December 2009 - Dual Output uModule DC/DC Regulator Produces High Efficiency 4A Outputs from a 4.5V to 26.5V Input

DESIGN IDEAS L
Dual Output µModule DC/DC
Regulator Produces High Efficiency
4A Outputs from a 4.5V to 26.5V Input
by Alan Chern
Dual System-in-a-Package
Regulator
Systems and PC boards that use FPGAs and ASICs are often very densely
populated with components and ICs.
This dense real estate (especially the
supporting circuitry for FPGAs, such
as DC/DC regulators) puts a burden
on system designers who aim to simplify layout, improve performance and
reduce component count. A new family
of DC/DC µModule regulator systems
with multiple outputs is designed
to dramatically reduce the number
of components and their associated
costs. These regulators are designed
to eliminate layout errors and to offer
a ready-made complete solution. Only
a few external components are needed
since the switching controllers, power
MOSFETs, inductors, compensation
and other support components are all
integrated within the compact surface
mount 15mm × 15mm × 2.82mm
LGA package. Such easy layout saves
board space and design time by implementing high density point-of-load
regulators.
The LTM4619 switching DC/DC
µModule converter regulates two 4A
outputs from a single wide 4.5V to
26.5V input voltage range. Each out-
put can be set between 0.8V and 5V
with a single resistor. In fact, only a
few components are needed to build a
complete circuit (see Figure 2).
Figure 2 shows the LTM4619
µModule regulator in an application
with 3.3V and 1.2V outputs. The out4.5V TO
26.5V
VOUT1
1.2V/4A
LTM4619
VOUT1
100µF
TK/SS1
FREQ/
PLLFLTR
VFB2
COMP2
RSET2
19.1k
22pF
VOUT2
100µF
TK/SS2
RUN1
RUN2
SGND
PGND
VOUT2
3.3V/4A
0.1µF
EXTVCC
24VIN
12VIN
POWER LOSS (W)
6VIN
2.5
2.0
1.5
1.0
65
60
COMP1
3.0
70
INTVCC
Figure 2. µModule regulator converts a 4.5V to 26.5V input to dual 3.3V and 1.2V outputs, each
with 4A maximum output current.
3.5
75
continued on page 35
VFB1
PGOOD
90
80
MODE/
PLLIN
VIN
RSET1
121k
0.1µF
4.0
6VIN
12VIN
24VIN
10µF
s2
22pF
95
85
EFFICIENCY (%)
Figure 1. The LTM4619 LGA package is only
15mm × 15mm × 2.82mm, yet it houses dual
DC/DC switching circuitry, inductors,
MOSFETs and support components.
put voltages can be adjusted with a
value change in RSET1 and RSET2.
Thus, the final design requires nothing
more than a few resistors and capacitors. Flexibility is achieved by pairing
outputs, allowing the regulator to form
different combinations such as single
input/dual independent outputs or
single input/parallel single output for
higher maximum current output.
The efficiency of the system design
for Figure 2 is shown in Figure 3 and
power loss is shown in Figure 4, both
at various input voltages. Efficiency at
light load operation can be improved
with selective pulse-skipping mode or
Burst Mode® operation by tying the
mode pin high or leaving it floating.
0.5
0
1
2
3
CURRENT (A)
4
5
Figure 3. Efficiency of the circuit in Figure 2
at different input voltage ranges for 3.3V and
1.2V outputs
Linear Technology Magazine • December 2009
0
0
1
2
3
CURRENT (A)
4
5
Figure 4. Power loss of the circuit in Figure 2
at different input voltages for 3.3V and 1.2V
outputs
Figure 5. Exceptional thermal performance
of a paralleled output LTM4619 µModule
regulator (12VIN with two channels paralleled
to 1.5VOUT at 8A load)
33
DESIGN IDEAS L
Battery Charger
The LTC3577/LTC3577-1 battery
charger can provide a charge current
up to 1.5A via VBUS or wall adapter
when available. The charger also has
an automatic recharge and a trickle
charge function. The battery charge/
no-charge status, plus the NTC status
can be read via the I2C bus. Since
Li-Ion/Polymer batteries quickly lose
capacity when both hot and fully
charged, the LTC3577/LTC3577-1
reduces the battery voltage when the
battery heats up, extending battery
life and improving safety.
