DESCRIPCIÓN

The LTC3545/LTC3545-1 are triple, high efficiency, monolithic synchronous buck regulators using a constant frequency, current mode architecture. The regulators operate independently with separate run pins. The 2.25V to 5.5V input voltage range makes the LTC3545/LTC3545-1 well suited for single Li-Ion battery-powered applications. Low ripple pulse skip mode or high efficiency Burst Mode operation is externally selectable. PWM pulse skip mode operation provides very low output ripple voltage while Burst Mode operation increases effi ciency at low output loads.

Switching frequency is internally set to 2.25MHz, or the switching frequency can be synchronized to an external 1MHz to 3MHz clock. Power good indicators easily allow power on sequencing between the three regulators.

The internal synchronous switches increase efficiency and eliminate external Schottky diodes. Low output voltages are supported with the 0.6V feedback reference voltage.

 

CARACTERÍSTICAS

Three 800mA Outputs

High Effi ciency: Up to 95%

2,25 V a 5,5 V Rango de tensión de entrada

Low Ripple (<20mVP-P) Burst Mode® Operation IQ: 58μA

2.25MHz Constant Frequency Operation or

Synchronizable to External 1MHz to 3MHz Clock

Power Good Indicators Ease Supply Sequencing

0.6V Reference Allows Low Output Voltages

Current Mode Operation/Excellent Transient Response

Low Profi le 16-Lead 3mm × 3mm QFN Package

 

APLICACIONES

Smart Phones

Wireless and DSL Modems

Digital Still Cameras

Portable Instruments

Point of Load Regulation

 

OPERACIÓN

MAIN CONTROL LOOP

The LTC3545/LTC3545-1 use a constant frequency, current mode step-down architecture. Both the main (P-channel MOSFET) and synchronous (N-channel MOSFET) switches are internal. During normal operation, the internal top power MOSFET is turned on each cycle when the oscillator sets the RS latch, and turned off when the current comparator, ICOMP , resets the RS latch. The peak inductor current at which ICOMP resets the RS latch, is controlled by the output of error amplifier EA. When the load current increases, it causes a slight decrease in the feedback voltage FB relative to the 0.6V reference, which in turn, causes the EA amplifier’s output voltage to increase until the average inductor current matches the new load current. While the top MOSFET is off, the bottom MOSFET is turned on until either the inductor current starts to reverse, as indicated by the current reversal comparator, IRCMP , or the beginning of the next clock cycle.

PULSE SKIPPING/Burst Mode OPERATION

At light loads, the inductor current may reach zero or reverse on each pulse. The bottom MOSFET is turned off by the current reversal comparator, IRCMP , and the switch voltage will ring. This is discontinuous mode operation, and is normal behavior for the switching regulator.

At very light loads, the LTC3545/LTC3545-1 will automatically begin operating in either pulse skipping or Burst Mode operation depending on the state of the MODE/SYNC pin (LTC3545). In either case the part will begin to skip cycles in order to maintain regulation.

 

OPERACIÓN

SOFT-START

Soft-start reduces surge currents on VIN and output overshoot during start-up. Soft-start on the LTC3545/LTC3545-1 is implemented by internally ramping the reference signal fed to the error amplifi er over approximately a 1ms period.

DROPOUT OPERATION

As the input supply voltage decreases to a value approaching the output voltage, the duty cycle increases toward the maximum on-time. Further reduction of the supply voltage forces the main switch to remain on for more than one cycle until it reaches 100% duty cycle. The output voltage will then be determined by the input voltage minus the voltage drop across the P-channel MOSFET and the inductor. An important detail to remember is that at low input supply voltages, the RDS(ON) of the P-channel switch increases. Therefore, the user should calculate the power dissipation when the LTC3545/LTC3545-1 is used at 100% duty cycle with low input voltage.

Inductor Core Selection

Different core materials and shapes will change the size/current and price/current relationship of an inductor. Toroid or shielded pot cores in ferrite or permalloy materials are small and don’t radiate much energy, but generally cost more than powdered iron core inductors with similar electrical characteristics. The choice of which style inductor to use often depends more on the price vs size requirements and any radiated fi eld/EMI requirements than on what the LTC3545/LTC3545-1 require to operate.