LTC1772BES6#TRMPBF Linear Technology, LTC1772BES6#TRMPBF Datasheet - Page 8
LTC1772BES6#TRMPBF
Manufacturer Part Number
LTC1772BES6#TRMPBF
Description
IC CTRLR DC/DC S-DWN SOT23-6
Manufacturer
Linear Technology
Type
Step-Down (Buck)r
Datasheet
1.LTC1772BES6TRMPBF.pdf
(16 pages)
Specifications of LTC1772BES6#TRMPBF
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
Dc To Dc Converter Type
Step Down
Pin Count
6
Input Voltage
2.5 to 9.8V
Output Voltage
0.8 to 9.8V
Output Current
5A
Package Type
TSOT-23
Mounting
Surface Mount
Operating Temperature Classification
Industrial
Operating Temperature (min)
-40C
Operating Temperature (max)
85C
Lead Free Status / RoHS Status
Lead free / RoHS Compliant
Power - Output
-
Lead Free Status / Rohs Status
Compliant
Other names
LTC1772BES6#TRMPBFTR
Available stocks
Company
Part Number
Manufacturer
Quantity
Price
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