LTC3552-1 Linear Technology, LTC3552-1 Datasheet - Page 11

• Manufacturer Part Number: LTC3552-1
• Description: Standalone Linear Li-Ion Battery Charger and Dual Synchronous Buck Converter
• Manufacturer: Linear Technology

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OPERATIO
Thermal Limiting
An internal thermal feedback loop reduces the pro-
grammed charge current if the die temperature attempts
to rise above a preset value of approximately 120°C. This
feature protects the charger from excessive temperature
and allows the user to push the limits of the power handling
capability of a given circuit board without risk of damag-
ing the charger. The charge current can be set according
to typical (not worst case) ambient temperature with the
assurance that the charger will automatically reduce the
current in worst-case conditions. DFN package power
considerations are discussed further in the Applications
Information section.
Undervoltage Lockout (UVLO)
An internal undervoltage lockout circuit monitors the in-
put voltage and keeps the charger in shutdown mode
until V
The UVLO circuit has a built-in hysteresis of 200mV.
Also, to protect against reverse current in the power
MOSFET, the UVLO circuit keeps the charger in shutdown
mode if V
UVLO comparator is tripped, the charger will not come
out of shutdown mode until V
BAT voltage.
Manual Shutdown
At any point in the charge cycle, the charger can be put
into shutdown mode by driving the ⎯ E ⎯ N pin high. This re-
duces the battery drain current to less than 2µA and the
V
mode, the ⎯ C ⎯ H ⎯ R ⎯ G pin is in the high impedance state. A new
charge cycle can be initiated by driving the ⎯ E ⎯ N pin low. An
internal resistor pull-down on this pin forces the charger
to be enabled if the pin is allowed to fl oat.
DUAL SWITCHING REGULATOR
The regulators use a current mode architecture with a con-
stant operating frequency of 2.25MHz. Both regulators share
the same clock and run in-phase. The output voltages are
fi xed at 1.8V for regulator 1 and at 1.575V for regulator 2.
The resistive divider feedback networks are integrated inside
the LTC3552-1. An error amplifi er compares the divided
IN
supply current to less than 50µA. When in shutdown
IN
rises above the undervoltage lockout threshold.
IN
falls to within 30mV of the BAT voltage. If the
U
IN
rises 100mV above the
output voltage (V
adjusts the peak inductor current accordingly.
Main Regulator Control Loop
During normal operation, the top power switch (P-channel
MOSFET) is turned on at the beginning of a clock cycle
when the V
voltage. The current fl ows into the inductor and the load in-
creases until the current limit is reached. The switch turns
off and energy stored in the inductor fl ows through the
bottom switch (N-channel MOSFET) into the load until the
next clock cycle. The peak inductor current is controlled
by the internally compensated I
output of the error amplifi er. This amplifi er compares the
V
load current increases, the V
below the reference. This decrease causes the error ampli-
fi er to increase the I
current matches the new load current. The main control
loop is shut down by pulling the RUN pin to ground.
Low Load Current Operation
When the load is relatively light, the regulator automati-
cally switches into Burst Mode operation, where the PMOS
switch operates intermittently based on load demand with
a fi xed peak inductor current. By running cycles periodi-
cally, 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 I
reducing the power. The output capacitor and the induc-
tor supply the power to the load until I
turning on the switch and the main control loop which
starts another cycle.
Dropout Operation
When the V
ing 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 important design consideration is that the R
FB
to the 0.6V reference (see Block Diagram). When the
OUT
TH
CC
is below 0.35V, shutting off the switch and
input supply voltage decreases approach-
feedback voltage is below the reference
FB
) with a reference voltage of 0.6V and
TH
voltage until the average inductor
FB
voltage decreases slightly
TH
LTC3552-1
voltage, which is the
TH
exceeds 0.65V,
DS(ON)
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