LTC3552 Datasheet, PDF(12/24 Page) Linear Technology – Standalone Linear Li-Ion Battery Charger and Dual Synchronous Buck Converter

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LTC3552 Datasheet, PDF (12/24 Pages) Linear Technology – Standalone Linear Li-Ion Battery Charger and Dual Synchronous Buck Converter

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LTC3552

OPERATIO

ï¬xed peak inductor current. By running cycles periodically,

the switching losses which are dominated by the gate

charge losses of the power MOSFETs are minimized. The

main control loop is interrupted when the output voltage

reaches the desired regulated value. A voltage comparator

trips when ITH is below 0.35V, shutting off the switch and

reducing the power. The output capacitor and the induc-

tor supply the power to the load until ITH exceeds 0.65V,

turning on the switch and the main control loop which

starts another cycle.

For lower ripple noise at low currents, the pulse-skipping

mode can be selected by grounding the MODE/SYNC pin. In

this mode, the regulator continues to switch at a constant

frequency down to very low currents, where it will begin

skipping pulses. The efï¬ciency in pulse-skipping mode can

be improved slightly by connecting the SW node to the

MODE/SYNC input which reduces the clock frequency by

approximately 30%. Do not ï¬oat the MODE/SYNC pin.

Dropout Operation

When the VCC input supply voltage decreases toward the

output 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 impor-

tant design consideration is that the RDS(ON) of the

P-channel switch increases with decreasing input supply

voltage (see Typical Performance Characteristics). There-

fore, the user should calculate the power dissipation when

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

voltage (see Thermal Considerations in the Applications

Information section).

Low Supply Voltage Operation

To prevent unstable operation, the regulators incorporate

an undervoltage lockout circuit which shuts them down

when the VCC voltage drops below approximately 1.65V.

3552f

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