7

LTC3801/LTC3801B

sn3801 3801fs

OPERATIO

(Refer to the Functional Diagram)

inductor current. The amount of reduction is given by the

curve in Figure 3.

Soft-Start

An internal default soft-start circuit is employed at power-

up and/or when coming out of shutdown. The soft-start

circuit works by internally clamping the voltage at the

gradually raising the clamp voltage such that the minimum

time required for the programmed switch current to reach

its maximum is approximately 0.6ms. After the soft-start

circuit has timed out, it is disabled until the part is put in

shutdown again or the input supply is cycled.

Figure 3. Maximum Current Limit Trip Voltage vs Duty Cycle

DUTY CYCLE (%)

20

75

95

100

3801 F03

55

35

40

60

80

30

50

70

90

115

65

85

45

105

70

50

30

100

60

80

40

90

LTC3801

LTC3801B

APPLICATIO S I FOR ATIO

The basic LTC3801/LTC3801B application circuit is shown

on the front page of this data sheet. External component

selection is driven by the load requirement and begins with

MOSFET and the output diode are selected followed by the

With the current comparator monitoring the voltage

tor determines the inductor’s peak current. The output

current the LTC3801 can provide is given by:

I

R

I

OUT

SENSE

RIPPLE

=−

0 117

2

.

(see Inductor Value Calculation section). For the LTC3801B

use 104mV in the previous equation and follow through

the analysis using that number.

A reasonable starting point for setting ripple current is

becomes:

R

I

SENSE

OUT

=<

1

10

()(

)

for Duty Cycle

40%

However, for operation that is above 40% duty cycle, slope

compensation effect has to be taken into consideration to

select the appropriate value to provide the required amount

R

SF

I

SENSE

OUT

=

()(

)(

)

10

100

where SF is the “Slope Factor.”

Inductor Value Calculation

The operating frequency and inductor selection are inter-

related in that higher operating frequencies permit the use

of a smaller inductor for the same amount of inductor

ripple current. However, this is at the expense of efficiency

due to an increase in MOSFET gate charge losses.

The inductance value also has a direct effect on ripple cur-

I

VV

fL

VV

VV

RIPPLE

IN

OUT

OUT

D

IN

D

=

−+

+









()

where f is the operating frequency. Accepting larger values

higher output voltage ripple and greater core losses. A

reasonable starting point for setting ripple current is