LTC3801B_15 Datasheet, PDF (7/12 Pages) Linear Technology – Micropower Constant Frequency Step-Down DC/DC Controllers in ThinSOT

U
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
I_TH/RUN pin to the corresponding zero current level and
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.
LTC3801/LTC3801B
115
100
105
90
LTC3801
95
80
LTC3801B
85
70
75
60
65
50
55
45 V_IN = 4.2V
T_A = 25°C
35
20 30 40 50 60 70 80
DUTY CYCLE (%)
40
30
90 100
3801 F03
Figure 3. Maximum Current Limit Trip Voltage vs Duty Cycle
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
the selection of the inductor and R_SENSE. Next, the power
MOSFET and the output diode are selected followed by the
input bypass capacitor C_IN and output bypass capacitor
C_OUT.
R_SENSE Selection for Output Current
R_SENSE is chosen based on the required output current.
With the current comparator monitoring the voltage
developed across R_SENSE, the threshold of the compara-
tor determines the inductor’s peak current. The output
current the LTC3801 can provide is given by:
I_OUT
=
0.117
R_SENSE
−
I_RIPPLE
2
where I_RIPPLE is the inductor peak-to-peak ripple current
(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_RIPPLE = (0.4)(I_OUT). Rearranging the above equation, it
becomes:
R_SENSE
=
1
(10)(I_OUT
)
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
of current. Using Figure 3, the value of R_SENSE is:
R_SENSE
=
SF
(10)(I_OUT )(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-
rent. The ripple current, I_RIPPLE, decreases with higher in-
ductance or frequency and increases with higher V_IN or
V_OUT. The inductor’s peak-to-peak ripple current is given by:
I_RIPPLE
=
V_IN
− V_OUT
f(L)


V_OUT + V_D
V_IN + V_D


where f is the operating frequency. Accepting larger values
of I_RIPPLE allows the use of low inductances, but results in
higher output voltage ripple and greater core losses. A
reasonable starting point for setting ripple current is
sn3801 3801fs
7