LTC3370 datasheet(9/26 Pages) AD

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LTC3307A

Rev. A

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PIN FUNCTIONS

AGND (Pin 1): The AGND pin is the output voltage remote

ground sense. Connect the AGND pin directly to the nega-

tive terminal of the output capacitor at the load. The AGND

pin is also the ground reference for the internal analog cir-

cuitry. Place a small analog bypass 0201 or 0402 ceramic

capacitor as close as possible to the VIN (Pin 3) and AGND

pins. Connect RT and FB returns to AGND as well.

EN (Pin 2): The EN pin has a precision IC enable thresh-

old with hysteresis. An external resistor divider, from

VIN or from another supply, can be used to program the

threshold below which the LTC3307A will shut down. If

the precision threshold is not required, tie EN directly

to VIN. When the EN pin is low the LTC3307A enters a

low current shutdown mode where all internal circuitry

is disabled. Do not float this pin.

VIN (Pins 3, 8): The VIN pins supply current to internal

circuitry and topside power switch. Connect both VIN pins

together with short wide traces and bypass to PGND and

AGND with low ESR capacitors located as close as pos-

sible to the pins.

PGND (Pins 4, 7, Exposed Pad Pin 13): The PGND pins

are the return path of the internal bottom side power

switch. Connect the negative terminal of the input capaci-

tors as close to the PGND pins as possible. For low para-

sitic inductance and good thermal performance, connect

Pin 4 and Pin 7 to a large continuous ground plane on

the printed circuit board directly under the LTC3307A.

The PGND exposed pad is the main electrical and thermal

highway and should be connected to large PCB ground

plane(s) with many vias.

SW (Pins 5, 6): The SW pins are the switching outputs of

the internal power switches. Connect these pins together

and to the inductor with a short, wide trace.

MODE/SYNC (Pin 9): The MODE/SYNC pin is a mode

selection and external clock synchronization input. Ground

this pin to enable Pulse Skip mode at light loads. For

higher efficiency at light loads, tie this pin to VIN to enable

the low-ripple Burst Mode operation. For faster transient

response, lower noise and full frequency operation over

a wide load range, float this pin to enable forced continu-

ous mode. Drive MODE/SYNC with an external clock to

synchronize the switcher to the applied frequency. While

synchronizing, the part operates in the forced continuous

mode. The slope compensation is automatically adapted

to the external clock frequency. In the absence of an

external clock the switching frequency is determined by

the RT pin.

RT (Pin 10): The RT pin sets the switching frequency with

an external resistor to AGND. If this pin is tied to VIN, the

buck will switch at the default oscillator frequency. If the

external clock is driving the MODE/SYNC pin, the RT pin

is ignored.

PGOOD (Pin 11): The PGOOD pin is the open drain output

of an internal power good comparator. When the regulated

output voltage falls below the PGOOD threshold or rises

above the overvoltage threshold, this pin is pulled low.

When VIN is above VIN UVLO and the part is in shutdown,

this pin is also pulled low.

FB (Pin 12): Program the output voltage and close the

control loop by connecting this pin to the middle node

of a resistor divider between the VOUT and AGND. The

LTC3307A regulates FB to 500mV (typical). A phase lead

capacitor connected between FB and VOUT may be used

to optimize transient response.

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