Datasheet

7
LTC1707
Frequency synchronization is inhibited when the feedback
voltage V
FB
is below 0.6V. This prevents the external clock
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
when V
IN
drops below 2.7V, making it ideal for single
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
switch is on continuously. Hence, the I
2
R loss is due
mainly to the R
DS(ON)
of the P-channel MOSFET. See
Efficiency Considerations in the Applications Information
section.
Below V
IN
= 4V, the output current must be derated as
shown in Figures 2a and 2b. For applications that require
500mA below V
IN
= 4V, select the LTC1627.
Figure 2a. Maximum Output Current
vs Input Voltage (Unsynchronized)
INPUT VOLTAGE (V)
2.5
0
OUTPUT CURRENT (mA)
200
400
600
6.5
1200
1000
1707 F02a
4.5
3.5
7.5
5.5 8.5
800
T
J
= 25°C
L = 15µH
V
OUT
= 5V
V
OUT
= 1.5V
V
OUT
= 2.5V
V
OUT
= 2.9V
V
OUT
= 3.3V
V
OUT
= 1.8V
INPUT VOLTAGE (V)
2.5
0
OUTPUT CURRENT (mA)
200
400
600
6.5
1200
1000
1707 F02b
4.5
3.5
7.5
5.5 8.5
800
T
J
= 25°C
L = 15µH
EXT SYNC AT 400kHz
V
OUT
= 5V
V
OUT
= 1.5V
V
OUT
= 2.9V
V
OUT
= 3.3V
V
OUT
= 1.8V
V
OUT
= 2.5V
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
MAXIMUM INDUCTOR PEAK CURRENT (mA)
WORST-CASE
EXTERNAL
CLOCK SYNC
WITHOUT
EXTERNAL
CLOCK SYNC
V
IN
= 4V
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
lower for V
OUT
/V
IN
> 0.4 than when V
OUT
/V
IN
< 0.4. See the
inductor peak current as a function of duty cycle graph in
Figure 3. The worst-case peak current reduction occurs
OPERATIO
U