LTC3550 datasheet(13/24 Pages) LINER

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LTC3550

3550fa

OPERATIO

mode and the charge current begins to decrease. Once

the charge current drops below the programmed termina-

tion threshold (set by the external resistor RITERM), the

internal P-channel MOSFET is shut off and the charger

enters standby mode.

In standby mode, the charger sits idle and monitors the

battery voltage using a comparator with a 6ms ﬁlter time

(tRECHRG). A charge cycle automatically restarts when the

battery voltage falls below 4.1V (which corresponds to ap-

proximately 80% to 90% battery capacity). This ensures

that the battery is kept near a fully charged condition and

eliminates the need for periodic charge cycle initiations.

Figure 1 uses a state diagram to describe the behavior of

the LTC3550 battery charger.

600mA Step-Down Regulator

The LTC3550 regulator uses a constant frequency, current

mode step-down architecture. Both the top (P-channel

MOSFET) and bottom (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 is turned off when the current com-

parator, 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 output voltage

(VOUT), relative to the internal reference, which in turn

causes the EA ampliﬁer’s output voltage to increase until

the average inductor current matches the new load cur-

rent. 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.

Burst Mode® Operation

The LTC3550 buck regulator is capable of Burst Mode

operation in which the internal power MOSFETs operate

intermittently based on load current demand.

In Burst Mode operation, the peak current of the inductor is

set to approximately 200mA regardless of the output load.

Each burst event can last from a few cycles at light loads

to almost continuously cycling with short sleep intervals

at moderate loads. In between these burst events, the

power MOSFETs and any unneeded circuitry are turned

off, reducing the quiescent current to 20µA. In this sleep

state, the load current is being supplied solely from the

output capacitor. As the output voltage droops, the EA

ampliﬁer’s output rises above the sleep threshold signaling

the BURST comparator to trip and turn the top MOSFET

on. This process repeats at a rate that is dependent on

the load demand.

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 LTC3550 is used at 100% duty cycle with low input

voltage (See Thermal Considerations in the Applications

Information section).

Short-Circuit Protection

When the regulator output is shorted to ground, the fre-

quency of the oscillator is reduced to about 210kHz, one

seventh the nominal frequency. This frequency foldback

ensures that the inductor current has more time to decay,

thereby preventing runaway. The oscillator’s frequency

will progressively increase to 1.5MHz when VFB rises

above 0V.

Burst Mode is a registered trademark of Linear Technology Corporation.

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