3509 データシートの表示（PDF） - Linear Technology

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3509

Dual 36V, 700mA Step-Down Regulator

Linear Technology 

LT3509

PIN FUNCTIONS (DFN/MSOP)

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 VIN 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.

VIN (Pin 4/Pins 4, 5): The VIN pins supply power to the

internal power switches and control circuitry. In the MSE

package the VIN 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 1μA 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 VIN.

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

RT resistor to set a free-run frequency at least 12% less

than the external clock frequency for correct operation.

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 performance.

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.

3509fc

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