LTC1772CS6#TR Linear Technology, LTC1772CS6#TR Datasheet - Page 6

• Manufacturer Part Number: LTC1772CS6#TR
• Description: IC CTRLR DC/DC STEPDOWN SOT23-6
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1772CS6#TR

• Internal Switch(s): No
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 0.8 ~ 9.8 V
• Current - Output: 1A
• Frequency - Switching: 550kHz
• Voltage - Input: 2.5 ~ 9.8 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: TSOT-23-6, TSOT-6
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

OPERATIO
LTC1772
Slope Compensation and Inductor’s Peak Current
The inductor’s peak current is determined by:
when the LTC1772 is operating below 40% duty cycle.
However, once the duty cycle exceeds 40%, slope com-
pensation begins and effectively reduces the peak induc-
tor current. The amount of reduction is given by the curves
in Figure 2.
APPLICATIONS
The basic LTC1772 application circuit is shown in Figure 1.
External component selection is driven by the load re-
quirement and begins with the selection of L1 and R
(= R1). Next, the power 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 LTC1772 can provide is given by:
where I
(see Inductor Value Calculation section).
A reasonable starting point for setting ripple current is
I
becomes:
6
RIPPLE
SENSE
SENSE
I
I
R
PK
OUT
SENSE
=
RIPPLE
= (0.4)(I
Selection for Output Current
is chosen based on the required output current.
=
10
V
R
=
ITH
(
0 12
SENSE
R
.
( )(
10
SENSE
is the inductor peak-to-peak ripple current
– . 0 7
SENSE
OUT
U
1
I
OUT
−
U
). Rearranging the above equation, it
)
I
, the threshold of the comparator
RIPPLE
(Refer to Functional Diagram)
)
2
for Duty Cycle
INFORMATION
U
IN
(= C1) and C
W
<
40%
OUT
U
(= C2).
SENSE
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 R
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:
OUT
R
I
RIPPLE
SENSE
. The inductor’s peak-to-peak ripple current is given
Figure 2. Maximum Output Current vs Duty Cycle
110
100
=
90
80
70
60
50
40
30
20
10
=
0
V
( )(
10
IN
10 20 30 40 50 60 70 80 90 100
−
f L
( )
I
I
AT 5% DUTY CYCLE
I
AT 5% DUTY CYCLE
V
OUT
V
RIPPLE
RIPPLE
IN
SF
OUT
= 4.2V
DUTY CYCLE (%)
)(
= 0.4I
= 0.2I
100
⎛
⎜
⎝
V
PK
PK
V
OUT
RIPPLE
)
IN
+
+
V
V
, decreases with higher
D
D
⎞
⎟
⎠
1772 F02
SENSE
is:
IN
1772fb
or