LTC3521 datasheet(7/20 Pages) LINER

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LTC3521

3521f

in overtemperature shutdown, the part is in undervoltage

lockout, or if the SHDN3 pin is pulled low.

PGOOD2 (Pin 5/Pin 3): This pin is an open-drain output

which pulls low under any of the following conditions:

VOUT2 buck output voltage is out of regulation, the part is

in overtemperature shutdown, the part is in undervoltage

lockout, or if the SHDN2 pin is pulled low.

PGOOD1 (Pin 6/Pin 4): This pin is an open-drain output

which pulls low under any of the following conditions:

VOUT1 buck-boost output voltage is out of regulation,

the part is in overtemperature shutdown, the part is in

undervoltagelockout,thebuck-boostconverterisincurrent

limit, or if the SHDN1 pin is pulled low. See the Operation

section of this data sheet for details on the functionality

of this pin in PWM mode.

VIN (Pin 7/Pin 5): Low Current Power Supply Connection

Used to Power the Internal Circuitry of the LTC3521. This

pin should be bypassed by a 4.7µF, or larger, ceramic

capacitor. The bypass capacitor should be placed as close

to the pin as possible and should have a short return path

to ground. Pins VIN, PVIN1, and PVIN2 must be connected

together in the application circuit.

GND (Pin 8/Pin 6): Small Signal Ground. This pin is used as

a ground reference for the internal circuitry of the LTC3521.

PWM (Pin 9/Pin 7): Logic Input Used to Choose Between

Burst Mode Operation and PWM Mode for All Three Con-

verters. This pin cannot be left floating.

PWM = Low: Burst Mode operation is enabled on all

three converters. The buck converters will operate in

Burst Mode operation at light current but will automati-

cally transition to PWM operation at high currents. The

buck converters can supply maximum output current

(600mA) in this mode. The buck-boost converter will

operate in variable frequency mode and can only supply

a reduced load current (typically 50mA).

PWM = High: All three converters are forced into PWM

mode operation. The buck converters will remain at

constant-frequency operation until their minimum on-

time is reached. The buck-boost converter will remain

in PWM mode at all load currents.

FB1 (Pin 10/Pin 8): Feedback Voltage for the Buck-Boost

Converter Derived from a Resistor Divider on the Buck-

Boost Output Voltage. The buck-boost output voltage is

given by the following equation, where R1 is a resistor

between FB1 and ground, and R2 is a resistor between

FB1 and the buck output voltage:

OUT1

0 6 1









SHDN3 (Pin 11/Pin 9): Forcing this pin above 1.4V en-

ables the buck converter output at SW3. Forcing this pin

below 0.4V disables the buck converter. This pin cannot

be left floating.

SHDN1 (Pin 12/Pin 10): Forcing this pin above 1.4V

enables the buck-boost converter. Forcing this pin below

0.4V disables the buck-boost converter. This pin cannot

be left floating.

PVIN1(Pin13/Pin11):Highcurrentpowersupplyconnec-

tion used to supply switch A of the buck-boost converter.

This pin should be bypassed by a 4.7µF, or larger, ceramic

cap. The bypass capacitor should be placed as close to

the pin as possible and should have a short return path

to ground. Pins VIN, PVIN1, and PVIN2 must be connected

together in the application circuit.

NC (Pin 13, UF Package Only): No Internal Connection.

SW1B (Pin 14/Pin 14): Buck-Boost Switch Node. This pin

must be connected to one side of the buck-boost inductor.

SW1A (Pin 15/Pin 15): Buck-Boost Switch Node. This pin

must be connected to one side of the buck-boost inductor.

VOUT1 (Pin 16/Pin 16): Buck-Boost Output Voltage Node.

This pin should be connected to a low ESR ceramic capaci-

tor. The capacitor should be placed as close to the IC as

possible and should have a short return to ground.

SW3 (Pin 17/Pin 17): Buck converter Switch Node. This

pin must be connected to the opposite side of the inductor

connected to VOUT3.

PGND2 (Pin 18/Pin 18): High Current Ground Connec-

tion for Both Buck Converters. The PCB trace connecting

this pin to ground should be made as short and wide as

possible.

SW2 (Pin 19/Pin 20): Buck Converter Switch Node. This

pin must be connected to the opposite side of the inductor

connected to VOUT2.

pin FuncTions

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