LTC3406B
OPERATIO
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
LTC3406B is used at 100% duty cycle with low input
voltage (See Thermal Considerations in the Applications
Information section).
Low Supply Operation
The LTC3406B 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.
MAXIMUM OUTPUT CURRENT (mA)
U
(Refer to Functional Diagram)
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 LTC3406B 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
800
600
400
200
0
V
OUT
= 1.8V
V
OUT
= 2.5V
V
OUT
= 1.5V
2.5
3.0
3.5
4.0
4.5
SUPPLY VOLTAGE (V)
5.0
5.5
3406B F02
Figure 2. Maximum Output Current vs Input Voltage
3406bfa
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