説明

The LT3995 is an adjustable frequency monolithic buck switching regulator that accepts a wide input voltage range up to 60V. Low quiescent current design consumes only 2.7µA of supply current while regulating with no load. Low ripple Burst Mode operation maintains high efficiency at low output currents while keeping the output ripple below 15mV in a typical application. The LT3995 can supply up to 3A of load current and has current limit foldback to limit power dissipation during short circuit. A low dropout voltage of 500mV is maintained when the input voltage drops below the programmed output voltage, such as during automotive cold crank.

An internally compensated current mode topology is used for fast transient response and good loop stability. A high efficiency 85mΩ switch is included on the die along with a boost Schottky diode and the necessary oscillator, control, and logic circuitry. An accurate 1.02V threshold enable pin can be driven directly from a microcontroller or used as a programmable undervoltage lockout. A capacitor on the SS pin provides a controlled inrush current (soft-start). A power good flag signals when VOUT reaches 91.6% of the programmed output voltage. The LT3995 is available in a small 16-lead MSOP package with exposed pad for low thermal resistance.

 

特徴

Ultralow Quiescent Current: 2.7µA IQ at 12VIN to 3.3VOUT

Low Ripple Burst Mode® Operation Output Ripple < 15mVP-P

Wide Input Range: Operation from 4.3V to 60V

3A Maximum Output Current

Excellent Start-Up and Dropout Performance

Adjustable Switching Frequency: 200kHz to 2MHz

Synchronizable Between 250kHz to 2MHz

Accurate Programmable Undervoltage Lockout

Low Shutdown Current: IQ = 700nA

Power Good Flag

Soft-Start Capability

Thermal Shutdown Protection

Current Limit Foldback with SS Override

Saturating Switch Design: 85mΩ On Resistance

Small, Thermally Enhanced 16-Lead MSOP Package

AEC-Q100 Qualified for Automotive Applications

 

アプリケーション

自動車用バッテリー規制

Portable Products

工業用品

 

オペレーション

The LT3995 is a constant frequency, current mode stepdown regulator. An oscillator, with frequency set by RT, sets an RS flip-flop, turning on the internal power switch. An amplifier and comparator monitor the current flowing between the VIN and SW pins, turning the switch off when this current reaches a level determined by the voltage at VC. An error amplifier measures the output voltage through an external resistor divider tied to the FB pin and servos the VC node. If the error amplifier’s output increases, more current is delivered to the output; if it decreases, less current is delivered. An active clamp on the VC pin provides current limit. The VC pin is also clamped by the voltage on the SS pin; soft-start is implemented by generating a voltage ramp at the SS pin using an external capacitor.

An internal regulator provides power to the control circuitry. The bias regulator normally draws power from the VIN pin, but if the OUT pin is connected to an external voltage higher than 3.2V, bias power will be drawn from the external source (typically the regulated output voltage). This improves efficiency.

 

アプリケーション情報

Step-down regulators draw current from the input supply in pulses with very fast rise and fall times. The input capacitor is required to reduce the resulting voltage ripple at the LT3995 and to force this very high frequency switching current into a tight local loop, minimizing EMI. A 4.7μF capacitor is capable of this task, but only if it is placed close to the LT3995. A second precaution regarding the ceramic input capacitor concerns the maximum input voltage rating of the LT3995. A ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. If the LT3995 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT3995’s voltage rating. If the input supply is poorly controlled or the user will be plugging the LT3995 into an energized supply, the input network should be designed to prevent this overshoot. See Linear Technology Application Note 88 for a complete discussion.