LTC1265/LTC1265-3.3/LTC1265-5
U
OPERATION (Refer to Functional Diagram)
capacitor discharges past VTH1 until its voltage drops
below VTH2. Comparator S then trips and a sleep signal is
generated. Thecircuitnowentersintosleepmodewiththe
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
comparatorV,theP-channelswitchturnsonagainandthe
process repeats itself. During Burst Mode operation the
To avoid the operation of the current loop interfering with
BurstModeoperation, abuilt-inoffsetVOS isincorporated
in the gain stage. This prevents the current from increas-
inguntiltheoutputvoltagehasdroppedbelowaminimum
threshold.
Using constant off-time architecture, the operating fre-
quency is a function of the voltage. To minimize the
frequencyvariationasdropoutisapproached,theoff-time
controller increases the discharge current as VIN drops
below VOUT + 2V. In dropout the P-channel MOSFET is
turned on continuously (100% duty cycle) providing low
dropout operation with VOUT VIN.
peak inductor current is set at 25mV/RSENSE
.
U
W U U
APPLICATIONS INFORMATION
The basic LTC1265 application circuit is shown in
Figure 1. External component selection is driven by the
150mV
25mV
2 • R
I
=
(Amps)
OUT(MAX)
–
R
SENSE
SENSE
loadrequirement, andbeginswiththeselectionofRSENSE
.
137.5mV
=
Once RSENSE is known, CT and L can be chosen. Next, the
(Amps)
R
SENSE
Schottky diode D1 is selected followed by CIN and COUT
.
Solving for RSENSE and allowing a margin of variations in
the LTC1265 and extended component values yields:
RSENSE Selection for Output Current
RSENSE is chosen based on the required output current.
Withthecurrentcomparatormonitoringthevoltagedevel-
oped across RSENSE, the threshold of the comparator
determines the peak inductor current. Depending on the
load current condition, the threshold of the comparator
liesbetween25mV/RSENSE and150mV/RSENSE. Themaxi-
mum output current of the LTC1265 is:
100mV
OUT(MAX)
R
=
(Ω)
SENSE
I
TheLTC1265isratedwithacapabilitytosupplyamaximum
of1.2Aofoutputcurrent.Therefore, theminimumvalueof
RSENSE that can be used is 0.083Ω. A graph for selecting
RSENSE versus maximum output is given in Figure 2.
150mV
I
RIPPLE
I
=
(Amps)
OUT(MAX)
–
0.5
0.4
0.3
0.2
0.1
0
R
2
SENSE
where IRIPPLE 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/RSENSE.TofullybenefitfromBurstModeoperation,
the inductor current should be continuous during burst
periods. Hence, the peak-to-peak inductor ripple current
must not exceed 25mV/RSENSE
.
Toaccountforlightandheavyloadconditions,theIOUT(MAX)
is then given by:
0
0.2
0.4
0.6
0.8
1
MAXIMUM OUTPUT CURRENT (A)
1265 G10
Figure 2. Selecting RSENSE
6
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