LTC3522 datasheet(6/20 Pages) LINER

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LTC3522

3522f

PIN FUNCTIONS

FB2 (Pin 1): Feedback Voltage for the Buck Converter De-

rived from a Resistor Divider on the Buck Output Voltage.

The buck output voltage is given by the following equation

where R1 is a resistor between FB2 and ground and R2 is

a resistor between FB2 and the buck output voltage:

OUT =+

⎛

⎝⎜

⎞

⎠⎟

0 594

PWM (Pin 2): Logic Input Used to Choose Between Burst

and PWM Mode Operation for Both Converters. This pin

cannot be left ﬂoating.

PWM = Low: Burst Mode operation is enabled on both

converters. The buck converter will operate in Burst

Mode operation at light current but will automatically

transition to PWM operation at higher currents. The

buck converter can supply its maximum output current

(200mA) 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: Both converters are forced into low noise

1MHz PWM mode operation. The buck converter will

remain at constant frequency operation until its mini-

mum on-time is reached. The buck-boost converter will

remain in PWM mode at all load currents.

GND (Pin 3): Small-Signal Ground Used as a Ground

Reference for the Internal Circuitry of the LTC3522.

PGOOD2 (Pin 4): This pin is an open-drain output which

will only pull low if the buck converter is enabled and one

or more of the following conditions occurs: the buck output

voltage is out of regulation, the part is in overtemperature

shutdown or the part is in undervoltage lockout.

FB1 (Pin 5): Feedback Voltage for the Buck-Boost Con-

verter 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-boost output voltage:

OUT =+

⎛

⎝⎜

⎞

⎠⎟

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

will only pull low if the buck-boost converter is enabled

and one or more of the following conditions occurs: the

buck-boost output voltage is out of regulation, the part is

in overtemperature shutdown, the part is in undervoltage

lockout or the buck-boost converter is in current limit. See

the Operation section of this data sheet for details on the

functionality of this pin in PWM mode.

SHDN1 (Pin 7): Buck-Boost Active-Low Shutdown Pin.

Forcing this pin above 1.4V enables the buck-boost con-

verter. Forcing this pin below 0.4V disables the buck-boost

converter. This pin cannot be left ﬂoating.

PVIN1(Pin8):HighCurrentPowerSupplyConnectionUsed

to Supply Switch A of the Buck-Boost Converter. 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 PVIN1 and PVIN2 must be connected together in the

application circuit.

PGND2 (Pin 9): High Current Ground Connection for the

Buck-Boost Switch C. The PCB trace connecting this pin to

ground should be made as short and wide as possible.

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