LTC3544EUD-TRPBF Ver la hoja de datos (PDF) - Linear Technology

Número de pieza

componentes Descripción

Lista de partido

LTC3544EUD-TRPBF

Quad Synchronous Step-Down Regulator: 2.25MHz, 300mA, 200mA, 200mA, 100mA

Linear Technology 

LTC3544

OPERATION (Refer to Functional Diagrams)

The LTC3544 uses a constant-frequency current mode

architecture. The operating frequency is set at 2.25MHz.

All channels share the same clock and run in-phase.

The output voltage for each regulator is set by an external

resistor divider returned to the VFB pin. An error ampli-

fier compares the divided output voltage with a reference

voltage of 0.8V and regulates the peak inductor current

accordingly.

Main Control Loop

During normal operation, the top power switch (P-channel

MOSFET) is turned on at the beginning of a clock cycle

when the VFB voltage is below the reference voltage. The

current into the inductor and the load increases until the

peak inductor current (controlled by ITH) is reached. The RS

latch turns off the top switch, turns on the bottom switch,

and energy stored in the inductor is discharged through

the bottom switch (N-channel MOSFET) into the load until

the next clock cycle begins, or until the inductor current

begins to reverse (sensed by the IRCMP comparator).

The peak inductor current is controlled by the internally

compensated ITH voltage, which is the output of the er-

ror amplifier. This amplifier regulates the VFB pin to the

internal 0.8V reference by adjusting the peak inductor

current accordingly.

Burst Mode Operation

To optimize efficiency, the LTC3544 automatically switches

from continuous operation to Burst Mode operation when

the load current is relatively light. During Burst Mode

operation, the peak inductor current (as set by ITH) re-

mains fixed at a low level and the PMOS switch operates

intermittently based on load demand. By running cycles

periodically, the switching losses are minimized.

The duration of each burst event can range from a few

cycles at light load to almost continuous cycling with

short sleep intervals at moderate loads. During the sleep

intervals, the load current is being supplied solely from

the output capacitor. As the output voltage droops, the

error amplifier output rises above the sleep threshold,

signaling the burst comparator to trip and turn the top

MOSFET on. This cycle repeats at a rate that is dependent

on load demand.

VOUT100

VOUT200A

VOUT200B

VOUT300

RUNx

VIN = 3.6V

200μs/DIV

TA = 25°C

ALL CHANNELS UNLOADED

3544 G12

Figure 1. Regulator Soft-Start

Soft-Start

Soft-start reduces surge currents on VIN and output

overshoot during start-up. Soft-start on the LTC3544 is

implemented by internally ramping the reference signal

fed to the error amplifier over approximately a 1ms period.

Figure 1 shows the behavior of the four regulator channels

during soft-start.

Short-Circuit Protection

Short-circuit protection is achieved by monitoring the in-

ductor current. When the current exceeds a predetermined

level, the main switch is turned off, and the synchronous

switch is turned on long enough to allow the current in the

inductor to decay below the fault threshold. This prevents

a catastrophic inductor current, run-away condition, but

will still provide current to the output. Output voltage

regulation in this condition is not achieved.

DROPOUT OPERATION

As the input supply voltage decreases to a value approach-

ing 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

(see Typical Performance Characteristics). Therefore,

the user should calculate the power dissipation when

the LTC3544 is used at 100% duty cycle with low input

voltage (see Thermal Considerations in the Applications

Information section).

3544fa

9

Share Link: