DN357 - Tiny, Highly Flexible, Dual Boost/Inverter Tracks Supplies

Tiny, Highly Flexible, Dual Boost/Inverter Tracks Supplies
Design Note 357
Jeffrey Huang
Introduction
Linear Technology’s LT®3471 is a space saving and
extremely versatile dual switching regulator available
in a tiny 3mm × 3mm DFN package. With a 2.4V to
16V input range, the device can easily implement any
combination of boost, inverting and SEPIC topologies
to generate two positive and/or negative outputs up to
±40V. It can readily make the two output voltages track
each other with minimal additional circuitry, or it can
independently regulate the outputs, thus saving space
by replacing two separate ICs. Either way, the LT3471
fits into tight spaces and is easily adaptable for a wide
range of applications.
LT3471 Features
The LT3471 combines two independent, in-phase, 1.3A,
42V switches with error amplifiers that can sense to
ground, providing boost and inverting capabilities.
Both inputs of each error amplifier are available as high
impedance feedback pins with a typical bias current
of only 60nA. An accurate 1.00V reference is also pin
accessible.
The LT3471 switches at a fixed 1.2MHz which allows for
the use of tiny, low profile inductors and capacitors. Its
internally compensated, current mode PWM architecture yields low, predictable output noise that is easy to
filter. Additionally, the device provides a programmable
soft-start function that requires only a simple external
RC circuit to mitigate high inrush current.
Easy-to-Implement ±15V Dual Tracking Supplies
Figure 1 shows a Li-Ion to ±15V dual tracking supply
application, highlighting the versatility of the LT3471
by taking advantage of the accessible error amplifier
inputs and reference voltage. The circuit generates
well-regulated ±15V outputs that track each other with
switchers 1 and 2 in boost and inverting configurations,
respectively.
The two converters can talk to each other through their
shared external feedback resistor network, specifically
R1. When regulating, the control loop of switcher 1 servos
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of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
L1
15µH
D1
VIN
VIN
2.8V TO 4.2V
CONTROL 2
1.8V
OV
8
C1
RSS 10µF
10k
SW1
VIN
FB1N
FB1P
9
C1
3.3µF
VREF
SHDN/SS2
GND
C1, C2: X5R OR X7R 6.3V
C3, C4: X5R OR X7R 16V
C5: X5R OR X7R 25V
D1, D2: ON SEMICONDUCTOR MBRM-120
L1: SUMIDA CR43-150
L2, L3: SUMIDA CLS62-100
SW2
11
VIN
FB2N
2
3
C2
0.1µF
C5
10µF
R1
1.58k
4
5
R3
23.7k
6
L2
10µH
L3
D2 10µH
Figure 1. Li-Ion to ±15V Dual Tracking Supplies
05/16/357
C3
22µF
1
SHDN/SS1 LT3471
FB2P
7
CPL1
2200pF
R2
22.1k
10
VOUT1
15V
100mA
CPL2
500pF
C4
22µF
VOUT2
–15V
100mA
DN357 F01
FB1N to 1V while that of switcher 2 servos FB2P to ground.
Selecting the value of R2 to be (14 • R1) and R3 to be
(15 • R1) produces ±15V outputs. In this way, the magnitudes of VOUT1 and VOUT2 track each other; that is,
a change in the +15V output causes a similar change
in the –15V output, and vice versa. Figure 2 shows the
circuit efficiency.
85
VIN = 3.6V
80
and 5 show the effect of soft-start on the output voltages (top two traces) and input current (bottom trace).
Figure 4 depicts the start-up waveforms of the circuit
without soft-start (CSS removed). The output voltages
reach ±15V very quickly but the inrush current sharply
peaks above 2A. With soft-start, as shown in Figure 5,
the input current peaks slightly to about 500mA, while
the output voltages comfortably rise and settle to their
steady-state values. In both cases, VOUT1 and VOUT2
track each other.
EFFICIENCY (%)
75
VOUT1
10V/DIV
70
65
VOUT2
10V/DIV
60
55
50
IIN
1A/DIV
0
80
60
100 120 140
20
40
LOAD CURRENT FOR EACH OUTPUT (mA)
DN357 F02
Figure 2. Efficiency of the Circuit in Figure 1
The interaction between the two converters may compromise the stability of each. To prevent this, phase lead
capacitors CPL1 and CPL2 improve the phase margin of
both switchers. Figure 3 shows the transient response
of the circuit under a constant 50mA load on VOUT1 and
50mA load step on VOUT2. The upper waveform depicts
the 15V output tracking the clean, damped response
of the –15V output, the middle waveform. The output
voltages remain within 0.3% of their nominal values
during the step transient. VOUT2 similarly tracks VOUT1
under a load step on switcher 1 and constant load on
switcher 2.
VOUT1
50mV/DIV
AC COUPLED
VOUT2
50mV/DIV
AC COUPLED
SW2 LOAD
CURRENT
50mA/DIV
0.5ms/DIV
DN357 F03
Figure 3. Transient Response of the Circuit in Figure 1
Both switchers implement a programmable soft-start
feature. The RC circuit at the SS1 and SS2 pins in
Figure 1 sets the input current ramp rate. Figures 4
Data Sheet Download
www.linear.com/LT3471
Linear Technology Corporation
5ms/DIV
DN357 F04
Figure 4. Without Soft-Start (CSS Removed in Figure 1),
the Output Voltages Quickly Reach Regulation, but the
LT3471 Draws Very High Inrush Current
VOUT1
10V/DIV
VOUT2
10V/DIV
IIN
1A/DIV
5ms/DIV
DN357 F05
Figure 5. With Soft-Start Enabled (via the RSS CSS
Network in Figure 1), the Output Voltages Slowly Reach
Regulation, While the Controlled Inrush Current Safely
Ramps Up with Little Peaking
Conclusion
The LT3471 is a tiny, dual switching regulator easily
configurable for a broad array of applications, including
organic and white LED drivers, TFT-LCD bias supplies
and other portable device and medical diagnostic
equipment functions. A flexible pinout makes it simple
to generate any combination of two positive and/or
negative outputs. The LT3471’s small footprint and
versatile, 2-switcher capability make it a great fit in a
variety of power management solutions.
For applications help,
call (408) 432-1900
dn357fa LT/TP 0516 REV A 305K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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 LINEAR TECHNOLOGY CORPORATION 2005
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