LT3509
6
3509fd
For more information www.linear.com/LT3509
pin FuncTions
DA1, DA2 (Pins 1, 7/Pins 1, 8): The DA pins are the anode
connections for the catch diodes. These are connected
internally to the exposed ground pad by current sensing
resistors.
BOOST1, BOOST2 (Pins 2, 6/Pins 2, 7): The BOOST pins
are used to dynamically boost the power transistor base
above V
IN
to minimize the voltage drop and power loss in
the switch. These should be tied to the associated switch
pins through the boost capacitors.
SW1, SW2 (Pins 3, 5/Pins 3, 6): The SW pins are the
internal power switch outputs. These should be connected
to the associated inductors, catch diode cathodes, and the
boost capacitors.
V
IN
(Pin 4/Pins 4, 5): The V
IN
pins supply power to the
internal power switches and control circuitry. In the MSE
package the V
IN
pins must be tied together. The input
capacitor should be placed as close as possible to the
supply pins.
FB1, FB2 (Pins 14, 8/Pins 16, 9): The FB pins are used
to set the regulated output voltage relative to the internal
reference. These pins should be connected to a resistor
divider from the regulated output such that the FB pin is
at 0.8V when the output is at the desired voltage.
RUN/SS1, RUN/SS2 (Pins 13, 9/Pins 15, 10): The RUN/SS
pins enable the associated regulator channel. If both pins
are pulled to ground, the device will shut-down to a low
power state. In the range 0.8V to 2V, the regulators are
enabled but the peak switch current and the DA pin maxi
-
mum current are limited to provide a soft-start function.
Above 2V, the full output current is available. The inputs
incorporate a 1
µA pull-up so that they will float high or
charge an external capacitor to provide a current limited
soft-start. The pins are pulled down by approximately 250µA
in the case of overvoltage or overtemperature conditions
in order to discharge the soft-start capacitors. The pins
can also be driven by a logic control signal of up to 5.0V.
In this case, it is necessary place a 10k to 50k resistor
in series along with a capacitor from the RUN/SS pin to
ground to ensure that there will be a soft-start for both
initial turn on and in the case of fault conditions. Do not
tie these pins to V
IN
.
RT (Pin 10/Pin 11): The RT pin is used to set the internal
oscillator frequency. A 40.2k resistor from RT to ground
results in a nominal frequency of 1MHz.
SYNC (Pin 11/Pin12):
The SYNC pin allows the switching
frequency to be synchronized to a external clock. Choose
R
T
resistor to set a free-run frequency at least 12% less
than the external clock frequency for correct operation.
The SYNC pin should not be allowed to float; if not used,
it should be tied low through a resistance 10kΩ or less.
BD (Pin 12/Pin 13): The BD pin is common anode connec
-
tion of the internal Schottky boost diodes. This provides
the power for charging the BOOST capacitors. It should
be locally bypassed for best per
formance.
Exposed Pad (Pin 15/Pin 17):
GND. This is the reference
and supply ground for the regulator. The exposed pad
must be soldered to the PCB and electrically connected to
supply ground. Use a large ground plane and thermal vias
to optimize thermal performance. The current in the catch
diodes also flows through the GND pad to the DA pins.
AGND (Pin 14, MSOP Package Only): This is the connected
to the ground connection of the chip and may be used as a
separate return for the low current control side components.
It should not be used as the only ground connection or as
a connection return for load side components.
(DFN/MSOP)
