7

LTC1707

Frequency synchronization is inhibited when the feedback

from interfering with the frequency foldback for short-

circuit protection.

Dropout Operation

When the input supply voltage decreases toward the out-

put voltage, the duty cycle increases toward the maximum

on-time. Further reduction of the supply voltage forces the

main switch to remain on for more than one cycle until it

reaches 100% duty cycle. The output voltage will then be

determined by the input voltage minus the voltage drop

across the P-channel MOSFET and the inductor.

In Burst Mode operation or pulse skipping mode operation

with the output lightly loaded, the LTC1707 transitions

through continuous mode as it enters dropout.

Undervoltage Lockout

A precision undervoltage lockout shuts down the LTC1707

lithium-ion battery applications. In lockout, the LTC1707

draws only several microamperes, which is low enough to

prevent deep discharge and possible damage to the lithium-

ion battery nearing its end of charge. A 100mV hysteresis

ensures reliable operation with noisy input supplies.

Low Supply Operation

The LTC1707 is designed to operate down to a 2.85V input

voltage. At this voltage the converter is most likely to be

running at high duty cycles or in dropout where the main

Efficiency Considerations in the Applications Information

section.

shown in Figures 2a and 2b. For applications that require

Figure 2a. Maximum Output Current

vs Input Voltage (Unsynchronized)

INPUT VOLTAGE (V)

2.5

0

200

400

600

6.5

1200

1000

1707 F02a

4.5

3.5

7.5

5.5

8.5

800

L = 15

µH

INPUT VOLTAGE (V)

2.5

0

200

400

600

6.5

1200

1000

1707 F02b

4.5

3.5

7.5

5.5

8.5

800

L = 15

µH

EXT SYNC AT 400kHz

Figure 2b. Maximum Output Current

vs Input Voltage (Synchronized)

Figure 3. Maximum Inductor Peak Current vs Duty Cycle

0

10 20 30 40 50 60 70 80 90 100

1000

900

800

700

600

500

DUTY CYCLE (%)

1707 F03

WORST-CASE

EXTERNAL

CLOCK SYNC

WITHOUT

EXTERNAL

CLOCK SYNC

Slope Compensation and Inductor Peak Current

Slope compensation provides stability by preventing sub-

harmonic oscillations. It works by internally adding a ramp

to the inductor current signal at duty cycles in excess of

40%. As a result, the maximum inductor peak current is

inductor peak current as a function of duty cycle graph in

Figure 3. The worst-case peak current reduction occurs

OPERATIO