LTC3405 datasheet(7/16 Pages) LINER

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LTC3405

3405fa

OPERATIO

until it reaches 100% duty cycle. The output voltage will then

be determined by the input voltage minus the voltage drop

across the P-channel MOSFET and the inductor.

Another important detail to remember is that at low input

supply voltages, the RDS(ON) of the P-channel switch

increases (see Typical Performance Characteristics). There-

fore, the user should calculate the power dissipation when

the LTC3405 is used at 100% duty cycle with low input

voltage (See Thermal Considerations in the Applications

Information section).

Low Supply Operation

The LTC3405 will operate with input supply voltages as

low as 2.5V, but the maximum allowable output current is

reduced at this low voltage. Figure 2 shows the reduction

in the maximum output current as a function of input

voltage for various output voltages.

(Refer to Functional Diagram)

Comparator OVDET guards against transient overshoots

> 6.25% by turning the main switch off and keeping it off

until the fault is removed.

Burst Mode Operation

The LTC3405 is capable of Burst Mode operation in which

the internal power MOSFETs operate intermittently based

on load demand. To enable Burst Mode operation, simply

connect the MODE pin to GND. To disable Burst Mode

operation and enable PWM pulse skipping mode, connect

the MODE pin to VIN or drive it with a logic high (VMODE >

1.5V). In this mode, the efficiency is lower at light loads,

but becomes comparable to Burst Mode operation when

the output load exceeds 25mA. The advantage of pulse

skipping mode is lower output ripple and less interference

to audio circuitry.

When the converter is in Burst Mode operation, the peak

current of the inductor is set to approximately 100mA re-

gardless of the output load. Each burst event can last from

a few cycles at light loads to almost continuously cycling

with short sleep intervals at moderate loads. In between

these burst events, the power MOSFETs and any unneeded

circuitry are turned off, reducing the quiescent current to

20µA. In this sleep state, the load current is being supplied

solely from the output capacitor. As the output voltage

droops, the EA amplifier’s output rises above the sleep

threshold signaling the BURST comparator to trip and turn

the top MOSFET on. This process repeats at a rate that is

dependent on the load demand.

Short-Circuit Protection

When the output is shorted to ground, the frequency of the

oscillator is reduced to about 210kHz, 1/7 the nominal

frequency. This frequency foldback ensures that the in-

ductor current has more time to decay, thereby preventing

runaway. The oscillator’s frequency will progressively

increase to 1.5MHz when VFB rises above 0V.

Dropout Operation

As the input supply voltage decreases to a value approach-

ing the output voltage, the duty cycle increases toward the

maximum on-time. Further reduction of the supply voltage

forces the main switch to remain on for more than one cycle

Slope Compensation and Inductor Peak Current

Slope compensation provides stability in constant fre-

quency architectures by preventing subharmonic oscilla-

tions at high duty cycles. It is accomplished internally by

adding a compensating ramp to the inductor current

signal at duty cycles in excess of 40%. Normally, this

results in a reduction of maximum inductor peak current

for duty cycles > 40%. However, the LTC3405 uses a

patent-pending scheme that counteracts this compensat-

ing ramp, which allows the maximum inductor peak

current to remain unaffected throughout all duty cycles.

Figure 2. Maximum Output Current vs Input Voltage

SUPPLY VOLTAGE (V)

2.5

600

500

400

300

200

100

3.0

3.5

4.0

4.5

3405 G23

5.0

5.5

VOUT = 1.8V

VOUT = 1.3V

VOUT = 2.5V

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