6
LTC1265/LTC1265-3.3/LTC1265-5
capacitor discharges past V
TH1
until its voltage drops
below V
TH2
. Comparator S then trips and a sleep signal is
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 160µA. 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
SENSE
.
To avoid the operation of the current loop interfering with
Burst Mode operation, a built-in offset V
OS
is incorporated
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
IN
drops
below V
OUT
+ 2V. In dropout the P-channel MOSFET is
turned on continuously (100% duty cycle) providing low
dropout operation with V
OUT
≅ V
IN
.
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
SENSE
.
Once R
SENSE
is known, C
T
and L can be chosen. Next, the
Schottky diode D1 is selected followed by C
IN
and C
OUT
.
R
SENSE
Selection for Output Current
R
SENSE
is chosen based on the required output current.
With the current comparator monitoring the voltage devel-
oped across R
SENSE
, the threshold of the comparator
determines the peak inductor current. Depending on the
load current condition, the threshold of the comparator
lies between 25mV/R
SENSE
and 150mV/R
SENSE
. The maxi-
mum output current of the LTC1265 is:
I
OUT(MAX)
=
150mV
R
SENSE
I
RIPPLE
2
–
(Amps)
where I
RIPPLE
is the peak-to-peak inductor ripple current.
At a relatively light load, the LTC1265 is in Burst Mode
operation. In this mode the peak inductor current is set at
25mV/R
SENSE
. To fully benefit from Burst Mode operation,
the inductor current should be continuous during burst
periods. Hence, the peak-to-peak inductor ripple current
must not exceed 25mV/R
SENSE
.
To account for light and heavy load conditions, the I
OUT(MAX)
is then given by:
I
OUT(MAX)
=
150mV
R
SENSE
25mV
2 • R
SENSE
137.5mV
R
SENSE
–
=
(Amps)
(Amps)
Solving for R
SENSE
and allowing a margin of variations in
the LTC1265 and extended component values yields:
R
SENSE
=
100mV
I
OUT(MAX)
(Ω)
The LTC1265 is rated with a capability to supply a maximum
of 1.2A of output current.
Therefore, the minimum value of
R
SENSE
that can be used is 0.083
Ω
.
A graph for selecting
R
SENSE
versus maximum output is given in Figure 2.
Figure 2. Selecting R
SENSE
(Refer to Functional Diagram)
MAXIMUM OUTPUT CURRENT (A)
0
R
SENSE
(Ω)
0.3
0.4
0.5
0.8
1265 G10
0.2
0.1
0
0.2
0.4
0.6
1
OPERATIO
U
APPLICATIO S I FOR ATIO
WUUU
