of Linear Technology Corporation. All other trademarks are the property ... lithium-ion powered systems to automotive, i
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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