ltc1772b Linear Technology Corporation, ltc1772b Datasheet - Page 6

• Manufacturer Part Number: ltc1772b
• Description: Constant Frequency Current Mode Step-down Dc/dc Controller In Sot-23
• Manufacturer: Linear Technology Corporation

OPERATIO
LTC1772B
Overvoltage Protection
As a further protection, the overvoltage comparator in the
LTC1772B will turn the external MOSFET off when the
feedback voltage has risen 7.5% above the reference
voltage of 0.8V. This comparator has a typical hysteresis
of 20mV.
Slope Compensation and Inductor’s Peak Current
The inductor’s peak current is determined by:
when the LTC1772B is operating below 40% duty cycle.
However, once the duty cycle exceeds 40%, slope
APPLICATIONS
The basic LTC1772B application circuit is shown
in Figure 1. External component selection is driven by the
load requirement and begins with the selection of L1 and
R
output diode D1 is selected followed by C
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 LTC1772B 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
OUT
SENSE
SENSE
SENSE
I
I
R
PK
OUT
SENSE
(= C2).
RIPPLE
= (0.4)(I
Selection for Output Current
(= R1). Next, the power MOSFET, M1 and the
is chosen based on the required output current.
10
V
ITH
R
0 105
SENSE
R
.
0 0875
– . 0 85
SENSE
is the inductor peak-to-peak ripple current
.
I
SENSE
OUT
OUT
U
U
). Rearranging the above equation, it
I
, the threshold of the comparator
RIPPLE
(Refer to Functional Diagram)
for Duty Cycle < 40%
2
INFORMATION
U
W
IN
(= C1)and
U
compensation begins and effectively reduces the peak
inductor current. The amount of reduction is given by the
curves in Figure 3.
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 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 3. Maximum Output Current vs Duty Cycle
110
100
90
80
70
60
50
40
30
20
10
0
V
( .
0 0875
I
IN
V
OUT
10 20 30 40 50
IN
f L
= 4.2V
I
AT 5% DUTY CYCLE
I
AT 5% DUTY CYCLE
RIPPLE
RIPPLE
V
100
OUT
)
SF
= 0.4I
= 0.2I
DUTY CYCLE (%)
PK
PK
V
V
OUT
RIPPLE
IN
60
V
70 80 90 100
, decreases with higher
V
D
D
1772 F03
SENSE
is:
IN
or