LT3471 Dual 1.3A, 1.2MHz Boost/Inverter in 3mm x 3mm DFN

Features

• 1.2MHz Switching Frequency
• Low V_CESAT Switches: 330mV at 1.3A
• High Output Voltage: Up to 40V
• Wide Input Range: 2.4V to 16V
• Inverting Capability
• 5V at 630mA from 3.3V Input
• 12V at 320mA from 5V Input
• -12V at 200mA from 5V Input
• Uses Tiny Surface Mount Components
• Low Shutdown Current: < 1µA
• Low Profile (0.75mm) 10-Lead 3mm × 3mm DFN Package

Applications

• Organic LED Power Supply
• Digital Cameras
• White LED Power Supply
• Cellular Phones
• Medical Diagnostic Equipment
• Local ±5V or ±12V Supply
• TFT-LCD Bias Supply
• xDSL Power Supply

Description

The LT3471 dual switching regulator combines two 42V, 1.3A switches with error amplifiers that can sense to ground, providing boost and inverting capability. The low V_CESAT bipolar switches enable the device to deliver high current outputs in a small footprint. The LT3471 switches at 1.2MHz, allowing the use of tiny, low cost and low profile inductors and capacitors. High inrush current at start-up is eliminated using the programmable soft-start function, where an external RC sets the current ramp rate. A constant frequency current mode PWM architecture results in low, predictable output noise that is easy to filter.

The LT3471 switches are rated at 42V, making the device ideal for boost converters up to ±40V, as well as SEPIC and flyback designs. Each channel can generate 5V at up to 630mA from a 3.3V supply, or 5V at 510mA from four alkaline cells in a SEPIC design. The device can be configured as two boosts, a boost and inverter, or two inverters.

The LT3471 is available in a low profile (0.75mm) 10-lead 3mm x 3mm DFN package.

Typical Application

OLED Driver

A typical application circuit for an OLED driver is shown. This circuit utilizes the LT3471 to generate a positive output voltage (V_OUT1 = 7V) and a negative output voltage (V_OUT2 = -7V).

OLED Driver Efficiency

The graph shows the efficiency of the OLED driver circuit across various output currents and input voltages. For V_OUT1 = 7V, the efficiency is high, reaching over 90% at moderate currents. For V_OUT2 = -7V, the efficiency is also good, though slightly lower.

Absolute Maximum Ratings

Pin Configuration

The LT3471 is available in a 10-lead plastic DFN package. The pinout is as follows:

• Pin 1 (FB1N): Negative Feedback Pin for Switcher 1.
• Pin 2 (FB1P): Positive Feedback Pin for Switcher 1.
• Pin 3 (VREF): 1.00V Reference Pin.
• Pin 4 (FB2P): Positive Feedback Pin for Switcher 2.
• Pin 5 (FB2N): Negative Feedback Pin for Switcher 2.
• Pin 6 (SW2): Switch Pin for Switcher 2.
• Pin 7 (SHDN/SS2): Shutdown and Soft-Start Pin for Switcher 2.
• Pin 8 (VIN): Input Supply.
• Pin 9 (SHDN/SS1): Shutdown and Soft-Start Pin for Switcher 1.
• Pin 10 (SW1): Switch Pin for Switcher 1.
• Pin 11 (Exposed Pad): Ground. Connect directly to a local ground plane.

Order Information

Consult LTC Marketing for parts specified with wider operating temperature ranges.

Electrical Characteristics

The following table details the electrical characteristics of the LT3471. Specifications apply over the full operating temperature range unless otherwise noted. T_A = 25°C.

Typical Performance Characteristics

The following graphs illustrate the typical performance characteristics of the LT3471:

• Quiescent Current vs Temperature: Shows quiescent current variation with temperature.
• V_REF Voltage vs Temperature: Displays the reference voltage stability over temperature.
• V_REF Voltage vs V_REF Current: Illustrates the relationship between the reference voltage and the current drawn from it.
• SHDN/SS Current vs SHDN/SS Voltage: Depicts the current drawn by the shutdown/soft-start pin as a function of its voltage.
• Current Limit vs Duty Cycle: Shows how the current limit varies with the duty cycle.
• Switch Saturation Voltage vs Switch Current: Illustrates the V_CESAT of the internal switches as a function of the switch current at different temperatures.

