LTC1874 데이터 시트보기 (PDF) - Linear Technology
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LTC1874 Datasheet PDF : 12 Pages
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OPERATIO
Slope Compensation and Inductor’s Peak Current
The inductor’s peak current is determined by:
( ) IPK
=
VITH – 0.7
10 RSENSE
when the controller is operating below 40% duty cycle.
However, once the duty cycle exceeds 40%, slope com-
pensation begins and effectively reduces the peak inductor
current. The amount of reduction is given by the curves in
Figure 2.
LTC1874
110
100
90
80
70
60
50
IRIPPLE = 0.4IPK
40
AT 5% DUTY CYCLE
30
IRIPPLE = 0.2IPK
AT 5% DUTY CYCLE
20 VIN = 4.2V
10
0 10 20 30 40 50 60 70 80 90 100
DUTY CYCLE (%)
1874 F02
Figure 2. Percentage of Maximum Output Current vs Duty Cycle
APPLICATIO S I FOR ATIO
The basic LTC1874 application circuit is shown in
Figure 1. External component selection for each control-
ler is driven by the load requirement and begins with the
selection of L1 and RSENSE (= R1). Next, the power
MOSFET (M1) and the output diode (D1) are selected
followed by CIN and COUT (= C1).
RSENSE Selection for Output Current
RSENSE is chosen based on the required output current.
With the current comparator monitoring the voltage devel-
oped across RSENSE, the threshold of the comparator
determines the inductor’s peak current. The output cur-
rent the controller can provide is given by:
IOUT
=
0.12V
RSENSE
−
IRIPPLE
2
where IRIPPLE is the inductor peak-to-peak ripple current
(see Inductor Value Calculation section).
A reasonable starting point for setting ripple current is
IRIPPLE = (0.4)(IOUT). Rearranging the above equation, it
becomes:
( )( ) RSENSE =
1
10 IOUT
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 2, the value of RSENSE is:
( )( )( ) RSENSE =
SF
10 IOUT 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
current. The ripple current, IRIPPLE, decreases with higher
inductance or frequency and increases with higher VIN or
VOUT. The inductor’s peak-to-peak ripple current is given
by:
( ) IRIPPLE
=
VIN
− VOUT
fL
VOUT + VD
VIN + VD
where f is the operating frequency. Accepting larger values
of IRIPPLE 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
7
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