LTC3312SAAVPBF datasheet(11/28 Pages) AD

11 / 28 page

LTC3312SA

Rev. 0

For more information www.analog.com

PIN FUNCTIONS

PGOOD2 (Pin 16): Power Good (Open-Drain) Output for

Buck Regulator 2. When the regulated output voltage

is outside the power-good voltage window, and PVIN is

above 2.25V, this pin is driven LOW. The PGOOD2 out-

put is also pulled low when VIN is above 2.25V and Buck

Regulator 2 is in shutdown.

FB2 (Pin 17): Feedback Input for Buck Regulator 2.

Program the output voltage of Buck 2 by connecting this

pin to the middle node of a resistor divider between the

output and ground. The FB2 pin is regulated to 500mV. A

phase lead capacitor connected between VOUT2 and FB2

may be used to optimize transient response.

EN2 (Pin 18): Active high Enable Input for Buck

Regulator 2. The EN2 pin has a precision threshold. An

external resistor divider from VIN, or from another sup-

ply, can be used to program when Buck Regulator 2 is

enabled. If the enable function is not required, tie EN2

directly to AVIN. The EN2 pin should not be floated.

RT (Pin 19): The RT pin sets the switching frequency with

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

the buck will switch at the default oscillator frequency. If

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

pin is ignored.

AGND (Pin 20): The AGND pin is the ground pin for

the internal bias circuits, including the bandgap refer-

ence. Connect the bottom resistor of the FB1 and FB2

resistor dividers to the AGND pin for accurate output

voltage regulation.

AVIN (Pin 21): Filtered Input Supply used to bias the inter-

nal bandgap reference and buck control circuits. There are

internal 20Ω resistors from PVIN1 to AVIN, and from PVIN2

to AVIN. Connect a 1µF ceramic capacitor from AVIN to

AGND to provide a filtered supply for the internal control

circuits. Do not externally load this pin.

SSTT2 (Pin 22): Soft-Start, Tracking, Temperature Monitor

Input for Buck Regulator 2. An internal 4µA current into

an external capacitor on the soft-start pin programs the

output voltage ramp rate for during start-up. When SSTT2

is below 0.5V, the FB2 pin voltage will track the SSTT2 pin

voltage. When SSTT2 is above 0.5V, the tracking function

is disabled, the internal reference resumes control of the

error amplifier and the SSTT2 pin servos to a voltage

proportional to junction temperature. For a clean recov-

ery from an output short circuit condition, the SSTT2

pin is pulled down to approximately 100mV above the

VFB voltage and a new soft-start cycle is initiated. During

shutdown and fault conditions, the SSTT2 pin is pulled to

ground. Short the SSTT2 pin to AVIN to use an internally

set default soft-start time of 1ms (typical).

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