5

LTC1265/LTC1265-3.3/LTC1265-5

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

The LTC1265 uses a constant off-time architecture to

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

operating frequency is then determined by the off time and

The output voltage is set by an internal resistive divider

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

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

off the switch and causing the capacitor to discharge. This

(Refer to Functional Diagram)

5pF

100k

SGND

SLEEP

PGND

SW

13k

ADJUSTABLE

VERSION

25mV TO 150mV

OFF-TIME

CONTROL

SHDN

V

C

R

S

Q

–

+

T

S

A3

REFERENCE

G

1265 FD

5

2

3

6

11

4

10

9

7

8

14

12

1, 13