LTC1772BES6#TRM Linear Technology, LTC1772BES6#TRM Datasheet - Page 8

• Manufacturer Part Number: LTC1772BES6#TRM
• Description: IC CTRLR DC/DC S-DWN SOT23-6
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1772BES6#TRM

• 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: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: TSOT-23-6, TSOT-6
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -
• Other names: LTC1772BES6#TRMTR

LTC1772B
APPLICATIONS INFORMATION
The basic LTC1772B application circuit is shown in Figure 1.
External component selection is driven by the load require-
ment and begins with the selection of L1 and R
Next, the power MOSFET, M1 and the output diode D1 is
selected followed by C
R
R
With the current comparator monitoring the voltage de-
veloped across R
determines the inductor’s peak current. The output current
the LTC1772B 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
select the appropriate value to provide the required amount
of current. Using Figure 3, 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 effi ciency due
to an increase in MOSFET gate charge losses.
8
RIPPLE
SENSE
SENSE
I
R
R
OUT
SENSE
SENSE
RIPPLE
= (0.4)(I
Selection for Output Current
is chosen based on the required output current.
=
R
=
=
0 105
SENSE
.
0 0875
( .
I
0 0875
is the inductor peak-to-peak ripple current
OUT
.
I
OUT
OUT
SENSE
( )
−
100
). Rearranging the above equation, it
I
)
RIPPLE
for Duty Cycle < 40%
SF
IN
, the threshold of the comparator
2
(= C1)and C
OUT
(= C2).
SENSE
SENSE
is:
(= R1).
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
in higher output voltage ripple and greater core losses.
A reasonable starting point for setting ripple current is
I
occurs at the maximum input voltage.
The ripple current is normally set such that the induc-
tor current is continuous down to approximately 1/4 of
maximum load current. This results in:
This implies a minimum inductance of:
A smaller value than L
however, the inductor current transitioning from continuous
to discontinuous will occur at a higher load current.
Power MOSFET Selection
An external P-channel power MOSFET must be selected
for use with the LTC1772B. The main selection criteria for
the power MOSFET are the threshold voltage V
the “on” resistance R
C
RIPPLE
OUT
RSS
(Use V
I
I
L
RIPPLE
RIPPLE
RIPPLE
MIN
. The inductor’s peak-to-peak ripple current is given
and total gate charge.
= 0.4(I
=
IN(MAX)
allows the use of low inductances, but results
=
f
≤
V
IN
V
R
R
OUT(MAX)
IN
0 03
0.03
SENSE
SENSE
f L
.
V
( )
= V
OUT
V
OUT
IN
DS(ON)
). Remember, the maximum I
)
V
MIN
V
OUT
V
IN
V
OUT
IN
, reverse transfer capacitance
could be used in the circuit;
RIPPLE
+ V
+ V
+ V
+ V
D
D
D
D
, decreases with higher
GS(TH)
RIPPLE
1772bfa
IN
and
or