ltc3586 Linear Technology Corporation, ltc3586 Datasheet - Page 19

• Manufacturer Part Number: ltc3586
• Description: High Ef? Ciency Usb Power Manager With Boost, Buck-boost And Dual Bucks
• Manufacturer: Linear Technology Corporation

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 from excessive temperature due to high power
operation or high ambient thermal conditions and allows
the user to push the limits of the power handling capability
with a given circuit board design without risk of damag-
ing the LTC3586 or external components. The benefit
of the LTC3586 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 fl ow chart of battery charger operation can be seen in
Figure 4.
Low Supply Operation
The LTC3586 incorporates an undervoltage lockout cir-
cuit on V
switching regulators when V
This UVLO prevents unstable operation.
FAULT Pin
FAULT is a bi-directional pin with an open-drain output used
to indicate a fault condition on any of the general purpose
regulators. If any of the four regulators are enabled, and
their corresponding FB pin voltage does not rise to within
8% of the internal reference voltage (0.8V) within 14ms, a
fault condition will be reported by FAULT going low. This,
in turn, will disable all of the regulators. Alternatively, the
regulators can be all disabled simultaneously by driving
FAULT low externally. This fault condition can be cleared
only if all of the ENABLE inputs are pulled low for at least
3.6μs. 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.
If any of the ENABLE pins is tied high during start-up,
the FAULT pin can erroneously report a fault condition.
To avoid such an event, the ENABLE pins should be tied
high through a lowpass fi lter (comprised of a 1k resistor
OUT
which shuts down all four general purpose
OUT
drops below V
OUTUVLO
.
and a 0.1μF capacitor) to the same power source to which
FAULT pin is pulled up.
General Purpose Buck Switching Regulators
The LTC3586 contains two 2.25MHz constant-frequency
current-mode buck switching regulators. Each buck regula-
tor can provide up to 400mA of output current. Both buck
regulators can be programmed for a minimum output volt-
age of 0.8V and can be used to power a microcontroller
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 available to
control the operation of the LTC3586’s buck regulators.
At moderate to heavy loads, the pulse skip 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 current when powering
on, short-circuit current protection and switch node slew
limiting circuitry to reduce radiated EMI. No external com-
pensation 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 fi xed feedback
servo voltage of 800mV. The buck regulator input supplies
V
load pin V
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:
where V
Typical values for R1 are in the range of 40k to 1M. The
capacitor C
IN1
V
OUTX
and V
FB
=
OUT
FB
IN2
is fixed at 0.8V and X = 1, 2. See Figure 4.
V
FBX
cancels the pole created by feedback resistors
.
will generally be connected to the system
⎛
⎜
⎝
R
R
2
1
+
1
⎞
⎟
⎠
LTC3586
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