LTC1266CS datasheet(9/20 Pages) LINER

9 / 20 page

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.

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

× 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.

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:

MAXIMUM OUTPUT CURRENT (A)

100

LTC1266 • F02

Figure 3. Timing Capacitor Value

FREQUENCY (kHz)

200

400

100

200

300

400

LTC1266 • F03

600

800

500

VIN = 12V

VIN = 5V

VOUT = 3.3V

LTC1266CS 数据表(PDF) 9 Page - Linear Technology

LTC1266CS 数据表(HTML) 9 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

数据表下载