BESCHREIBUNG

The LT1930 and LT1930A are the industry’s highest power SOT-23 switching regulators. Both include an internal 1A, 36V switch allowing high current outputs to be generated in a small footprint. The LT1930 switches at 1.2MHz, allowing the use of tiny, low cost and low height capacitors and inductors. The faster LT1930A switches at 2.2MHz, enabling further reductions in inductor size. Complete regulator solutions approaching one tenth of a square inch in area are achievable with these devices. Multiple output power supplies can now use a separate regulator for each output voltage, replacing cumbersome quasi-regulated approaches using a single regulator and custom transformers.

A constant frequency internally compensated current mode PWM architecture results in low, predictable output noise that is easy to filter. Low ESR ceramic capacitors can be used at the output, further reducing noise to the millivolt level. The high voltage switch on the LT1930/LT1930A is rated at 36V, making the device ideal for boost converters up to 34V as well as for single-ended primary inductance converter (SEPIC) and flyback designs. The LT1930 can generate 5V at up to 480mA from a 3.3V supply or 5V at 300mA from four alkaline cells in a SEPIC design.

The LT1930/LT1930A are available in the 5-lead ThinSOT package.

 

FEATURES

1.2MHz Switching Frequency (LT1930)

2.2MHz Switching Frequency (LT1930A)

Low VCESAT Switch: 400mV at 1A

High Output Voltage: Up to 34V

5V at 480mA from 3.3V Input (LT1930)

12V at 250mA from 5V Input (LT1930A)

Wide Input Range: 2.6V to 16V

Uses Small Surface Mount Components

Low Shutdown Current: <1mA

Low Profile (1mm) ThinSOTTM Package

Pin-for-Pin Compatible with the LT1613

 

ANWENDUNGEN

TFT-LCD Bias Supply

Digital Cameras

Cordless Phones

Battery Backup

Medizinisch-diagnostische Geräte

Local 5V or 12V Supply

External Modems

PC Cards

xDSL Power Supply

 

OPERATION

The LT1930 uses a constant frequency, current-mode control scheme to provide excellent line and load regulation. Operation can be best understood by referring to the block diagram. At the start of each oscillator cycle, the SR latch is set, which turns on the power switch Q1. A voltage proportional to the switch current is added to a stabilizing ramp and the resulting sum is fed into the positive terminal of the PWM comparator A2. When this voltage exceeds the level at the negative input of A2, the SR latch is reset turning off the power switch. The level at the negative input of A2 is set by the error amplifier A1, and is simply an amplified version of the difference between the feedback voltage and the reference voltage of 1.255V. In this manner, the error amplifier sets the correct peak current level to keep the output in regulation. If the error amplifier’s output increases, more current is delivered to the output; if it decreases, less current is delivered. The LT1930 has a current limit circuit not shown. The switch current is constantly monitored and not allowed to exceed the maximum switch current (typically 1.2A). If the switch current reaches this value, the SR latch is reset regardless of the state of comparator A2. This current limit helps protect the power switch as well as the external components connected to the LT1930.

The block diagram for the LT1930A (not shown) is identical except that the oscillator frequency is 2.2MHz.

 

ANWENDUNGSINFORMATIONEN

Switching Frequency

The key difference between the LT1930 and LT1930A is the faster switching frequency of the LT1930A. At 2.2MHz, the LT1930A switches at nearly twice the rate of the LT1930. Care must be taken in deciding which part to use. The high switching frequency of the LT1930A allows smaller cheaper inductors and capacitors to be used in a given application, but with a slight decrease in efficiency and maximum output current when compared to the LT1930. Generally, if efficiency and maximum output current are critical, the LT1930 should be used. If application size and cost are more important, the LT1930A will be the better choice. In many applications, tiny inexpensive chip inductors can be used with the LT1930A, reducing solution cost.