LM2735XMFSLASHNOPB datasheet(9/59 Pages) TI

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LM2735, LM2735-Q1

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SNVS485G – JUNE 2007 – REVISED AUGUST 2015

7 Detailed Description

7.1 Overview

The LM2735 device is highly efficient and easy-to-use switching regulator for boost and SEPIC applications. The

device provides regulated DC output with fast transient response. Device architecture (current mode control) and

internal compensation enable solutions with minimum number of external components. Additionally high

switching frequency allows for use of small external passive components (chip capacitors, SMD inductors) and

enables power solutions with very small PCB area. LM2735 also provides features such as soft start, pulse-by-

pulse current-limit, and thermal shutdown.

7.1.1 Theory of Operation

The LM2735 is a constant-frequency PWM boost regulator IC that delivers a minimum of 2.1 A peak switch

current. The regulator has a preset switching frequency of either 520 kHz or 1.6 MHz. This high frequency allows

the device to operate with small surface mount capacitors and inductors, resulting in a DC-DC converter that

requires a minimum amount of board space. The LM2735 is internally compensated, so it is simple to use, and

requires few external components. The device uses current-mode control to regulate the output voltage. The

following operating description of the LM2735 will refer to the simplified internal block diagram (Functional Block

Diagram), the simplified schematic (Figure 13), and its associated waveforms (Figure 14). The LM2735 supplies

a regulated output voltage by switching the internal NMOS control switch at constant frequency and variable duty

cycle. A switching cycle begins at the falling edge of the reset pulse generated by the internal oscillator. When

this pulse goes low, the output control logic turns on the internal NMOS control switch. During this on-time, the

SW pin voltage (VSW) decreases to approximately GND, and the inductor current (IL) increases with a linear

slope. IL is measured by the current sense amplifier, which generates an output proportional to the switch

current. The sensed signal is summed with the corrective ramp of the regulator and compared to the error

amplifier’s output, which is proportional to the difference between the feedback voltage and VREF. When the

PWM comparator output goes high, the output switch turns off until the next switching cycle begins. During the

switch off-time, inductor current discharges through diode D1, which forces the SW pin to swing to the output

voltage plus the forward voltage (VD) of the diode. The regulator loop adjusts the duty cycle (D) to maintain a

constant output voltage .

Figure 13. Simplified Schematic

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