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LTC3401

3401fa

Rt (Pin 1): Timing Resistor to Program the Oscillator

Frequency.

OSC

31010

•

MODE/SYNC (Pin 2): Burst Mode Select and Oscillator

Synchronization.

MODE/SYNC = High. Enable Burst Mode operation. The

inductor peak inductor current will be 1/3 the current

limit value and return to zero current on each cycle.

During Burst Mode operation the operation is variable

frequency, providing a significant efficiency improve-

ment at light loads. It is recommended the Burst Mode

operation only be entered once the part has started up.

MODE/SYNC = Low. Disable Burst Mode operation and

maintain low noise, constant frequency operation.

MODE/SYNC = External CLK. Synchronization of the

internal oscillator and Burst Mode operation disable. A

clock pulse width of 100ns to 2

µs is required to

synchronize.

VIN (Pin 3): Input Supply Pin.

SW (Pin 4): Switch Pin. Connect inductor and Schottky

diode here. For applications with output voltages over

4.3V, a Schottky diode is required to ensure that the SW

pin voltage does not exceed its absolute maximum rating.

Minimize trace length to keep EMI down. For discontinu-

ous inductor current, a controlled impedance is placed

PI FU CTIO S

from SW to VIN from the IC to eliminate high frequency

ringing due to the resonant tank of the inductor and SW

node capacitance, therefore reducing EMI radiation.

GND (Pin 5): Signal and Power Ground for the IC.

PGOOD (Pin 6): Power Good Comparator Output. This

open-drain output is low when VFB < – 9% from its

regulation voltage.

VOUT (Pin 7): Output of the Synchronous Rectifier and

Bootstrapped Power Source for the IC. A ceramic capaci-

tor of at least 1

µF is required and should be located as

close to the VOUT and GND pins as possible (Pins 7 and 5).

FB (Pin 8): Feedback Pin. Connect resistor divider tap

here. The output voltage can be adjusted from 2.6V to

5.5V. The feedback reference voltage is typically 1.25V.

VC (Pin 9): Error Amp Output. A frequency compensation

network is connected to this pin to compensate the loop.

See the section “Compensating the Feedback Loop” for

guidelines.

SHDN (Pin 10): Shutdown. Grounding this pin shuts down

the IC. Tie to >1V to enable (VIN or digital gate output). To

operate with input voltages below 1V once the converter

has started, a 1M resistor from SHDN to VIN, and a 5M

resistor from SHDN to VOUT will provide sufficient hyster-

esis During shutdown the output voltage will hold up to VIN

minus a diode drop due to the body diode of the PMOS

synchronous switch. If the application requires a com-

plete disconnect during shutdown then refer to section

“Output Disconnect Circuits”.

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