LTC3405A-1.8_15 Datasheet, PDF (7/16 Pages) Linear Technology – 1.5V, 1.8V, 1.5MHz, 300mA Synchronous Step-Down Regulators in ThinSOT

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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
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 V_OUT 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.
LTC3405A-1.5/LTC3405A-1.8
600
V_OUT = 1.5V
500
V_OUT = 1.8V
400
300
200
100
0
2.5
3.0 3.5 4.0 4.5 5.0 5.5
SUPPLY VOLTAGE (V)
3405A1518 F02
Figure 2. Maximum Output Current vs Input Voltage
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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