Download DN500 - Wide Input Voltage Range Boost/Inverting/SEPIC

Wide Input Voltage Range Boost/Inverting/SEPIC Controller
Works Down to an Input Voltage of 1.6V
Design Note 500
Zhongming Ye
Introduction
Many of today’s electronic devices require an inverting
or noninverting converter or sometimes both. They also
need to operate from a variety of power sources including USB, wall adapters, alkaline and lithium batteries.
To produce various polarity outputs from variable input
voltages, power supply designers often use a variety
of regulator ICs, which makes for a long inventory list.
The LT®3759 operates over an input voltage range from
1.6V to 42V and controls either positive or negative
outputs using the same feedback pin, thus shortening
the inventory list and simplifying design. It also packs
many popular features such as soft-start, adjustable
frequency and synchronization into a small footprint.
The LT3759 comes in a 5mm × 4mm 12-pin MSE package and can be used in multiple configurations such as
boost, SEPIC, flyback and Cuk topologies.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
Wide Input Voltage Range with Internal LDO
The LT3759’s wide input range simplifies the design of
power supplies that must be compatible with a wide array
of power sources. There is no need to add an external
regulator or a slow-charge hysteretic start scheme
because the LT3759 includes two internal low dropout
(LDO) voltage regulators powered from VIN and DRIVE
respectively, allowing simple start-up and biasing. The
LT3759’s internal INTVCC current limit function protects
the IC from excessive on-chip power dissipation.
Sensing Output Voltage Made Easier
LT3759 features a novel FBX pin architecture that simplifies the design of inverting and noninverting converters.
It contains two internal error amplifiers—one senses
positive outputs and the other negative—allowing the
FBX pin to be connected directly to a divider from either
a positive output or a negative output, eliminating any
confusion associated with positive or negative output
sensing and simplifying the board layout. Simply decide the output polarity and topology and the LT3759
does the rest.
100
VIN
2.5V TO 15V
VIN
PGOOD
EN_UVLO
887k
LT3759
SYNC
D1
VOUT
12V, 2.5A
M1
1.8mΩ
0.5W
IL2
L2
102k
1%
COUT1
47µF
×4
FBX
GND INTVCC
VC
0.1µF
GATE
0.02Ω
SS
80
CDC
47µF ×4
16V
VSW
SENSE
RT
27.4k
300kHz
IL1
100k L1
DRIVE
1.4k
22nF
4.7µF
10V
X5R
15.8k
1%
DN500 F01
M1: BSC027N04LS
L1, L2: XAL-1010-682-EMB
D1: PMEG4030
Figure 1. SEPIC Converter Produces 12V Output from 2.5V to 15V Inputs
EFFICIENCY (%)
348k
CIN
22µF
×4
03/12/500
VIN = 12V
90
70
60
50
40
30
20
10
0
0
0.5
1.5
1
LOAD CURRENT (A)
2
2.5
DN500 F02
Figure 2. Efficiency for the
Converter in Figure 1
Adjustable/Synchronizable Switching Frequency
It is often necessary to operate a converter at a particular frequency, especially if the converter is used
in an RF communications product that is sensitive to
spectral noise in certain frequency bands. Also, if the
area available for a converter is limited, operating at
higher frequencies allows the use of smaller component sizes, reducing the real estate required and the
output ripple. If power loss is a concern, switching at
a lower frequency reduces switching losses, improving
efficiency. The switching frequency can be set from
100kHz to 1MHz via a single resistor from the RT pin
to ground. The device can also be synchronized to an
external clock via the SYNC pin.
Precision UVLO and Soft-Start
Input supply undervoltage (UVLO) for sequencing or
start-up overcurrent protection is easily achieved by
driving the UVLO with a resistor divider from the VIN
supply. The divider output produces 1.22V at the UVLO
pin when VIN is at the desired UVLO rising threshold
voltage. The UVLO pin has an adjustable input hysteresis,
which allows the IC to ignore a settable input supply
droop before disabling the converter. During a UVLO
event, the IC is disabled and VIN quiescent current
drops to 1µA or lower.
The SS pin provides access to the soft-start feature,
which reduces the peak input current and prevents
output voltage overshoot during start-up or recovery
from a fault condition. The SS pin reduces the inrush
current by lowering the switch peak current. In this
way soft-start allows the output capacitor to charge
gradually toward its final value.
VIN
1.8V TO 4.5V
A 2.5V to 15V to 12V SEPIC Converter
Figure 1 shows a 2.5V to 15V input, 12V/2.5A output
SEPIC power supply using the LT3759. The typical efficiency for this converter is shown in Figure 2. Figure 3
shows the switch waveform during an output shortcircuit event. Notice how the switching frequency folds
back to one-third of the regular frequency as soon as
the output voltage is shorted to ground. This feature
enhances short-circuit performance of both Cuk and
SEPIC converters.
A 1.8V to 4.5V to 5V/2A Boost Converter
Figure 4 shows a 5V, 2A output converter that takes an
input of 4.5V to as low as 1.8V. The LT3759 is configured as a boost converter for this application, where
the converter output voltage is higher than the input
voltage. The 500kHz operating frequency allows the
use of small inductor and output capacitors.
Conclusion
The LT3759 is a versatile IC that integrates a rich set
of unique features in a tiny 5mm × 4mm 12-pin MSE
package. It accepts wide input voltage range from
1.6V to 42V, with low shutdown current and frequency
foldback at output short-circuit. The LT3759 is ideal
for wide input voltage applications from single-cell,
lithium-ion powered systems to automotive, industrial
and telecommunications power supplies. The high level
of integration yields a simple, low parts-count solution
for boost, SEPIC and Inverting converters.
CIN
47µF
6.3V
X5R
R3
59k
VIN
SYNC
IL1 + IL2
10A/DIV
LT3759
SS
RT
16.9k
500kHz
VOUT
10V/DIV
20µs/DIV
DN500 F03
Figure 3. Short-Circuit Event
for the Converter in Figure 1
CSS
0.1µF
GATE
M1
VOUT
5V
2A
R2
34k
1%
FBX
GND INTVCC
VC
CC2
100pF
D1
SENSE
RT
VSW
20V/DIV
L1
2.2µH
PGOOD
EN_UVLO
R4
124k
R5
10k
DRIVE
RC
7.5k
CC1
22nF
CVCC
4.7µF
10V
X5R
RS
5mΩ
0.5W
M1: Si7858ADP
L1: WÜRTH ELEKTRONIK 7443552200
D1: SBR3U20SA
R1
15.8k
1%
COUT2
100µF
6.3V
X5R
×2
DN500 F04
Figure 4. Boost Converter Produces 5V/2A
Output from 1.8V to 4.5V Input
Data Sheet Download
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Linear Technology Corporation
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call (408) 432-1900, Ext. 3798
dn500fa LT/AP 0312 196K • PRINTED IN THE USA
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