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

W

FUNCTIONAL DIAGRA (Pin 9 connection shown for LTC1265-3.3 and LTC1265-5; change create LTC1265)

1, 13 PWR VIN

12 PGND

14 SW

V

SENSE+

8

SENSE–

7

VFB

9 ADJUSTABLE

VERSION

SLEEP

S

VTH2

5 CT

R

Q

S

VTH1 –

T

+

OFF-TIME

CONTROL

C

25mV TO 150mV

13k

ITH 6

2 VIN

SENSE–

VFB

3 LB0UT

A3

11 SGND

5pF

VOS

G

100k

REFERENCE

10

SHDN

4 LBIN

1265 FD

U

OPERATION (Refer to Functional Diagram)

The LTC1265 uses a constant off-time architecture to

switch its internal P-channel power MOSFET. The off time

is set by an external timing capacitor at CT (Pin 5). The

operating frequency is then determined by the off time and

the difference between VIN and VOUT.

The output voltage is set by an internal resistive divider

(LTC1265-3.3 and LTC1265-5) connected to SENSE–

(Pin 7) or an external divider returned to VFB (Pin 9 for

LTC1265). A voltage comparator V, and a gain block G,

compare the divided output voltage with a reference

voltage of 1.25V.

To optimize efficiency, the LTC1265 automatically switches

between continuous and Burst Mode operation. The volt-

age comparator is the primary control element when the

device is in Burst Mode operation, while the gain block

controls the output voltage in continuous mode.

When the load is heavy, the LTC1265 is in continuous

operation. During the switch ON time, current comparator

C monitors the voltage between Pins 7 and 8 connected

across an external shunt in series with the inductor. When

the voltage across the shunt reaches the comparator’s

threshold value, its output signal will change state, setting

the flip flop and turning the internal P-channel MOSFET off.

The timing capacitor connected to Pin 5 is now allowed to

discharge at a rate determined by the off-time controller.

When the voltage on the timing capacitor has discharged

past VTH1, comparator T trips, sets the flip flop and causes

the switch to turn on. Also, the timing capacitor is re-

charged. The inductor current will again ramp up until the

current comparator C trips. The cycle then repeats.

When the load current increases, the output voltage de-

creases slightly. This causes the output of the gain stage

(Pin 6) to increase the current comparator threshold, thus

tracking the load current.

When the load is relatively light, the LTC1265 automati-

cally goes into Burst Mode operation. The current loop is

interrupted when the output voltage exceeds the desired

regulated value. The hysteretic voltage comparator V trips

when VOUT is above the desired output voltage, shutting

off the switch and causing the capacitor to discharge. This

5

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