LINEAR TECHNOLOGY LT3501 Technical data

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A Compact Dual Step-Down Converter with V

OUT

Sequencing – Design Note 403

Tiger Zhou and David Canny

Introduction

Typical industrial and automotive applications require
multiple high current, low voltage power supply solutions
to drive everything from disc drives to microproces­sors. For many of these applications, particularly those

®

that have size constraints, the LT3501

dual step-down
converter is an attractive solution because it’s compact
and inexpensive compared to a 2-chip solution. The dual
converter accommodates a 3V to 25V input voltage range
and is capable of supplying up to 3A per channel. The
circuit in Figure 1 produces 3.3V and 1.8V.

LT3501 Dual Converter Features

• The LT3501 is feature- rich and comes with internal 3. 5A

switches and sense resistors to minimize solution size
and cost.

• The LT3501 operates at a fi xed frequency between

250kHz and 1.5MHz, programmed using a single re­sistor or synchronized to an external clock, allowing
optimization of effi ciency and solution size.

V

IN

4.5V TO 24V

3A, V

IN

2.8A, V

IN

> 16V*

V

OUT1

3.3V

< 16V

4.7µF
X5R

47µF
X5R

24.9k

8.06k

4.7µH

PMEG4005PMEG4005

10pF

*FOR 3A CAPABILITY ABOVE 16V, INCREASE INDUCTOR VALUE BY 33%.

0.47µF

B360A B360A

100k

40.2k

470pF

• A 180° phase relationship between the channels is
maintained to reduce input voltage ripple and input
capacitor size.

• Independent input voltage, feedback, soft-start and
power good functions for each converter simplify the
implementation of all the tracking and sequencing
options available.

• Minimum input-to-output voltage ratios are extended
by allowing the switch to stay on through multiple
clock cycles resulting in a 95% maximum duty cycle
regardless of switching frequency.

• The LT3501 automatica lly resets the sof t-start func tion
if the output drops out of regulation so that a short
circuit or brownout event is graceful and controlled.

• One or both converters can be shutdown at any time
if they’re not being used, reducing input power drain.

• The LT3501 is available in a 20-pin TSSOP package
with an exposed pad for low thermal resistance.

, LT, LTC and LTM are registered trademarks of Linear Technology
Corporation. All other trademarks are the property of their respective owners.

= 500kHz)

(f

V

IN1

SHDN

BST1

SW1

IND1
V

OUT1

PG1
FB1
V

C1

SS/TRACK1

0.1µF

LT3501

GND

V

IN2

R

/SYNC

T

BST2

SW2

IND2

V

OUT2

PG2

FB2
V

SS/TRACK2

SW

61.9k

3.3µH

0.47µF

100k

C2

40.2k

470pF

Tracking and

V

OUT2

1.8V
< 16V

3A, V

IN

> 16V*

2.8A, V

100µF
X5R

IN

10k

8.06k

47pF

DN403 F01

11/06/403

Figure 1. Compact Dual 3A Step-Down Converter with Ceramic Capacitors

Output Supply Tracking and Sequencing

Output voltage tracking and sequencing between chan­nels can be implemented using the LT3501’s soft-start
and power good pins as shown in Figures 2(a) to 2(c).
Output sequencing can also be implemented as shown
in Figure 2(d).

V

OUT1

PG1

PG2

1V/DIV

V

OUT2

1V/DIV

High Current Single V

, Low Ripple 6A Output

OUT

The LT3501 can generate a single, low ripple 6A output
as shown in Figure 3 with the dual converters sharing a
s i n g l e o u t p u t c a p a c i t o r . W i t h t h i s s o l u t i o n , r i p p l e c u r r e n t s
at the input and output are reduced, thus reducing volt­age ripple and allowing the use of smaller, less expensive
capacitors.

V

OUT1

PG1

PG2

1V/DIV
V

OUT2

1V/DIV

10ms/DIV

(2a) Independent

10ms/DIV

(2c) Absolute

V

OUT

3.3V

< 16V

6A, V

IN

5.6A, V

> 16V

IN

DN403 F02a

V

OUT1

1V/DIV
V

OUT2

PG2

PG1

1V/DIV

DN403 F02c

Figure 2. Output Voltage Tracking and Sequencing

V

IN

100µF
X5R

24.9k

8.06k

4.5V TO 24V

4.7µH

10pF

470pF

4.7µF
X5R

B360APMEG4005 PMEG4005B360A

100k

40.2k

V

IN1

SHDN

BST1

0.47µF 0.47µF
SW1

LT3501

IND1
V

OUT1

PG1
FB1
V

C1

SS/TRACK1

GND

V

IN2

R

/SYNC

T

BST2

SW2

IND2

V

OUT2

PG2

FB2
V

SS/TRACK2

10ms/DIV

(2b) Ratiometric

PG2

10ms/DIV

(2d) Output Sequencing

(fSW = 500kHz)

61.9k

4.7µH

C2

DN403 F02b

DN403 F02d

V

OUT1

1V/DIV
V

OUT2

1V/DIV

Figure 3. 4.5V to 24VIN, 3.3V

Data Sheet Download

www.linear.com

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

[phone]

●

FAX: [phone] ● www.linear.com

0.1µF

DN403 F03

/6A Step-Down Converter

OUT

For applications help,

call [phone], Ext. 3223

dn403f LT 1106 409K • PRINTED IN THE USA

© LINEAR TECHNOLOGY CORPORATION 2006