BESCHREIBUNG

The LT3973 is an adjustable frequency monolithic buck switching regulator that accepts a wide input voltage range up to 42V, and consumes only 2.5µA of quiescent current. A high efficiency switch is included on the die along with the catch diode, boost diode, and the necessary oscillator, control and logic circuitry. Low ripple Burst Mode operation maintains high efficiency at low output currents while keeping the output ripple below 10mV in a typical application. A minimum dropout voltage of 530mV is maintained when the input voltage drops below the programmed output voltage, such as during automotive cold crank. Current mode topology is used for fast transient response and good loop stability. A catch diode current limit provides protection against shorted outputs and overvoltage conditions, with thermal shutdown providing additional fault protection. An accurate programmable undervoltage lockout feature is available, producing a low shutdown current of 0.75µA. A power good flag signals when VOUT reaches 90% of the programmed output voltage. The LT3973 is available in small, thermally enhanced 10-lead MSOP and 3mm × 3mm DFN packages.

 

FEATURES

Ultralow Quiescent Current

2.5µA IQ at 12VIN to 3.3VOUT

Low Ripple Burst Mode® Operation

Output Ripple < 10mVP-P

Wide Input Voltage Range: 4.2V to 42V Operating

Adjustable Switching Frequency: 200kHz to 2.2MHz

Integrated Boost and Catch Diodes

750mA Output Current

Excellent Start-Up and Dropout Performance

Fixed Output Voltages: 3.3V, 5V

1.9µA IQ at 12VIN

Accurate Programmable Undervoltage Lockout

Low Shutdown Current: IQ = 0.75µA

Internal Catch Diode Current Limit

Power Good Flag

Thermische Abschaltung

Small, Thermally Enhanced 10-Lead MSOP and (3mm × 3mm) DFN Packages

 

ANWENDUNGEN

Regulierung von Autobatterien

Strom für tragbare Produkte

Industriebedarf

Gate Drive Bias

 

OPERATION

The LT3973 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.

If the input voltage decreases towards the programmed output voltage, the LT3973 will start to skip switch-off times and decrease the switching frequency to maintain output regulation up to a maximum duty cycle of approximately 97.5%. When the OUT pin is tied to VOUT, the LT3973 regulates the output such that it stays more than 530mV below VIN; this sets a minimum dropout voltage. This enforced minimum dropout voltage limits the duty cycle and keeps the boost capacitor charged during dropout conditions. Since sufficient boost voltage is maintained, the internal switch can fully saturate yielding good dropout performance.

The LT3973 contains a power good comparator which trips when the FB pin is at 90% of its regulated value. The PG output is an open-drain transistor that is off when the output is in regulation, allowing an external resistor to pull the PG pin high. Power good is valid when the LT3973 is enabled and VIN is above 4.2V.

 

Eingangskondensator

Bypass the input of the LT3973 circuit with a ceramic capacitor of X7R or X5R type. Y5V types have poor performance over temperature and applied voltage, and should not be used. A 4.7µF ceramic capacitor is adequate to bypass the LT3973 and will easily handle the ripple current. Note that larger input capacitance is required when a lower switching frequency is used (due to longer on-times). If the input power source has high impedance, or there is significant inductance due to long wires or cables, additional bulk capacitance may be necessary. This can be provided with a low performance electrolytic capacitor.