LTC1707 LINER [Linear Technology], LTC1707 Datasheet - Page 7

• Manufacturer Part Number: LTC1707
• Description: High Efficiency Monolithic Synchronous Step-Down Switching Regulator
• Manufacturer: LINER [Linear Technology]

OPERATIO
Frequency synchronization is inhibited when the feedback
voltage V
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
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
IN
FB
1200
1000
drops below 2.7V, making it ideal for single
800
400
600
200
is below 0.6V. This prevents the external clock
Figure 2a. Maximum Output Current
vs Input Voltage (Unsynchronized)
0
2.5
V
OUT
V
OUT
V
U
OUT
3.5
= 1.5V
= 1.8V
V
V
OUT
= 2.9V
OUT
INPUT VOLTAGE (V)
4.5
= 3.3V
= 2.5V
5.5
V
OUT
6.5
= 5V
T
L = 15 H
7.5
J
= 25 C
1707 F02a
8.5
switch is on continuously. Hence, the I
mainly to the R
Efficiency Considerations in the Applications Information
section.
Below V
shown in Figures 2a and 2b. For applications that require
500mA below V
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
inductor peak current as a function of duty cycle graph in
Figure 3. The worst-case peak current reduction occurs
Figure 3. Maximum Inductor Peak Current vs Duty Cycle
IN
1200
1000
1000
OUT
800
400
600
200
= 4V, the output current must be derated as
900
800
700
600
500
0
Figure 2b. Maximum Output Current
vs Input Voltage (Synchronized)
2.5
0
/V
V
V
OUT
IN
IN
10 20 30 40 50 60 70 80 90 100
IN
DS(ON)
V
V
= 4V
3.5
OUT
OUT
= 1.5V
= 4V, select the LTC1627.
> 0.4 than when V
V
V
= 2.9V
= 1.8V
OUT
OUT
WORST-CASE
INPUT VOLTAGE (V)
CLOCK SYNC
4.5
= 2.5V
EXTERNAL
DUTY CYCLE (%)
= 3.3V
of the P-channel MOSFET. See
5.5
T
L = 15 H
EXT SYNC AT 400kHz
J
= 25 C
6.5
V
OUT
WITHOUT
EXTERNAL
CLOCK SYNC
= 5V
OUT
7.5
1707 F02b
/V
1707 F03
LTC1707
8.5
2
IN
R loss is due
< 0.4. See the
7