LTC3801 LINER [Linear Technology], LTC3801 Datasheet - Page 7

• Manufacturer Part Number: LTC3801
• Description: Micropower Constant Frequency Step-Down DC/DC Controllers in ThinSOT
• Manufacturer: LINER [Linear Technology]

OPERATIO
APPLICATIO S I FOR ATIO
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
I
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.
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
the selection of the inductor and R
MOSFET and the output diode are selected followed by the
input bypass capacitor C
C
R
R
With the current comparator monitoring the voltage
developed across R
tor determines the inductor’s peak current. The output
current the LTC3801 can provide is given by:
where I
(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
I
becomes:
TH
RIPPLE
OUT
SENSE
SENSE
I
R
/RUN pin to the corresponding zero current level and
OUT
SENSE
.
RIPPLE
= (0.4)(I
Selection for Output Current
is chosen based on the required output current.
R
0 117
SENSE
.
( )(
10
is the inductor peak-to-peak ripple current
OUT
U
1
I
OUT
U
). Rearranging the above equation, it
SENSE
I
RIPPLE
(Refer to the Functional Diagram)
)
2
for Duty Cycle
IN
, the threshold of the compara-
U
and output bypass capacitor
SENSE
W
. Next, the power
40%
U
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
of current. Using Figure 3, the value of R
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-
rent. The ripple current, I
ductance or frequency and increases with higher V
V
where f is the operating frequency. Accepting larger values
of I
higher output voltage ripple and greater core losses. A
reasonable starting point for setting ripple current is
OUT
Figure 3. Maximum Current Limit Trip Voltage vs Duty Cycle
I
R
RIPPLE
RIPPLE
SENSE
. The inductor’s peak-to-peak ripple current is given by:
allows the use of low inductances, but results in
115
105
95
85
75
65
55
45
35
20
V
( )(
IN
10
V
T
IN
A
= 25 C
30
f L
= 4.2V
( )
I
V
OUT
OUT
40
SF
LTC3801/LTC3801B
LTC3801B
DUTY CYCLE (%)
)(
50
RIPPLE
100
V
V
OUT
60
IN
)
, decreases with higher in-
70
LTC3801
V
V
D
D
80
90
3801 F03
SENSE
100
100
90
80
70
60
50
40
30
is:
IN
7
3801f
or