LTC3406-1.5_15 Datasheet, PDF (7/16 Pages) Linear Technology – 1.5MHz, 600mA Synchronous Step-Down Regulator in ThinSOT

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OPERATIO (Refer to Functional Diagram)
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_FB or V_OUT 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
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.
An important detail to remember is that at low input supply
voltages, the R_DS(ON) of the P-channel switch increases
(see Typical Performance Characteristics). Therefore, the
user should calculate the power dissipation when the
LTC3406 is used at 100% duty cycle with low input voltage
(See Thermal Considerations in the Applications Informa-
tion section).
Low Supply Operation
The LTC3406 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
LTC3406
LTC3406-1.5/LTC3406-1.8
in the maximum output current as a function of input
voltage for various output voltages.
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 LTC3406 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.
1200
1000
V_OUT = 1.8V
800
V_OUT = 1.5V
600
V_OUT = 2.5V
400
200
0
2.5 3.0 3.5 4.0 4.5 5.0 5.5
SUPPLY VOLTAGE (V)
3406 F02
Figure 2. Maximum Output Current vs Input Voltage
3406fa
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