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

design ideas
Dual Output High Efficiency Converter Produces
3.3V and 8.5V Outputs from a 9V to 60V Rail
Victor Khasiev
The LTC3890 dual output
DC/DC controller brings
a unique combination of
high performance features
to applications that require
low voltage outputs from
high voltage inputs. It can
produce two output voltages
ranging from 0.8V to 24V
from an input voltage of 4V
to 60V. It is also very efficient,
with a no-load quiescent
current of only 50µA.
Many high-input-voltage step-down
DC/DC converter designs use a transformerbased 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.
1µF
Si 7850DP
TG1
0.1µF
4.7µH
VIN
INTVCC
BOOST1
SW1
Si 7850DP
BG1
+
1000pF
31.6k
= DFLS1100
34.8k
+
Figure 2. Transient response of 3.3V channel
8µH
Si 7850DP
BG2
LTC3890
PGND
SENSE2+
SENSE1–
VFB1
ITH1
SENSE2–
VFB2
ITH2
100k
470pF
TRACK/SS1 SGND TRACK/SS2
34.8k
0.1µF
0.1µF
+
10.5k
VOUT2
8.5V
3A
COUT2
150µF
= 6TPE150
Figure 1. High efficiency dual 8.5V/3.3V output step-down converter
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 ideal choice for high stepdown ratio applications. Power loss and
(continued on page 35)
95
100
94
98
EFFICIENCY (%)
92
91
90
87
10VIN
96
30VIN
94
92
60VIN
90
88
88
ILOAD1 = 1A TO 5A
Si 7850DP
0.01Ω
100k
COUT1
150µF
VIN
9V TO 60V
SW2
SENSE1+
89
1ms/DIV
0.1µF
0.01Ω
VOUT1
3.3V
5A
EFFICIENCY (%)
ILOAD1
2A/DIV
TG2
BOOST2
93
VOUT1
50mV/DIV
22µF
50V
VIN = 36V
0
2
1
LOAD CURRENT (A)
3
Figure 3. Efficiency of the converter in Figure 1 for
the VOUT2 8.5V channel
86
1
2
3
4
LOAD CURRENT (A)
5
6
Figure 4. Efficiency of the LTC3890 configured as a
2-phase single output of 8.5V at up to 6A
July 2010 : LT Journal of Analog Innovation | 33
product briefs
The LTC4361 overvoltage and overcurrent protection controller utilizes a 2%
accurate 5.8V overvoltage threshold to detect an overvoltage event and responds
quickly within 1µs (max) to isolate the downstream components from the input.
The LTC4361 controls a low cost external
N-channel MOSFET so that under normal
operation it provides a low loss path
from the input to the load. Inrush current limiting is achieved by controlling
the voltage slew rate of the gate. If the
voltage at the input exceeds the overvoltage threshold of 5.8V, the GATE is pulled
low within 1µs to protect the load. While
the IC operates from supplies between
2.5V and 5.5V, the input pins can withstand 80V transients or DC overvoltages.
The LTC4361 features a soft shutdown
controlled by the ON pin and provides a
gate drive output for an optional external P-channel MOSFET for reverse voltage
protection. A power good output pin
indicates gate turn-on. Following an overvoltage condition, the LTC4361 automatically restarts with a start-up delay. The
LTC4361 is available in two options; the
LTC4361-1 latches off after an overcurrent
event, where as the LTC4361-2 performs
an auto-retry following a 130ms delay.
The new LTC4360 overvoltage protection
controller is recommended for applications that do not require overcurrent
protection. While offering many of the
same features as the LTC4361, the two
LTC4360 versions are differentiated by pin
functions. The LTC4360-1 features soft
shutdown control with low shutdown
current of 1.5µA, while the LTC4360-2
can drive an optional external P-channel
MOSFET for negative voltage protection.
The LTC4361 is offered in 8-lead
(2mm × 2mm) DFN and SOT-23 packages,
and the LTC4360 is offered in a tiny 8-lead
SC70 package.
180MHZ, 1mA POWER EFFICIENT
RAIL-TO-RAIL I/O OP AMPS
The LTC6246/LTC6247/LTC6248 are single/
dual/quad low power, high speed unity
gain stable rail-to-rail input/output
operational amplifiers. On only 1mA of
supply current, they feature an impressive
180MHz gain-bandwidth product, 90V/µs
slew rate and a low 4.2nV/√Hz of inputreferred noise. The combination of high
bandwidth, high slew rate, low power consumption and low broadband noise makes
these amplifiers unique among rail-to-rail
input/output op amps with similar supply
currents. They are ideal for lower supply
voltage high speed signal conditioning
systems. The LTC6246 family maintains
high efficiency performance from supply
voltage levels of 2.5V to 5.25V and is fully
specified at supplies of 2.7V and 5.0V. For
applications that require power-down,
the LTC6246 and the LTC6247 in MS10
offer a shutdown pin, which disables the
amplifier and reduces current consumption to 42µA. The LTC6246 family can be
used as a plug-in replacement for many
commercially available op amps. n
(LTC3890 continued from page 33)
supply noise are minimized by operating the two output stages out-of-phase.
Less than 10% mismatch can be
achieved, as shown in Figure 5.
DUAL OUTPUT APPLICATION
CONCLUSION
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).
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. n
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 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.
Current mode control provides good
current balance between the phases.
IL1, IL2
1A/DIV
0A
1µs/DIV
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.
July 2010 : LT Journal of Analog Innovation | 35
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