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LTC3535

3535fa

Burst Mode OPERATION

Each channel of the LTC3535 will enter Burst Mode

operationatlightloadcurrentandreturntofixedfrequency

PWM mode when the load increases. Refer to the Typical

Performance Characteristics to see the output load Burst

Mode threshold current vs VIN. The 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 LTC3535 still switches at a

fixed frequency of 1MHz, 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 delivered to the output until it reaches the

nominal regulation value, then the LTC3535 transitions

to sleep mode where the outputs are off and the LTC3535

consumes only 9µA of quiescent current from VOUT for

each channel. When the output voltage droops slightly,

switching resumes. This maximizes efficiency at very light

loadsbyminimizingswitchingandquiescentlosses.Burst

Mode output voltage ripple, which is typically 1% peak-to-

peak, can be reduced by using more output capacitance

(10µF or greater), or with a small capacitor (10pF to 50pF)

connected between VOUT and FB.

As the load current increases, the LTC3535 will automati-

cally leave Burst Mode operation. Note that larger output

capacitor values may cause this transition to occur at

lighter loads. Once the LTC3535 has left Burst Mode op-

eration and returned to normal operation, it will remain

there until the output load is reduced below the burst

threshold current.

Burst Mode operation is inhibited during start-up and

soft-start and until VOUT is at least 0.24V greater than VIN.

Note that each channel can enter or leave Burst Mode

operation independent of the other channel.

OPERATION (Refer to Block Diagram)

APPLICATIONS INFORMATION

VIN > VOUT OPERATION

The LTC3535 will maintain voltage regulation even when

the input voltage is above the desired output voltage. Note

that the efficiency is much lower in this mode, and the

maximum output current capability will be less. Refer to

the Typical Performance Characteristics.

SHORT-CIRCUIT PROTECTION

The LTC3535 output disconnect feature allows output

short circuit while maintaining a maximum internally set

current limit. To reduce power dissipation under short-

circuit conditions, the peak switch current limit is reduced

to 400mA (typical per channel).

SCHOTTKY DIODE

Althoughnotrecommended,addingaSchottkydiodefrom

SW to VOUT will improve efficiency by about 2%. Note

that this defeats the output disconnect and short-circuit

protection features.

PCB LAYOUT GUIDELINES

The high speed operation of the LTC3535 demands careful

attention to board layout. A careless layout will result in

reduced performance. Figure 2 shows the recommended

component placement. A large ground pin copper area

will help to lower the die temperature. A multilayer board

with a separate ground plane is ideal, but not absolutely

necessary.

Figure 2. Recommended Component Placement

VIN2

VIN1

VOUT2

VOUT1

GND

SHDN

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