LTC1265 LINER [Linear Technology], LTC1265 Datasheet - Page 6

• Manufacturer Part Number: LTC1265
• Description: 1.2A, High Efficiency Step-Down DC/DC Converter
• Manufacturer: LINER [Linear Technology]

OPERATION
The basic LTC1265 application circuit is shown in
Figure 1. External component selection is driven by the
load requirement, and begins with the selection of R
Once R
Schottky diode D1 is selected followed by C
R
R
With the current comparator monitoring the voltage devel-
oped across R
determines the peak inductor current. Depending on the
load current condition, the threshold of the comparator
lies between 25mV/R
mum output current of the LTC1265 is:
where I
At a relatively light load, the LTC1265 is in Burst Mode
operation. In this mode the peak inductor current is set at
25mV/R
the inductor current should be continuous during burst
periods. Hence, the peak-to-peak inductor ripple current
must not exceed 25mV/R
To account for light and heavy load conditions, the I
is then given by:
LTC1265/LTC1265-3.3/LTC1265-5
capacitor discharges past V
below V
generated. The circuit now enters into sleep mode with the
power MOSFET turned off. In sleep mode, the LTC1265 is
in standby and the load current is supplied by the output
capacitor. All unused circuitry is shut off, reducing quies-
cent current from 2mA to 160 A. When the output capaci-
tor discharges by the amount of the hysteresis of the
comparator V, the P-channel switch turns on again and the
process repeats itself. During Burst Mode operation the
peak inductor current is set at 25mV/R
APPLICATIONS
6
SENSE
SENSE
I
OUT(MAX)
SENSE
RIPPLE
Selection for Output Current
is chosen based on the required output current.
TH2
SENSE
. Comparator S then trips and a sleep signal is
is known, C
= 150mV
. To fully benefit from Burst Mode operation,
is the peak-to-peak inductor ripple current.
SENSE
R
U
SENSE
U
SENSE
, the threshold of the comparator
(Refer to Functional Diagram)
T
INFORMATION
–
SENSE
U
and L can be chosen. Next, the
and 150mV/R
I
RIPPLE
TH1
2
.
until its voltage drops
W
(Amps)
SENSE
SENSE
IN
.
. The maxi-
and C
U
OUT(MAX)
SENSE
OUT
.
.
To avoid the operation of the current loop interfering with
Burst Mode operation, a built-in offset V
in the gain stage. This prevents the current from increas-
ing until the output voltage has dropped below a minimum
threshold.
Using constant off-time architecture, the operating fre-
quency is a function of the voltage. To minimize the
frequency variation as dropout is approached, the off-time
controller increases the discharge current as V
below V
turned on continuously (100% duty cycle) providing low
dropout operation with V
Solving for R
the LTC1265 and extended component values yields:
The LTC1265 is rated with a capability to supply a maximum
of 1.2A of output current. Therefore, the minimum value of
R
R
SENSE
SENSE
I
=
R
OUT(MAX)
SENSE
137.5mV
R
SENSE
that can be used is 0.083 . A graph for selecting
versus maximum output is given in Figure 2.
OUT
0.5
0.4
0.1
0.3
0.2
= 100mV
0
+ 2V. In dropout the P-channel MOSFET is
0
SENSE
I
= 150mV
OUT(MAX)
(Amps)
R
Figure 2. Selecting R
MAXIMUM OUTPUT CURRENT (A)
SENSE
0.2
and allowing a margin of variations in
0.4
( )
OUT
–
2 • R
0.6
25mV
V
IN
SENSE
.
SENSE
0.8
OS
1265 G10
(Amps)
is incorporated
1
IN
drops