LTC1626CS datasheet(6/12 Pages) LINER

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LTC1626

APPLICATIONS INFORMATION

inductor current should be continuous during burst peri-

ods. Hence, the peak-to-peak inductor ripple current must

not exceed 25mV/RSENSE.

To account for light load conditions, the IOUT(MAX) is then

given by:

IOUT(MAX) = 150mV/RSENSE – 25mV/2RSENSE (A)

= 137.5mV/RSENSE (A)

Solving for RSENSE and allowing a margin of variations in

the LTC1626 and external component values yields:

RSENSE = 100mV/IOUT(MAX) (Ω)

The LTC1626 switch is capable of supplying a maximum

of 1.2A of output current.

Therefore, the minimum value

of RSENSE that can be used is 0.083Ω. A graph for

selecting RSENSE versus maximum output current is given

in Figure 2.

Operating Frequency Considerations

For most applications, the LTC1626 should be operated in

the 100kHz to 300kHz range. This range can be extended,

however, up to 600kHz, to accommodate smaller size/

valued inductors, such as low profile types, with a slight

decrease in efficiency due to gate charge losses. Some

experimentation may be required to determine the opti-

mum operating frequency for a particular set of external

components and operating conditions.

CT and L Selection

The value of CT is calculated from the desired continuous

mode operating frequency:

OUT

−

()

()( ) −

()( ) ()

3300

where VD is the drop across the Schottky diode D1 and VBE

is a base-emitter voltage drop (0.6V).

The complete expression for operating frequency is given

by:

OFF

OUT

≈









−







 ()

where:

OFF

()( ) −

() ( )

3300

sec

Figure 3 is a graph of operating frequency versus power

supply voltage for the 2.5V regulator circuit shown in

Figure 1 (CT = 270pF). Note that the frequency is relatively

constant with supply voltage but drops as the supply

voltage approaches the regulated output voltage.

To maintain continuous inductor current at light load, the

inductor must be chosen to provide no more than 25mV/

RSENSE of peak-to-peak ripple current. This results in the

following expression for L:

LRSENSE

≥

()





()( )(

) ( )

52 105

REG

Using an inductance smaller than the above value will

result in inductor current being discontinuous. As a con-

MAXIMUM OUTPUT CURRENT (A)

0.5

0.4

0.3

0.2

0.1

1626 F02

0.6

0.8

1.0

0.4

0.2

Figure 2. Selecting RSENSE

During a short circuit of the regulator output to ground, the

peak current is determined by:

ISC = 150mV/RSENSE (A)

In this condition, the LTC1626 automatically extends the

off-time period of the P-channel MOSFET switch to allow

the inductor current to decay far enough to prevent any

current buildup. The resulting ripple current causes the

average current to be approximately IOUT(MAX).

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