LT3653
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
of the LT3653 regulator control node, V
C
, and overrides
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
across the current sense resistor R
SENSE1
. The compara-
tor compares the amplified current signal with the output
(V
C
) of the error amplifier (G1). The switch is turned off
when this current exceeds a value determined by the
V
C
voltage. The error amplifier monitors the V
OUT
volt-
age through an internal resistor divider and, when not
driven externally servos the V
C
voltage to regulate V
OUT
.
If the V
OUT
voltage drops, the V
C
voltage will be driven
higher increasing the output current and V
OUT
voltage.
An active clamp (not shown) on the V
C
node provides
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.
The switch driver operates from either V
IN
or BOOST to
ensure startup.
An internal regulator provides power to the control circuitry.
This regulator includes input undervoltage and overvoltage
protection which disables switching action when V
IN
is less
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,
R
SENSE2
, is compared to a voltage programmed by ex-
ternal resistor R1 on the I
LIM
pin. A capacitor averages
the inductor ripple current. If the averaged inductor cur-
rent exceeds the programmed value then the V
C
voltage
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.
3653f
7
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