LTC3586-2 Datasheet, PDF (20/36 Pages) –

LTC3586-2/LTC3586-3
OPERATION
Thermal Regulation
To optimize charging time, an internal thermal feedback
loop may automatically decrease the programmed charge
current. This will occur if the die temperature rises to
approximately 110°C. Thermal regulation protects the
LTC3586-2/LTC3586-3 from excessive temperature due to
high power operation or high ambient thermal conditions
and allows the user to push the limits of the power han-
dling capability with a given circuit board design without
risk of damaging the LTC3586-2/LTC3586-3 or external
components. The benefit of the LTC3586-2/LTC3586-3
thermal regulation loop is that charge current can be set
according to actual conditions rather than worst-case
conditions with the assurance that the battery charger will
automatically reduce the current in worst-case conditions.
A flow chart of battery charger operation can be seen in
Figure 4.
Low Supply Operation
The LTC3586-2/LTC3586-3 incorporate an undervoltage
lockout circuit on V_OUT which shuts down all four general
purpose switching regulators when V_OUT drops below
V_OUTUVLO. This UVLO prevents unstable operation.
FAULT Pin
FAULT is an open-drain output used to indicate a fault
condition on any of the general purpose regulators. If
the FB pin voltage of any of the enabled regulators stays
below 92% of the internal reference voltage (0.8V) for
more than 14ms, a fault condition will be reported by
FAULT going low. Since FAULT is an open-drain output,
it requires a pull-up resistor to the input voltage of the
monitoring microprocessor or another appropriate power
source such as LD03V3.
General Purpose Buck Switching Regulators
The LTC3586-2/LTC3586-3 contain two 2.25MHz constant-
frequency current mode buck switching regulators. Each
buck regulator can provide up to 400mA of output current.
Both buck regulators can be programmed for a minimum
output voltage of 0.8V and can be used to power a micro-
controller core, microcontroller I/O, memory, disk drive or
other logic circuitry. Both buck converters support 100%
duty cycle operation (low dropout mode) when their input
voltage drops very close to their output voltage. To suit a
variety of applications, selectable mode functions can be
used to trade-off noise for efficiency. Two modes are avail-
able to control the operation of the LTC3586-2/LTC3586-3’s
buck regulators. At moderate to heavy loads, the pulse-
skipping mode provides the least noise switching solution.
At lighter loads, Burst Mode operation may be selected.
The buck regulators include soft-start to limit inrush cur-
rent when powering on, short-circuit current protection
and switch node slew limiting circuitry to reduce radiated
EMI. No external compensation components are required.
The operating mode of the buck regulators can be set by
the MODE pin. The buck converters can be individually
enabled by the EN1 and EN2 pins. Both buck regulators
have a fixed feedback servo voltage of 800mV. The buck
regulator input supplies V_IN1 and V_IN2 will generally be
connected to the system load pin V_OUT.
Buck Regulator Output Voltage Programming
Both buck regulators can be programmed for output volt-
ages greater than 0.8V. The output voltage for each buck
regulator is programmed using a resistor divider from the
buck regulator output connected to the feedback pins (FB1
and FB2) such that:
V_OUTX
=
V_FBX


R1
R2
+ 1^{}
where V_FB is fixed at 0.8V and X = 1, 2. See Figure 5.
Typical values for R1 are in the range of 40k to 1M. The
capacitor C_FB cancels the pole created by feedback resistors
and the input capacitance of the FBx pin and also helps
to improve transient response for output voltages much
greater than 0.8V. A variety of capacitor sizes can be used
for C_FB but a value of 10pF is recommended for most ap-
plications. Experimentation with capacitor sizes between
2pF and 22pF may yield improved transient response.
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