7

LTC3408

3408f

off and on the bypass P-channel MOSFET with a frequency

of approximately 50kHz to 100kHz at 1.6A peak current.

This will continue until the short is removed. While the

bypass P-channel MOSFET is pulsing intermittently, the

inherent current limit of the step-down regulator limits its

peak current to about 1A.

Dropout Operation

the LTC3408 enters dropout operation. During dropout,

the main switch remains on continuously and operates at

100% duty cycle. If the voltage at REF is less than 1.2V, the

bypass P-channel MOSFET will stay off even in dropout

operation. The output voltage is then determined by the

input voltage minus the voltage drop across the main switch

and the inductor. If the voltage at REF is greater than 1.2V,

Figure 2. Maximum Output Current vs Input Voltage

SUPPLY VOLTAGE (V)

2.5

1200

1000

800

600

400

200

0

3.0

3.5

4.0

4.5

3408 F02

5.0

5.5

on, but the main switch will be off. For best performance

An important detail to remember is that at low input

increases (see Typical Performance Characteristics).

Therefore, the user should calculate the power dissipa-

tion when the LTC3408 is used at 100% duty cycle with

low input voltage (See Thermal Considerations in the

Applications Information section).

Low Supply Operation

The LTC3408 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 LTC3408 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.

APPLICATIO S I FOR ATIO

The basic LTC3408 application circuit is shown in Fig-

ure 1. External component selection is driven by the load

requirement and begins with the selection of L followed by

Inductor Selection

For most applications, the value of the inductor will fall in

the range of 4

µH to 6µH. Its value is chosen based on the

desired ripple current. Large value inductors lower ripple

current and small value inductors result in higher ripple

currents. As Equation 1 shows, a greater difference be-

Where these voltages are subject to change, the highest

current. A reasonable starting point for setting ripple

∆=









I

fL

V

V

V

L

OUT

OUT

IN

1

1

()( )

–

(1)