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A Compact Dual Step-Down Converter with VOUT Tracking and
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 fixed frequency between
250kHz and 1.5MHz, programmed using a single resistor or synchronized to an external clock, allowing
optimization of efficiency and solution size.
• 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 automatically resets the soft-start function
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.
VIN
4.5V TO 24V
4.7µF
X5R
VIN1
4.7µH
SHDN
RT/SYNC
BST1
BST2
SW1
SW2
0.47µF
B360A
PMEG4005
VOUT1
3.3V
3A, VIN < 16V
2.8A, VIN > 16V*
VIN2
(fSW = 500kHz)
61.9k
0.47µF 3.3µH
B360A
LT3501
PMEG4005
IND1
VOUT1
100k
24.9k
8.06k
47µF
X5R
40.2k
10pF
470pF
100k
PG1
PG2
FB1
FB2
VC1
VC2
SS/TRACK1
SS/TRACK2
GND
0.1µF
VOUT2
1.8V
3A, VIN < 16V
2.8A, VIN > 16V*
IND2
VOUT2
10k
40.2k
470pF
8.06k
47pF
DN403 F01
*FOR 3A CAPABILITY ABOVE 16V, INCREASE INDUCTOR VALUE BY 33%.
Figure 1. Compact Dual 3A Step-Down Converter with Ceramic Capacitors
11/06/403
100µF
X5R
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).
High Current Single VOUT, Low Ripple 6A Output
The LT3501 can generate a single, low ripple 6A output
as shown in Figure 3 with the dual converters sharing a
single output capacitor. With this solution, ripple currents
at the input and output are reduced, thus reducing voltage ripple and allowing the use of smaller, less expensive
capacitors.
VOUT1
1V/DIV
PG1
VOUT2
1V/DIV
PG2
PG2
DN403 F02a
10ms/DIV
VOUT1
1V/DIV
VOUT2
1V/DIV
PG1
10ms/DIV
(2a) Independent
DN403 F02b
(2b) Ratiometric
VOUT1
1V/DIV
VOUT2
1V/DIV
PG2
VOUT1
1V/DIV
VOUT2
1V/DIV
PG1
PG2
DN403 F02c
10ms/DIV
10ms/DIV
(2c) Absolute
DN403 F02d
(2d) Output Sequencing
Figure 2. Output Voltage Tracking and Sequencing
VIN
4.5V TO 24V
4.7µH
PMEG4005
VOUT
3.3V
6A, VIN < 16V
5.6A, VIN > 16V
4.7µF
X5R
VIN1
RT/SYNC
BST1
BST2
SW1
SW2
0.47µF
B360A
0.47µF 4.7µH
PMEG4005
IND2
VOUT2
100k
24.9k
8.06k
(fSW = 500kHz)
61.9k
B360A
LT3501
IND1
VOUT1
100µF
X5R
VIN2
SHDN
10pF
40.2k
PG1
PG2
FB1
FB2
VC2
VC1
SS/TRACK1 SS/TRACK2
GND
470pF
0.1µF
DN403 F03
Figure 3. 4.5V to 24VIN, 3.3VOUT/6A Step-Down Converter
Data Sheet Download
For applications help,
call (408) 432-1900, Ext. 3223
www.linear.com
Linear Technology Corporation
dn403f LT 1106 409K • 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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