LTC3405A-1.5 datasheet(7/16 Pages) LINER

7 / 16 page

LTC3405A-1.5/LTC3405A-1.8

sn3405a1518 3405a1518fs

OPERATIO (Refer to Functional Diagram)

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

µA. 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 VOUT rises above 0V.

Low Supply Operation

The LTC3405A series parts 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 both fixed output voltages.

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

3405A1518 F02

5.0

5.5

VOUT = 1.8V

VOUT = 1.5V

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 LTC3405A series

parts use a patent-pending scheme that counteracts this

compensating ramp, which allows the maximum inductor

peak current to remain unaffected throughout all duty

cycles.

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