LTC3539/LTC3539-2

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For more information www.linear.com/3539/3539-2

OPERATION

Zero Current Comparator: The zero current comparator

monitors the inductor current to the output and shuts off

the synchronous rectifier once this current reduces to ap-

proximately40mA.Thispreventstheinductorcurrentfrom

reversing in polarity, improving efficiency at light loads.

Synchronous Rectifier: The P-channel MOSFET synchro-

This is to control inrush current and to prevent inductor

current from running away

Anti-ringing Control: The anti-ringing control connects

a resistor across the inductor to prevent high frequency

ringing on the SW pin during discontinuous current mode

operation. The ringing of the resonant circuit formed by L

cause EMI radiation.

Output Disconnect: The LTC3539 is designed to allow true

outputdisconnectbyeliminatingbodydiodeconductionof

to go to zero volts during shutdown, drawing no current

from the input source. It also provides inrush current

limiting at turn-on, minimizing surge currents seen by the

input supply. Note that to obtain the advantages of output

disconnect, there must not be an external Schottky diode

Thermal Shutdown: If the die temperature exceeds 160°C,

the device will go into thermal shutdown. All switches will

be turned off and the internal soft-start capacitor will be

discharged. The device will be enabled again when the die

temperature drops by about 15°C.

Burst Mode Operation

When Burst Mode operation is enabled by bringing the

MODE pin above 0.88V, the LTC3539 will automatically

enter Burst Mode operation at light load current, then

return to fixed frequency PWM mode when the load in-

creases. Refer to the typical performance characteristics

load current at which Burst Mode operation is entered can

be changed by adjusting the inductor value. Raising the

inductor value will lower the load current at which Burst

Mode operation is entered.

In Burst Mode operation, the LTC3539 still switches at a

fixed frequency, using the same error amplifier and loop

compensation for peak current mode control. This control

method eliminates any output transient when switching

between modes. In Burst Mode operation, energy is deliv-

ered to the output until it reaches the nominal regulation

value, then the LTC3539 transitions to Sleep mode where

the outputs are off and the LTC3539 consumes only 10µA

droops slightly, switching resumes. This maximizes ef-

ficiency at very light loads by minimizing switching and

quiescent current losses. Burst Mode output voltage

ripple, which is application sensitive and typically 1% to

2% peak-to-peak if data sheet recommendations are fol-

lowed, can be reduced by using more output capacitance

(47µF or greater).

As the load increases, the LTC3539 will automatically

leave Burst Mode operation. Once the LTC3539 has left

Burst Mode operation and returned to normal operation,

it will remain there until the output load is reduced below

the burst threshold.

Burst Mode operation is inhibited during start-up until

When the MODE pin is below 0.3V, the LTC3539 features

continuous PWM operation. In this mode, at very light

loads, the LTC3539 will exhibit pulse-skip operation. If

by 0.5V, the MODE pin will sink additional current.

Due to the high inductor current slew rate in 5V output

applications, the LTC3539-2 may not enter Burst Mode

operation for input voltages less than 1.8V. For 5V applica-

tions requiring Burst Mode operation, the 1MHz LTC3539

is recommended. Refer to the Typical Performance

Characteristics curves for the Burst Mode thresholds for

different input and output voltages.