Three Bucks, Two LDOs
and a Boost/LED Driver
VOUT
2.7V TO 5.5V
ILED_FS
LTC3577/
LTC3577-1
C1
22µF
6.3V
L2
47nH
IFC-0805-47
L1
10µH
LPS4018-103M
D2
BAT54S
C4
10µF
25V
C2
10µF
25V
SW
D1
BAT54S
C3
10µF
25V
ILED
R1
301k
VBOOST
–12V
35mA
VBOOST
12V
35mA
R2
21.5k
LED_OV
DN470 F02
Figure 2. Dual polarity boost converter
A patented switching slew rate control
feature, set via the I2C interface, allows
the reduction of EMI noise in exchange
for efficiency.
The LTC3577/LTC3577-1 LED
boost driver can be used to drive up
to 10 series white LEDs at up to 25mA
or be configured as a constant voltage
boost converter. As a LED driver, the
current is controlled by a 6-bit, 60dB
logarithmic DAC, which can be further
reduced via internal PWM control. The
LED current smoothly ramps up and
down at one of four different rates.
Overvoltage protection prevents the
internal power transistor from damage if an open circuit fault occurs.
Alternatively, the LED boost driver can
be configured as a fixed voltage boost,
providing up to 0.75W at 36V.
Many circuits require a dual polarity
voltage to bias op amps or other analog
devices. A simple charge pump circuit,
as shown in Figure 2, can be added
to the boost converter switch node to
provide a dual polarity supply. Two
forward diodes are used to account
for the two diode voltage drops in the
inverting charge pump circuit and
provide the best cross-regulation.
For circuits where cross-regulation is
not important, or with relatively light
negative loads, using a single forward
diode for the boost circuit provides the
best efficiency.
Multiphase Operation
Thermal Performance
Conclusion
For a 4-phase, 4-rail output voltage system, use two LTM4619s and
drive their MODE_PLLIN pins with a
LTC6908-2 oscillator, such that the
two µModule devices are synchronized
90° out of phase. Reference Figure 21 in
the LTM4619 data sheet. Synchronization also lowers voltage ripple, reducing the need for high voltage capacitors
whose bulk size consumes board
space. The design delivers four different output voltage rails (5V, 3.3V,
2.5V and 1.8V) all with 4A maximum
load.
Exceptional thermal performance
is shown in Figure 5 where the
unit is operating in parallel output mode; single 12VIN to a single
1.5VOUT at 8A. Both outputs tied
together create a combined output
current of 8A with both channels
running at full load (4A each). Heat
dissipation is even and minimal, yielding good thermal results. If additional
cooling is needed, add a heat sink on
top of the part or use a metal chassis
to draw heat away.
The LTM4619 dual output µModule
regulator makes it easy to convert
a wide input voltage range (4.5V to
26.5V) to two or more 4A output
voltage rails (0.8V to 5V) with high
efficiency and good thermal dissipation. Simplicity and performance are
achieved through dual output voltage
regulation from a single package,
making the LTM4619 an easy choice
for system designs needing multiple
voltage rails. L
The LTC3577/LTC3577-1 contains
five resistor-adjustable step-down
regulators: two bucks, which can provide up to 500mA each, a third buck,
which can provide up to 800mA, and
two LDO regulators, which provide
up to 150mA each and are enabled
via the I2C interface. Individual LDO
supply inputs allow the regulators to be
connected to low voltage buck regulator outputs to improve efficiency. All
regulators are capable of low voltage
operation, adjustable down to 0.8V.
The three buck regulators are sequenced at power up (VOUT1, VOUT2
then VOUT3) via the pushbutton controller or via a static input pin. Each
buck can be individually selected to
run in Burst Mode operation to optimize efficiency or pulse-skipping mode
for lower output ripple at light loads.
Conclusion
The high level of integration of the
LTC3577/LTC3577-1 reduces the
number of components, required
board real estate and overall cost of
power systems for portable electronics.
It greatly simplifies power path design
with built-in solutions to a number of
complex power flow logic and control
problems. L
LTM4619, continued from page 33
Linear Technology Magazine • December 2009
35
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