DN479 - Dual Output High Efficiency Converter Produces 3.3V and 8.5V Outputs from a 9V to 60V Rail

Dual Output High Efficiency Converter Produces 3.3V and 8.5V
Outputs from a 9V to 60V Rail – Design Note 479
Victor Khasiev
Introduction
Among the many step-down DC/DC switching regulator controllers, the LTC3890 stands out because of its
unique features. This 50μA quiescent current device can
produce two output voltages ranging from 0.8V to 24V
when powered from an input voltage of 4V to 60V.
Many high input voltage step-down DC/DC converter
designs use a transformer-based topology or external high
side drivers to operate from up to 60VIN. Others use an
intermediate bus converter requiring an additional power
stage. However, the LTC3890 simplifies design, with its
smaller solution size, reduced cost and shorter development time compared to other design alternatives.
Feature Rich
The LTC3890 is a high performance synchronous buck
DC/DC controller with integrated N-channel MOSFET drivers. It uses a current mode architecture and operates from
a phase-lockable fixed frequency from 50kHz to 900kHz.
The device features up to 99% duty cycle capability for
low voltage dropout applications, adjustable soft-start or
voltage tracking and selectable continuous, pulse-skipping or Burst Mode ® operation with a no-load quiescent
current of only 50μA. These features, combined with a
minimum on-time of just 95ns, make this controller an
L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered
trademarks of Linear Technology Corporation. All other trademarks are the property of
their respective owners.
22μF
50V
1μF
VIN
INTVCC
TG1
0.1μF
TG2
BOOST1
4.7μH
BOOST2
SW1
0.1μF
8μH
SW2
BG1
BG2
LTC3890
PGND
SENSE1+
SENSE2+
SENSE1–
VFB1
ITH1
SENSE2–
0.01Ω
0.01Ω
VOUT1
3.3V
5A
+
VIN
9V TO 60V
100k
470μF
1000pF
31.6k
34.8k
VFB2
ITH2
TRACK/SS1 SGND TRACK/SS2
0.1μF
0.1μF
100k
470pF
10.5k
+
VOUT2
8.5V
3A
330μF
34.8k
dnVK F01
Figure 1. High Efficiency Dual 8.5V/3.3V Output Step-Down Converter
06/10/DN479
ideal choice for high step-down ratio applications. Power
loss and supply noise are minimized by operating the two
output stages out-of-phase.
together. For example, a 9V to 60V input can be converted
to an 8.5V output at 6A. Figure 4 shows the efficiency of this
configuration at input voltages of 10V, 30V and 60V.
Dual Output Application
Figure 1 shows the LTC3890 operating in an application
that converts a 9V to 60V input into 3.5V/5A and 8.5V/3A
outputs. The transient response for the 3.3V output with a
4A load step is less than 50mV (as shown in Figure 2).
Current mode control provides good current balance
between the phases. Less than 10% mismatch can be
achieved, as shown in Figure 5.
Figure 3 shows the efficiency of the 8.5V channel with a
36V input voltage.
Single Output Application
The LTC3890 can also be configured as a 2-phase single
output converter by simply connecting the two channels
Conclusion
Although there are many choices in dual-output controllers, the LTC3890 brings a new level of performance with
its high voltage operation, high efficiency conversion and
ease of design.
100
98
10V
EFFICIENCY (%)
96
50mV/DIV
2A/DIV
30V
94
92
60V
90
dnvk F02
1ms/DIV
Figure 2. Transient Response of 3.3V Channel (IOUT1: 1A to 5A)
88
86
1
2
3
4
AMPS (A)
5
6
dnvk F04
Figure 4. Efficiency of the LTC3890 Configured as a
2-Phase Single Output of 8.5V at Up to 6A from a 10V to
60V Input
95
VIN = 36V
94
EFFICIENCY (%)
93
92
91
1A/DIV
90
89
0A
88
87
1μs/DIV
0
1
2
dnvk F05
3
AMPS (A)
dnvk F03
Figure 3. Efficiency of the Converter in Figure 1 for the
VOUT2A 8.5V Channel
Data Sheet Download
www.linear.com
Figure 5. The Inductor Current in a 2-Phase Single Output
Converter. Currents in Both Inductors Shown with a 24V
Input and 8.5V at 6A Output.
For applications help,
call (408) 432-1900, Ext. 3161
Linear Technology Corporation
dn479f LT/TP 0610 116K • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2010
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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