LTC1874EGN Linear Technology, LTC1874EGN Datasheet - Page 7

• Manufacturer Part Number: LTC1874EGN
• Description: IC CTRLR DC/DC DUAL STPDN 16SSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1874EGN

• Internal Switch(s): No
• Synchronous Rectifier: No
• Number Of Outputs: 2
• Voltage - Output: 0.8 ~ 9.8 V
• Frequency - Switching: 550kHz
• Voltage - Input: 2.5 ~ 9.8 V
• Operating Temperature: -65°C ~ 150°C
• Mounting Type: Surface Mount
• Package / Case: 16-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Current - Output: -
• Power - Output: -

OPERATIO
APPLICATIO S I FOR ATIO
Slope Compensation and Inductor’s Peak Current
The inductor’s peak current is determined by:
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.
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 R
MOSFET (M1) and the output diode (D1) are selected
followed by C
R
R
With the current comparator monitoring the voltage devel-
oped across R
determines the inductor’s peak current. The output cur-
rent the controller can provide is given by:
where I
(see Inductor Value Calculation section).
A reasonable starting point for setting ripple current is
I
becomes:
However, for operation that is above 40% duty cycle, slope
compensation effect has to be taken into consideration to
RIPPLE
SENSE
SENSE
I
I
R
OUT
PK
SENSE
RIPPLE
= (0.4)(I
Selection for Output Current
is chosen based on the required output current.
10
V
R
0 12
ITH
SENSE
.
R
IN
SENSE
is the inductor peak-to-peak ripple current
10
– . 0 7
SENSE
V
OUT
and C
U
1
I
U
). Rearranging the above equation, it
OUT
I
, the threshold of the comparator
RIPPLE
OUT
2
SENSE
for Duty Cycle < 40%
U
(= C1).
(= R1). Next, the power
W
U
select the appropriate value to provide the required amount
of current. Using Figure 2, 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
current. The ripple current, I
inductance or frequency and increases with higher V
V
by:
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
Figure 2. Percentage of Maximum Output Current vs Duty Cycle
OUT
R
I
RIPPLE
RIPPLE
SENSE
. The inductor’s peak-to-peak ripple current is given
allows the use of low inductances, but results in
110
100
90
80
70
60
50
40
30
20
10
V
0
IN
10
V
10 20 30 40 50
IN
f L
= 4.2V
I
AT 5% DUTY CYCLE
I
AT 5% DUTY CYCLE
RIPPLE
RIPPLE
I
V
OUT
OUT
SF
= 0.4I
= 0.2I
DUTY CYCLE (%)
100
V
PK
PK
V
OUT
RIPPLE
IN
60
V
70 80 90 100
V
, decreases with higher
D
D
1874 F02
SENSE
LTC1874
is:
IN
7
or