LTC3405A datasheet(7/16 Pages) LINER

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LTC3406

LTC3406-1.5/LTC3406-1.8

3406fa

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 VFB or VOUT 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 RDS(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

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.

SUPPLY VOLTAGE (V)

2.5

1200

1000

800

600

400

200

3.0

3.5

4.0

4.5

3406 F02

5.0

5.5

VOUT = 1.8V

VOUT = 1.5V

VOUT = 2.5V

Figure 2. Maximum Output Current vs Input Voltage

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