DN1014 - 80µA Quiescent Current Synchronous Step-Down Controller Extends Battery Life in Automotive Applications

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80µA Quiescent Current Synchronous Step-Down Controller Extends Battery Life in Automotive Applications – Design Note 1014
Julian Zhu
Introduction
Battery powered applications that have a significant
amount of time in standby mode, require electrical circuits
to operate with a low quiescent current to preserve battery life. The LTC3835 synchronous step-down controller
is an excellent solution with its ultralow quiescent current (80µA). Other features make it uniquely qualified to
satisfy the needs of automotive applications. A wide 4V
to 36V input voltage range protects the supply against
high input voltage transients and is compatible with low
voltage cold crank conditions. The constant frequency
current-mode architecture with high-side inductor current sensing provides excellent line and load regulation
and reliable current limiting and sharing. The 0.8V to
10V output voltage range provides flexibility for a variety
of applications. Solution size is small due to the 650kHz
switching frequency, which reduces external component
sizes, and the integrated synchronous FET driver, which
keeps component count low. Furthermore, the LTC3835
provides high efficiency over a wide load range up to 20A.
Other important features include:
CLKOUT
• The output voltage can be ramped during start-up with
an adjustable soft-start or it can track an external ramp
signal.
• The controller can be synchronized to an external clock
via a phase-locked loop.
• The CLKOUT pin and current mode control allow interleaved operation of multiple parallel controllers for
delivery of high output current with accurate current
sharing.
The LTC3835 has overvoltage and overcurrent (shortcircuit) protection, a power good monitor and an EXTVCC
option for improved efficiency when VOUT > 4.7V. All
these are packed in a small 20-pin exposed pad TSSOP or
4mm × 5mm QFN package. The LTC3835-1 fits in a smaller
16-pin SSOP package or 5mm × 3mm DFN package,
by eliminating CLKOUT, EXTVCC and the power good
indicator.
, LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technology
Corporation. All other trademarks are the property of their respective owners.
VIN
PLLLPF
TRACK/SS
ITH
LTC3835
1nF
215k
BOOST
SW
3.3µH
VOUT
3.3V
5A
0.012Ω
CMDSH-4
68.1k
SGND
INTVCC
PLLIN/MODE
EXTVCC
VFB
VIN
4V TO 36V
0.22µF
PGOOD
1200pF
150pF
10k
Si4840DY
TG
RUN
0.1µF
10µF
50V
CERAMIC
BG
+
4.7µF
COUT
150µF
10V
Si4840DY
SENSE–
SENSE+
PGND
39pF
L: SUMIDA, CDEP105-3R2MC-88
COUT: SANYO, 10TPD150M
DN1014 F01
Figure 1. High Efficiency Synchronous Step-Down Converter Consumes only 80µA in Standby Mode
12/06/1014
High Efficiency at Standby
The LTC3835 uses synchronous rectification and N-channel MOSFETs to achieve high efficiency over a wide load
range. At light load it minimizes the IC supply current with
Burst Mode® operation, where the controller switches in
fixed duty cycle bursts, as needed, to reduce the IC supply current. When the load current is extremely low, the
controller enters sleep mode by turning off both external
MOSFETs and most of its internal circuitry, further reduc100000
100
EFFICIENCY
VIN = 12V; VOUT = 3.3V
90
80
10000
EFFICIENCY (%)
1000
60
50
100
40
POWER LOSS
30
20
10
POWER LOSS (mW)
70
1
10
0
0.001 0.01
0.1
0.1
1
10 100 1000 10000
LOAD CURRENT (mA)
DN1014 F02
Figure 2. Efficiency and Power Loss vs
Load Current for the Circuit in Figure 1.
100
VIN = 4V
ing quiescent current. To maximize efficiency during Burst
Mode operation the bottom MOSFET is turned off when
the instantaneous inductor current is close to zero. The
IC is also designed with an EXTVCC pin to further increase
conversion efficiency.
3.3V/5A Synchronous DC/DC Converter
Figure 1 shows an automotive application that converts a
4V to 36V input to a 3.3V/5A output. As Figure 2 shows,
the converter achieves high efficiency, up to 92%, at
12V input and maintains light-load efficiency, above
65%, down to 1mA load current. The no-load IC supply
quiescent current is kept below 80µA. For low dropout
operation the efficiency can reach 96.5% at 4V input as
illustrated in Figure 3.
The controller uses OPTI-LOOP compensation to optimize
the transient response and thus minimize the output
capacitance. Figure 4 shows the transient response for
the circuit in Figure 1.
Conclusion
The LTC3835 controller is especially well suited to automotive and other battery-powered applications that demand
low quiescent current and light-load efficiency—important for preserving battery life in applications that spend
significant time in standby. The LTC3835 also offers a
wide input and output voltage range and many other
advanced features in either leaded or leadless thermally
efficient packages.
95
EFFICIENCY (%)
VIN = 12V
90
85
VOUT
100mV/
DIV AC
COUPLED
80
75
70
0.1
1
LOAD CURRENT (A)
10
IL
2A/DIV
DN1014 F04
DN1014 F03
Figure 3. Efficiency vs Load Current at VIN = 4V
and VIN = 12V for the Circuit in Figure 1.
20µs/DIV
Figure 4. Load Step Transient with
12V Input for the Circuit in Figure 1.
Data Sheet Download
For applications help,
call (408) 432-1900, Ext. 3598
www.linear.com
Linear Technology Corporation
dn1014f LT 1206 • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2006
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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