LTC1266-3.3 Ver la hoja de datos (PDF) - Linear Technology
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LTC1266-3.3 Datasheet PDF : 20 Pages
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LTC1266
LTC1266-3.3/LTC1266-5
APPLICATIO S I FOR ATIO
RSENSE Selection for Output Current
RSENSE is chosen based on the required output current.
The LTC1266 series current comparator has a threshold
range which extends from a minimum of 25mV/RSENSE
(when Burst Mode operation is enabled) to a maximum of
155mV/RSENSE. The current comparator threshold sets
the peak of the inductor ripple current, yielding a maxi-
mum output current IMAX equal to the peak value less half
the peak-to-peak ripple current. For proper Burst Mode
operation, IRIPPLE(P-P) must be less than or equal to the
minimum current comparator threshold.
Since efficiency generally increases with ripple current,
the maximum allowable ripple current is assumed, i.e.,
IRIPPLE(P-P) = 25mV/RSENSE (see CT and L Selection for
Operating Frequency). Solving for RSENSE and allowing
a margin for variations in the LTC1266 series and
external component values yields:
RSENSE
=
100mV
IMAX
A graph for selecting RSENSE vs maximum output
current is given in Figure 2.
100
75
50
25
0
0
2
4
6
8
10
MAXIMUM OUTPUT CURRENT (A)
1266 F02
Figure 2. Selecting RSENSE
The load current, below which Burst Mode operation
commences, (IBURST), and the peak short-circuit cur-
rent, (ISC(PK)), both track IMAX. Once RSENSE has been
chosen, IBURST and ISC(PK) can be predicted from the
following:
IBURST
≈
15mV
RSENSE
ISC(PK) =
155mV
RSENSE
The LTC1266 series automatically extends tOFF during a
short circuit to allow sufficient time for the inductor
current to decay between switch cycles. The resulting
ripple current causes the average short-circuit current
ISC(AVG) to be reduced to approximately IMAX.
L and CT Selection for Operating Frequency
The LTC1266 series uses a constant off-time architecture
with tOFF determined by an external timing capacitor CT.
Each time the topside MOSFET switch turns on, the
voltage on CT is reset to approximately 3.3V. During the
off-time, CT is discharged by a current which is propor-
tional to VOUT. The voltage on CT is analogous to the
current in inductor L, which likewise decays at a rate
proportional to VOUT. Thus the inductor value must track
the timing capacitor value.
The value of CT is calculated from the desired continuous
mode operating frequency, f:
CT
=
2.6
•
1
104
•
f
assumes VIN = 2VOUT, (Figure 1 circuit).
A graph for selecting CT vs frequency including the effects
of input voltage is given in Figure 3.
800
VOUT = 3.3V
600
400
VIN = 12V
200
VIN = 5V
0
0
100 200 300 400 500
FREQUENCY (kHz)
1266 F03
Figure 3. Timing Capacitor Value
9
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