DN469 - Triple Output DC/DC uModule® Regulator in 15mm × 15mm × 2.8mm Surface Mount Package Replaces Up to 30 Discrete Components

Triple Output DC/DC µModule® Regulator in 15mm × 15mm ×
2.8mm Surface Mount Package Replaces Up to 30 Discrete
Components Design Note 469
Eddie Beville and Alan Chern
Introduction
When space and design-time are tight in multivoltage
systems, the solution is a multioutput DC/DC regulator
IC. For more space and time constraint systems, a better
solution is an already-fabricated compact multioutput
DC/DC system that includes not only the regulator ICs
but the supporting components such as the inductors,
compensation circuits, capacitors and resistors.
Dual Switching 4A and 1.5A VLDO™ Regulators
The LTM ®4615 offers three separate power supply
regulators in a 15mm × 15mm × 2.8mm LGA surface
mount package: two switching DC/DC regulators and
one very low dropout VLDO linear regulator (Figure 1).
MOSFETs, inductors, and other support components are
all built in. Each power supply can be powered individually
or together, to form a single input, three output design.
Moreover, for an otherwise complex triple output circuit
design, the task is eased to designing with only one device
while the layout is as simple as copying and pasting the
LTM4615’s package layout. One LTM4615 replaces up to
30 discrete components when compared to a triple-output
high efficiency DC/DC circuit.
The two switching regulators, operating at a 1.25MHz
switching frequency, accept input voltages between 2.35V
to 5.5V and each delivers a resistor-set output voltage
of 0.8V to 5V at 4A of continuous current (5A peak). The
output voltages can track each other or another voltage
source. Other features include, low output voltage ripple
and low thermal dissipation.
The VLDO regulator input voltage (1.14V to 3.5V) is capable of up to 1.5A of output current with an adjustable
output range of 0.4V to 2.6V, also via a resistor. The VLDO
regulator has a low voltage dropout of 200mV at maximum load. The regulator can be used independently, or
in conjunction with either of the two switching regulators
to create a high efficiency, low noise, large-ratio stepdown supply—simply tie one of the switching regulator’s
outputs to the input of the VLDO regulator.
Multiple Low Noise Outputs
The LTM4615 is capable of operating with all three regulators at full load while maintaining optimum efficiency.
A typical LTM4615 design (Figure 2) for a 3.3V input to
three outputs has the VLDO input driven by VOUT2. The
efficiency of this design is shown in Figure 3.
L, LT, LTC, LTM, Linear Technology, the Linear logo and μModule are registered
trademarks, and VLDO is a trademark of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
VIN
3.3V
+
C2
82μF
VOUT1
1.8V, 4A
C7
22μF
C1
10μF
s2
C6
100μF
VIN1
R2
4.02k
VIN
VOUT2
C3
10μF
PGOOD1
PGOOD2
VOUT1
VOUT2
FB1
FB2
LTM4615
COMP1
COMP2
TRACK1
TRACK2
RUN/SS1
RUN/SS2
LDO_IN
LDO_OUT
EN3
FB3
BOOST3
PGOOD3
GND1
Figure 1. Three DC/DC Circuits in One Package
08/09/469
GND2 GND3
VOUT2
1.2V, 4A
C5
100μF
C4
22μF
VIN2
R4
10k
VIN
R1
3.32k
VOUT3
1V, 1.5A
C8
4.7μF
DN469 F02
Figure 2. Triple Output LTM4615: 3.3V Input,
1.8V (4A), 1.2V (4A), 1.0V (1.5A)
100
3.3VIN TO 1.8VOUT
90
80
3.3VIN TO 1.2VOUT
EFFICIENCY (%)
70
1.2VIN TO 1.0VOUT
60
1.8V
1.6mVP-P
1.2V
2.0mVP-P
1.0V
1.4mVP-P
50
40
30
20
10
0
0
1
3
2
OUTPUT CURRENT (A)
4
DN469 F04
0.5μs/DIV
DN469 F03
Figure 3. Efficiency of the Circuit in Figure 2,
1.8V, 1.2V and 1.0V (VLDO)
Figure 4. Low Output Voltage Ripple (3.3V Input)
VIN
2.5V to
5.5V
+
C2
82μF
VOUT1
1.8V,
4A
C7
22μF
C1
10μF
s2
C6
100μF
R2
4.02k
VIN
VOUT2
C3
10μF
VIN1
VIN2
PGOOD1
PGOOD2
VOUT1
VOUT2
LTM4615
FB1
FB2
COMP1
COMP2
TRACK1
TRACK2
RUN/SS1
RUN/SS2
LDO_IN
LDO_OUT
EN3
FB3
BOOST3
PGOOD3
GND1 GND2
Figure 5. Top View Thermal Imaging of the Unit at Full
Load in Ambient Temperature with No Airflow. Even
Temperatures (Cursors 1 and 3) Indicate Balanced
Thermal Conductivity Between the Two Switching
Regulators. 3.3V Input, 1.8V (4A) and 1.2V (4A).
The LTM4615 comes prepackaged with ceramic capacitors and additional output capacitors are only needed
under full 4A load and if the input source impedance is
compromised by long inductive leads or traces.
The VLDO regulator provides a particularly low noise 1.0V
supply as it is driven by the output of the 1.2V switching
regulator (VOUT2). The low output voltage ripple for all
three outputs is shown in Figure 4.
Thermally Enhanced Packaging
The LGA packaging allows heatsinking from both the top
and bottom. This design utilizes the PCB copper layout
to draw heat away from the part and into the board. Additionally, a heat sink can be placed on top of the device,
such as a metal chassis, to promote thermal conductivity.
Thermal dissipation is well balanced between the two
switching regulators (Figure 5).
Data Sheet Download
www.linear.com
VOUT2
C5 1.2V, 4A
100μF
C4
22μF
GND3
R5
R4
4.99k
VOUT1
10k
VIN
VOUT3
1V,
1.5A
R1
R6
10k
3.32k
DN469 F06
Figure 6. Output Voltage Tracking Design
VOUT2 (1.2V) Tracks VOUT1 (1.8V)
Output Voltage Tracking
A tracking design (Figure 6) and output (Figure 7) can be
programmed using the TRACK1 and TRACK2 pins. Divide
down the master regulator’s output with an external resistor
divider that is the same as the slave regulator’s feedback
divider on the slave’s TRACK pin for coincident tracking.
1.8VOUT1
1.2VOUT2
0.5V/DIV
DN469 F07
Figure 7. Start-Up Voltage for Figure 5 Circuit VOUT1 (1.8V)
Coincidentally Tracks VOUT2 (1.2V) for Coincident Tracking
For applications help,
call (408) 432-1900, Ext. 2593
Linear Technology Corporation
dn469f LT/TP 0809 116K • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2009
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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