LTC1265I Ver la hoja de datos (PDF) - Linear Technology

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Fabricante

LTC1265I

1.2A, High Efficiency Step-Down DC/DC Converter

Linear Technology 

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. 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/RSENSE.

To avoid the operation of the current loop interfering with

Burst Mode operation, a built-in offset VOS 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 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.

APPLICATIONS INFORMATION

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 RSENSE.

Once RSENSE is known, CT and L can be chosen. Next, the

Schottky diode D1 is selected followed by CIN and COUT.

RSENSE Selection for Output Current

RSENSE is chosen based on the required output current.

With the current comparator monitoring the voltage devel-

oped across RSENSE, the threshold of the comparator

determines the peak inductor current. Depending on the

load current condition, the threshold of the comparator

lies between 25mV/RSENSE and 150mV/RSENSE. The maxi-

mum output current of the LTC1265 is:

IOUT(MAX)

=

150mV

RSENSE

–

IRIPPLE

2

(Amps)

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. 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/RSENSE.

To account for light and heavy load conditions, the IOUT(MAX)

is then given by:

IOUT(MAX)

=

150mV

RSENSE

–

25mV

2 • RSENSE

(Amps)

= 137.5mV (Amps)

RSENSE

Solving for RSENSE and allowing a margin of variations in

the LTC1265 and extended component values yields:

RSENSE

=

100mV

IOUT(MAX)

(Ω)

The LTC1265 is rated with a capability to supply a maximum

of 1.2A of output current. Therefore, the minimum value of

RSENSE that can be used is 0.083Ω. A graph for selecting

RSENSE versus maximum output is given in Figure 2.

0.5

0.4

0.3

0.2

0.1

0

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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