March 2009 - uModule Regulator Fits a (Nearly) Complete Buck-Boost Solution in 15mm × 15mm × 2.8mm for 4.5V–36V VIN to 0.8V–34V VOUT

DESIGN IDEAS L
µModule Regulator Fits a (Nearly)
Complete Buck-Boost Solution
in 15mm × 15mm × 2.8mm for
4.5V–36V VIN to 0.8V–34V VOUT
Linear Technology offers a number of
high efficiency synchronous 4-switch
buck-boost DC/DC converter solutions for applications where VOUT falls
within the range of VIN. The LTM4605,
LTM4607 and LTM4609 µModule
regulators are nearly self-contained
buck-boost solutions that share pincompatible 15mm × 15mm × 2.8mm
packages. The package includes the
controller, four power FETs and a
number of other discrete components.
Only an external inductor, a sensing
resistor, a voltage setting resistor and
a few input and output capacitors are
needed to complete a high efficiency
buck-boost converter.
Table 1 shows the input voltage,
output voltage and current specifications of these three buck-boost
µModule regulators. The LTM4609
is the latest addition to this family.
It satisfies the needs of high output
voltage applications with an output
range of 0.8V–34V.
by Judy Sun, Sam Young and Henry Zhang
VIN
10V TO 36V
OPTIONAL
CLOCK SYNC
4.7µF
×2
50V
ON/OFF
VIN
RUN
PLLIN V
OUT
FCB
LTM4609
+
150µF
×2
35V
VOUT
30V
3A
SW1
SW2
RSENSE
L1: SUMIDA CDEP147
SENSE+
10nF
7mΩ
SS
SGND
SENSE–
PGND
VFB
2.74k
Figure 1. Just a few components form a complete 10V to 36V input, 30V/3A output converter
using the LTM4609.
As with all Linear Technology µModule
regulators, the LTM4609 requires only
a few external components to complete
a wide input range buck-boost converter. Figure 1 shows a 10V to 36V
input, 30V output design. The output
current capability is 3A at 10V VIN,
and 8A with 36V input.
Figure 2 shows the efficiency of
this converter, up to 98% in buck
mode and 95% in boost mode. The
low profile LGA package features low
thermal resistance from junction to
pin, thus maintaining an acceptable
junction temperature even at high
output power. The LTM4609’s high
VIN = 36V, VOUT = 30V, IOUT = 8A
VIN = 24V, VOUT = 30V, IOUT = 6A
High Performance with
Minimum Component Count
10µF
×2
35V
L1
3.4µH
100
95
EFFICIENCY (%)
Introduction
36VIN
24VIN
10VIN
90
85
80
CONTINUOUS CURRENT MODE
VOUT = 30V
fSW = 275kHz
0
2
4
6
LOAD CURRENT (A)
8
10
Figure 2. Efficiency of the 30V
buck-boost converter
VIN = 12V, VOUT = 30V, IOUT = 3A
Figure 3. Thermal-graph taken with the LTM4609 running at different input voltages. The LTM4609 is on the left, the inductor (Sumida CDEP147)
is on the right. No heat sink or forced air flow. Ambient temperature = 25°C.
Linear Technology Magazine • March 2009
39
L DESIGN IDEAS
efficiency combined with its excellent
thermal management capability enables it to deliver up to 240W output
power without a heat sink or forced
airflow. Figure 3 shows the thermalgraphs taken with three different input
voltages and loads at 25°C ambient
temperature. With 240W output and
36V input, the maximum temperature
rise of the LTM4609 is only 52.8°C.
L1
VIN
L1,L2: FAIR-RITE 2518065007Y6
L2
CBULK
100µF
+
CIN1
10µF
CIN2
10µF
VIN
LTM4609
GND
Figure 4. The LTM4609 µModule regulator with an input π filter.
Input Ripple Reduction
One way to improve efficiency in a
switching DC/DC converter is to minimize the turn-on and turn-off times
of the MOSFET—shorter transitions
correspond to lower switch losses.
However, fast transitions also lead
to high frequency switching noise,
which can pollute the input power
source. For the applications where the
input voltage ripple must be limited,
a simple LC π filter can be inserted at
the input side to attenuate the high
frequency input voltage noise. Figure
4 shows the LTM4609 with an input
π filter. The filter includes two 10µF
low ESR ceramic capacitors and two
very small magnetic beads. For lower
output power applications, only one
magnetic bead is necessary.
Figure 5 shows the input ripple
reduction with the π filter. Figure 5a
shows the input ripple with 100µF
aluminum electrolytic plus 2 × 4.7µF
VIN
200mV/DIV
VIN
200mV/DIV
VIN = 10V
VOUT = 30V
IOUT = 3A
10µs/DIV
CBULK = 100µF
CIN1, CIN2 = 4.7µF
5a. Input voltage waveform without
the input π filter shown in Figure 4
ceramic input capacitors. Figure 5b
shows the input ripple with the filter
shown in Figure 4. Both waveforms
are measured across the 100µF aluminum capacitor. A 67% reduction in
input ripple is obtained with the input
π filter, which requires only two small
additional magnetic beads.
LTM4605
LTM4607
LTM4609
VIN
4.5V ~ 20V
4.5V ~ 36V
4.5V ~ 36V
VOUT
0.8V ~ 16V
0.8V ~ 24V
0.8V ~ 34V
IOUT
5A
(12A in buck mode)
5A
(10A in buck mode)
4A
(10A in buck mode)
LT3755/56, continued from page 32
15mm × 15mm × 2.8mm LGA
PWM on- and off-times are 1µs as
with the other circuits. Figure 7 shows
the waveforms during a short circuit
fault on the output. The input current remains in control as the switch
current ramps up to the set limit of
10A, then skips the next few cycles
while the current sensed by the LED
40
5b. Input voltage waveform with
input π filter as shown in Figure 4
Figure 5. The input π filter shown in Figure 4 effectively reduces
the input voltage spike caused by switching action of the MOSFETs.
Table 1. Specification comparison of the LTM4605, LTM4607 and LTM4609
Package
10µs/DIV
VIN = 10V
CBULK = 100µF
VOUT = 30V
CIN1, CIN2 = 10µF
IOUT = 3A
L1, L2: FAIR-RITE 2518065007Y6
resistor ramps down to 1.5A. This
faulted mode of circuit operation can
continue indefinitely without damage
to the components.
Conclusion
The LT3755 and LT3756 offer unparalleled performance for an LED
Conclusion
Buck-boost µModule regulators
are easy-to-use, high performance
solutions for applications where a
regulated output voltage sits within the
range of the input voltage. The 15mm
× 15mm × 2.8mm LTM4609 widens the
input/output voltage range of the pin
compatible LTM4605 and LTM4607.
The advanced package technology, as
well as the high efficiency design of
the LTM4609, allows it to deliver up
to 240W of output power without heat
sinks or forced airflow. For applications that require low input voltage
ripple, a simple π filter can be added
by inserting one or two small magnetic
beads to significantly reduce the high
frequency input noise. L
controller generating PWM pulse
widths as narrow as 1µs, which
enables 50:1 PWM dimming at frequencies above the audible range.
Other features include open LED
protection, an open LED status indicator, and programmability of the LED
current via an analog input. L
Linear Technology Magazine • March 2009