LTC3547B datasheet(7/16 Pages) LINER

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LTC3547B

3547bfb

OPERATIO

The LTC3547B uses a constant-frequency current mode

architecture. The operating frequency is set at 2.25MHz.

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

The output voltage is set by an external resistor divider

returned to the VFB pins. An error ampliﬁer compares the

divided output voltage with a reference voltage of 0.6V 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 synchronous 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 0.6V reference by adjusting the peak inductor

current accordingly.

At very low load currents, the LTC3547B automatically

transitions from constant frequency operation to discon-

tinuous operation, where the regulator begins skipping

pulses. Even though the total power loss decreases with

load, the efﬁciency of the switching regulator will drop,

because the power delivered to the output becomes very

small. For applications where light load efﬁciency is a

priority, consider using the LTC3547 instead.

Dropout Operation

When the input supply voltage decreases toward the out-

put voltage the duty cycle increases to 100%, which is the

dropout condition. In dropout, the PMOS switch is turned

on continuously with the output voltage being equal to the

input voltage minus the voltage drops across the internal

P-channel MOSFET and the inductor.

An important design consideration is that the RDS(ON)

of the P-channel switch increases with decreasing input

supply voltage (see Typical Performance Characteristics).

Therefore, the user should calculate the worst-case power

dissipation when the LTC3547B is used at 100% duty cycle

with low input voltage (see Thermal Considerations in the

Applications Information Section).

Soft-Start

In order to minimize the inrush current on the input by-

pass capacitor, the LTC3547B slowly ramps up the output

voltage during start-up. Whenever the RUN1 or RUN2 pin is

pulled high, the corresponding output will ramp from zero

to full-scale over a time period of approximately 650μs. This

prevents the LTC3547B from having to quickly charge the

output capacitor and thus supplying an excessive amount

of instantaneous current.

Short-Circuit Protection

When either regulator output is shorted to ground, the

corresponding internal N-channel switch is forced on for

a longer time period for each cycle in order to allow the

inductor to discharge, thus preventing current runaway.

This technique has the effect of decreasing switching

frequency. Once the short is removed, normal operation

resumes and the regulator output will return to its nominal

voltage.

(Refer to Functional Diagram )

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