Space-Efficient 12V Boost DC-DC Regulator using LM2735X

This is a Step-up DC-DC converter that converts 3V to 5.5V input to 12V output for up to 200mA load current. The project consists of LM2735X 1.6MHz DC-DC switching converter.

Features

• Input Supply 3V to 5.5V
• Output 12V DC
• Load Current Up to 200mA
• Operating Frequency 1.6Mhz
• Internal Soft Start
• Internal Compensation
• Current-Mode Control
• 4Pin Header Connector for Input and Output
• PCB Dimensions 21.75 x 11.91mm

The LM2735 device is an easy-to-use, space-efficient 2.1-A low-side switch regulator, ideal for Boost and SEPIC DC/DC regulation. The device provides all the active functions to provide local DC-DC conversion with fast-transient response and accurate regulation in the smallest PCB area. Switching frequency is internally set to either 520 kHz or 1.6 MHz, allowing the use of extremely small surface mount inductor and chip capacitors, while providing efficiencies of up to 90%. Current-mode control and internal compensation provide ease-of-use, minimal component count, and high-performance regulation over a wide range of operating conditions. External shutdown features an ultra-low standby current of 80 nA, ideal for portable applications. Tiny SOT-23, WSON, and MSOP-PowerPAD packages provide space savings. Additional features include internal soft start, circuitry to reduce inrush current, pulse-by-pulse current limit, and thermal shutdown.

Current Limit

SW GND The LM2735 uses cycle-by-cycle current limiting to protect the internal NMOS switch. It is important to note that this current limit will not protect the output from excessive current during an output short circuit. The input supply is connected to the output by the series connection of an inductor and a diode. If a short circuit is placed on the output, excessive current can damage both the inductor and diode.

Thermal Shutdown

Thermal shutdown limits total power dissipation by turning off the output switch when the IC junction temperature exceeds 160°C. After thermal shutdown occurs, the output switch does not turn on until the junction temperature drops to approximately 150°C.

Soft Start

This function forces VOUT to increase at a controlled rate during start-up. During soft start, the reference voltage of the error amplifier ramps to its nominal value of 1.255 V in approximately 4 ms. This forces the regulator output to ramp up in a more linear and controlled fashion, which helps reduce inrush current.

Compensation

The LM2735 uses constant-frequency peak current mode control. This mode of control allows for a simple external compensation scheme that can be optimized for each application. A complicated mathematical analysis can be completed to fully explain the internal and external compensation of the LM2735, but for simplicity, a graphical approach with simple equations will be used. Below is a Gain and Phase plot of a LM2735 that produces a 12-V output from a 5-V input voltage. The Bode plot shows the total loop Gain and Phase without external compensation.

Connections

• CN1: Pin 1 = 3 To 5.5V DC Input, Pin 2 = GND
• CN2: Pin 1 = 12V Output, Pin 2 = GND

Schematic

Parts List

Connections

LM2735X Efficiency Graph

LM2735X Block Diagram

Gerber View

Photos

LM2735X Datasheet