LTC1143CS datasheet(9/20 Pages) LINER

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LTC1143/LTC1143L

LTC1143L-ADJ

APPLICATIONS INFORMATION

MAXIMUM OUTPUT CURRENT (A)

0.20

0.15

0.10

0.05

1143 F02

Figure 2. Selecting RSENSE

The load current below which Burst Mode operation

commences, IBURST, and the peak short circuit current,

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) =

150mV

RSENSE

The LTC1143 series automatically extends tOFF during a

short circuit to allow sufficient time for the inductor

current to decay between switch cycles. The resulting

ISC(AVG) to be reduced to approximately IMAX.

L and CT Selection for Operating Frequency

Each regulator section of the LTC1143 series uses a

constant off-time architecture with tOFF determined by an

external timing capacitor CT. Each time the P-channel

MOSFET switch turns on the voltage on CT is reset to

approximately 3.3V. During the off-time, CT is discharged

by a current that is proportional 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:

OUT









−









13 10

where VD is the drop across the diode.

A graph for selecting CT versus frequency including the

effects of input voltage is given in Figure 3.

As the operating frequency is increased the gate charge

losses will be higher, reducing efficiency (see Efficiency

Considerations). The complete expression for operating

frequency of the circuit in Figure 1 is given by:

OFF

OUT

≈−









where:

OFF

REG

OUT

















13 104

VREG is the desired output voltage (i.e., 5V, 3.3V). VOUT

is the measured output voltage. Thus VREG/VOUT = 1 in

regulation.

Note that as VIN decreases, the frequency decreases.

When the input-to-output voltage differential drops below

1.5V for a particular section, the LTC1143 series reduces

tOFF in that section by increasing the discharge current in

CT. This prevents audible operation prior to dropout.

FREQUENCY (kHz)

100

150

200

LTC1143 F03

250

1000

800

600

400

200

300

VIN = 12V

VIN = 10V

VIN = 7V

VSENSE = VOUT = 5V

Figure 3. Timing Capacitor Value

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