LT3509 LINER [Linear Technology], LT3509 Datasheet - Page 6

• Manufacturer Part Number: LT3509
• Description: Dual 36V, 700mA Step-Down Regulator
• Manufacturer: LINER [Linear Technology]

PIN FUNCTIONS
LT3509
DA1, DA2 (Pins 1, 7 / Pins 1, 8): The DA pins are the
anode connections for the catch diodes. These are con-
nected 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
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
internal power switches and control circuitry. In the MSE
package the V
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 0.7V to 2.0V, 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 1μA pull-up so that they will fl oat high or
charge an external capacitor to provide a current limited
6
IN
(Pin 4 / Pins 4, 5): The V
IN
IN
to minimize the voltage drop and power
pins must be tied together. The input
(DFN/MSE)
IN
pins supply power to the
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
R
oscillator frequency. A 40.2k resistor from R
results in a nominal frequncy of 1MHz.
SYNC (Pin 11 / Pin12): The SYNC pin allows the switching
frequency to be synchronized to a external clock. Choose
R
than the external clock frequency for correct operation.
BD (Pin 12 / Pin 13): The BD pin is common anode con-
nection of the internal Schottky boost diodes. This provides
the power for charging the BOOST capacitors. It should
be locally bypassed for best performance.
GND (Exposed Pad): 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
fl ows through the GND pad to the DA pins.
AGND (Pin 14, MSE 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.
T
T
(Pin 10 / Pin 11): The R
resistor to set a free-run frequency at least 12% less
IN
.
T
pin is used to set the internal
T
to ground
3509f