficiency of the 12V to 3.3V Application
(Circuit Shown in Figure 1)
and ISN pins does not exceed 50mV. The ICTRL pin
allows the user to control this limit between 0mV and
50mV by applying 0V to 1V to the ICTRL pin. The IMON
pin outputs a ground-referenced voltage that is 20 •
(ISP – ISN), which allows easy monitoring and may be
used as an input to an A/D.
The LT8611 current sense and monitoring functionality
may be used to limit short-circuit current or to create
constant-current, constant-voltage (CCCV) supplies.
Figure 3 shows well controlled current during a shortcircuit event. The LT8611 can also be combined with a
microcontroller with A/D and D/A to create sophisticated
power systems. Typical apps include maximum power
point tracking (MPPT) for solar charging and programmable LED current source.
5W
20V/DIV
INDUCTOR
CURRENT
1A/DIV
dn504 F03
Figure 3. Short-Circuit Current is Well Regulated at 42V
with the LT8611
Wide Input Range Operation at 2MHz
It is well known that higher switching frequencies allow
for smaller solution sizes. In fact, a 2MHz switching
frequency is often used in automotive applications to
avoid the AM band and minimize solution footprint.
High switching frequencies, though, come with some
trade-offs, including reduced ability to handle wide
input voltage range commonly found in automotive
and industrial environments. However, the LT8610 and
LT8611 minimize these restrictions by allowing both
high switching frequencies and high conversion ratios.
This is due to their low minimum on-times (50ns typical) and low dropout, resulting in a wide input range,
even at 2MHz. Figure 4 shows a 5V, 2A, 2MHz circuit
that can accept 5.4V to 42V inputs. The circuit has a
2A output current limit.
Data Sheet Download
www.linear.com
VIN
5.4V TO 42V
VIN
4.7μF ON OFF
BST
0.1μF
EN/UV
0.025Ω
SYNC
SW
IMON
μP
ICTRL
OPTIONAL
LT8611
INTVCC
L1
3.3μH
ISP
ISN
BIAS
10pF
PG
TR/SS
0.1μF
1μF
18.2k
VOUT
5V
2A
RT
1M
FB
PGND GND
C1
47μF
243k
dn504 F04
Figure 4. LT8611 Running at 2MHz Reduces Solution
Size, Avoids AM Band, and Still Allows High Duty Cycle
Low Dropout Operation
As the input voltage decreases toward the programmed
output voltage, the LT8610 and LT8611 maintain regulation by skipping switch-off times and decreasing the
switching frequency up to a maximum duty cycle of
99.8%. If the input voltage decreases further, the output
voltage remains 450mV below the input voltage (at 2A
load). The boost capacitor is charged during dropout
conditions, maintaining high efficiency. Figure 5 shows
the dropout performance.
IL = 500mA/DIV
VIN = 1V/DIV
VO = 1V/DIV
VSW = 2V/DIV
dn504 F05
Figure 5. LT8610/LT8611 Dropout Performance
Conclusion
LT8610 and LT8611 are 42V, 2.5A synchronous stepdown regulators that offer 2.5μA quiescent current,
high efficiency, fault robustness and constant current
(LT8611 only), constant voltage operation in small
packages. This combination of features makes them
ideal for the harsh environment commonly found in
automotive and industrial applications.
For applications help,
call (408) 432-1900, Ext. 3513
Linear Technology Corporation
dn504f LT/AP 0712 196K • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2012
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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