LT3653

7

3653f

OPERATION

Please refer to the Block Diagram. The LT3653 is specifi-

cally tailored for use with a battery charger power path

controller. The LT3653 regulates the high voltage input to

a lower voltage to provide power to the system load and to

the single-cell Li-Ion battery charger. High efficiency in the

battery charger is achieved by keeping the voltage across

it low. To accomplish this, the battery charger takes control

the error amp, G1. The output voltage is regulated by the

battery charger to a voltage slightly above the battery,

typically 300mV.

HVOK is a status pin which indicates to the charger that a

high voltage input is present and that the LT3653 is ready to

start providing power to the system load. When the HVOK

pin is low, the LT3653 is not switching and the system

output is not supported by the LT3653 regulator.

The LT3653 is a constant frequency, current mode step

down regulator. A switch cycle is initiated when the

1.5MHz oscillator enables the RS flip flop, turning on

the internal power switch, Q1. The sense amplifier (A1)

monitors the switch current via the voltage dropped

tor compares the amplified current signal with the output

when this current exceeds a value determined by the

age through an internal resistor divider and, when not

current limit. The LT3653 is internally compensated with

a pole zero combination.

An external capacitor and internal diode, D2, are used to

generate a voltage at the BOOST pin that is higher than

the input supply. This allows the driver to fully saturate the

internal bipolar NPN power switch for efficient operation.

ensure startup.

An internal regulator provides power to the control circuitry.

This regulator includes input undervoltage and overvoltage

than 7V and greater than 33V, typical. When switching is

disabled, the LT3653 safely sustains input voltages up to

60V. Note that while switching is disabled the output will

discharge.

Output current limiting is provided via the servo action

of amplifier G2. The voltage across the sense resistor,

the inductor ripple current. If the averaged inductor cur-

is pulled low, reducing the current in the regulator. The

output current limit circuit allows for lower current rated

power path components and provides better control of

system power dissipation.