Block Diagram

The block diagram shows the internal architecture of the LT3471, including the two switching regulators, error amplifiers, ramp generators, and drivers.

Operation

The LT3471 employs a constant frequency, current mode control scheme for excellent line and load regulation. Each oscillator cycle begins with the SR latch setting, turning on the power switch (Q1/Q2). The switch current, proportional to a stabilizing ramp, is fed into the PWM comparator (A2/A4). When this exceeds the level at the negative input of A2/A4, the SR latch resets, turning off the power switch. The error amplifier (A1/A3) sets the peak current level to maintain output regulation. Enabling the part is achieved by raising either SHDN/SS pin above 1.8V, while grounding both pins disables the device. The soft-start feature limits inrush current by controlling the voltage rise at the output of the error amplifiers, thereby limiting peak switching current. This is achieved by slowly ramping the SHDN/SS pin voltage using an external RC network.

Applications Information

Soft-Start and Shutdown Features

To shut down the part, ground both SHDN/SS pins. To shut down one switcher but not the other, ground that switcher's SHDN/SS pin. The soft-start feature provides a way to limit the inrush current drawn from the supply upon start-up. To use the soft-start feature for either switcher, slowly ramp up that switcher's SHDN/SS pin. The rate of voltage rise at the switcher's comparator (A1 or A3) tracks the rate of voltage rise at the SHDN/SS pin once the SHDN/SS pin has reached about 1.1V. The soft-start function will go away once the voltage at the SHDN/SS pin exceeds 1.8V. The rate of voltage rise at the SHDN/SS pin can be controlled with a simple RC network connected between the control signal and the SHDN/SS pin. Typical values for the RC network are 4.7kΩ and 0.33µF, giving start-up times on the order of milliseconds. This RC time constant can be adjusted to give different start-up times.

Capacitor Selection

Low ESR (equivalent series resistance) capacitors should be used at the output to minimize the output ripple voltage. Multi-layer ceramic capacitors are an excellent choice, as they have extremely low ESR and are available in very small packages. X5R dielectrics are preferred, followed by X7R, as these materials retain the capacitance over wide voltage and temperature ranges. A 4.7µF to 15µF output capacitor is sufficient for most applications, but systems with very low output currents may need only a 1µF or 2.2µF output capacitor. Solid tantalum or OS-CON capacitors can be used, but they will occupy more board area than a ceramic and will have a higher ESR. Always use a capacitor with a sufficient voltage rating.

Ceramic capacitors also make a good choice for the input decoupling capacitor, which should be placed as close as possible to the LT3471. A 4.7µF to 10µF input capacitor is sufficient for most applications. Table 2 shows a list of several ceramic capacitor manufacturers.

Diode Selection

A Schottky diode is recommended for use with the LT3471. For high efficiency, a diode with good thermal characteristics at high currents should be used such as the On Semiconductor MBRM120. This is a 20V diode. Where the switch voltage exceeds 20V, use the MBRM140, a 40V diode. These diodes are rated to handle an average forward current of 1.0A. In applications where the average forward current of the diode is less than 0.5A, use the Philips PMEG 2005, 3005, or 4005 (a 20V, 30V or 40V diode, respectively).

Layout Hints

The high speed operation of the LT3471 demands careful attention to board layout. Figure 5 shows the recommended component placement.

Compensation Theory

Like all other current mode switching regulators, the LT3471 needs to be compensated for stable and efficient operation. Two feedback loops are used in the LT3471: a fast current loop which does not require compensation, and a slower voltage loop which does. Standard Bode plot analysis can be used to understand and adjust the voltage feedback loop.

Typical Applications

Li-Ion OLED Driver

This circuit shows a typical application for driving an OLED display using the LT3471. It generates both positive and negative output voltages required for OLED operation.

Li-Ion 20 White LED Driver

This circuit demonstrates the LT3471 used to drive a string of 20 white LEDs from a Li-Ion battery.

Li-Ion or 4-Cell Alkaline to 3.3V and 5V SEPIC

This application shows how the LT3471 can be configured as a SEPIC converter to provide 3.3V and 5V outputs from a Li-Ion or 4-cell alkaline battery.

5V to ±12V Dual Supply Boost/Inverting Converter

This circuit shows a dual supply configuration, generating both a positive 12V and a negative -12V output from a 5V input.

Package Description

The LT3471 is supplied in a 10-lead plastic DFN package with dimensions of 3mm x 3mm.

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