LINEAR TECHNOLOGY LTC3835 Technical data

advertisement

y

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 signifi cant
amount of time in st andby mode, require electric al 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 qualifi ed 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 fl exibility 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 effi ciency over a wide load range up to 20A.
Other important features include:

0.1µF

68.1k

215k

39pF

1200pF

10k

150pF

1nF

CLKOUT

PLLLPF

RUN

PGOOD

TRACK/SS
I

TH

LTC3835

SGND

PLLIN/MODE

V

FB

–

SENSE

+

SENSE

BOOST

INTV

EXTV

PGND

V

TG

SW

BG

IN

CC

CC

• 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 EXTV
option for improved effi ciency when V

> 4.7V. All

OUT

CC

these are packed in a small 20-pin exposed pad TSSOP or
4mm × 5mm QFN package. The LTC3835-1 fi ts in a smaller
16-pin SSOP package or 5mm × 3mm DFN package,
by eliminating CLKOUT, EXTV

and the power good

CC

indicator.

, LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technolog

Corporation. All other trademarks are the property of their respective owners.

V

IN

+

4V TO 36V

V

OUT

3.3V
5A

C

OUT

150µF
10V

0.22µF

CMDSH-4

4.7µF

Si4840DY

3.3µH

Si4840DY

0.012Ω

10µF
50V
CERAMIC

12/06/1014

L: SUMIDA, CDEP105-3R2MC-88
C

: SANYO, 10TPD150M

OUT

DN1014 F01

Figure 1. High Effi ciency Synchronous Step-Down Converter Consumes only 80µA in Standby Mode

High Effi ciency at Standby

The LTC3835 uses synchronous rectifi cation and N-chan­nel MOSFETs to achieve high effi ciency over a wide load
range. At light load it minimizes the IC supply current with

®

Burst Mode

operation, where the controller switches in
fi xed 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 reduc-

DN1014 F02

100000

10000

POWER LOSS (mW)

1000

100

10

1

0.1

100

90

80

70

60

50

40

EFFICIENCY (%)

30

20

10

0

0.01 0.1 1 10 100 1000 10000

0.001

EFFICIENCY

= 12V; V

V

IN

LOAD CURRENT (mA)

= 3.3V

OUT

POWER LOSS

Figure 2. Effi ciency and Power Loss vs
Load Current for the Circuit in Figure 1.

100

95

90

VIN = 4V

VIN = 12V

ing quiescent current. To maximize effi ciency 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 EXTV

pin to further increase

CC

conversion effi ciency.

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 effi ciency, up to 92%, at
12V input and maintains light-load effi ciency, above
65%, down to 1mA load current. The no-load IC supply
quiescent current is kept below 80µA. For low dropout
operation the effi ciency can reach 96.5% at 4V input as
illustrated in Figure 3.

The controller us es OPTI-LOOP compensation to opt imize
the transient response and thus minimize the output
capacitance. Figure 4 shows the transient response for
the circuit in Figure 1.

Conclusion

The LTC3835 contro ller is especially well suited to automo ­tive and other battery-powered applications that demand
low quiescent current and light-load effi ciency—impor­tant for preserving battery life in applications that spend
signifi cant 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
effi cient packages.

85

EFFICIENCY (%)

80

75

70

0.1

110

LOAD CURRENT (A)

DN1014 F03

Figure 3. Effi ciency vs Load Current at VIN = 4V
and VIN = 12V for the Circuit in Figure 1.

Data Sheet Download

www.linear.com

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

[phone]

●

FAX: [phone] ● www.linear.com

V

OUT

100mV/

DIV AC

COUPLED

I

L

2A/DIV

20µs/DIV

DN1014 F04

Figure 4. Load Step Transient with
12V Input for the Circuit in Figure 1.

For applications help,

call [phone], Ext. 3598

dn1014f LT 1206 • PRINTED IN THE USA

© LINEAR TECHNOLOGY CORPORATION 2006