LTC1707 데이터 시트보기 (PDF) - Linear Technology
부품명
상세내역
제조사
LTC1707 Datasheet PDF : 16 Pages
| |||
U
OPERATIO
Frequency synchronization is inhibited when the feedback
voltage VFB 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 VIN 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
1200
1000
800
VOUT = 1.8V
VOUT = 1.5V
600
VOUT = 5V
400
200
0
2.5
VOUT = 3.3V
VOUT = 2.5V
VOUT = 2.9V
3.5 4.5 5.5 6.5
INPUT VOLTAGE (V)
TJ = 25°C
L = 15µH
7.5 8.5
1707 F02a
Figure 2a. Maximum Output Current
vs Input Voltage (Unsynchronized)
LTC1707
switch is on continuously. Hence, the I2R loss is due
mainly to the RDS(ON) of the P-channel MOSFET. See
Efficiency Considerations in the Applications Information
section.
Below VIN = 4V, the output current must be derated as
shown in Figures 2a and 2b. For applications that require
500mA below VIN = 4V, select the LTC1627.
1200
1000
800
VOUT = 1.8V
VOUT = 1.5V
600
VOUT = 5V
400
200
0
2.5
VOUT = 3.3V
VOUT = 2.5V
VOUT = 2.9V
TJ = 25°C
L = 15µH
EXT SYNC AT 400kHz
3.5 4.5 5.5 6.5 7.5 8.5
INPUT VOLTAGE (V)
1707 F02b
Figure 2b. Maximum Output Current
vs Input Voltage (Synchronized)
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 VOUT/VIN > 0.4 than when VOUT/VIN < 0.4. See the
inductor peak current as a function of duty cycle graph in
Figure 3. The worst-case peak current reduction occurs
1000
900
800
700
WITHOUT
EXTERNAL
CLOCK SYNC
WORST-CASE
EXTERNAL
CLOCK SYNC
600
VIN = 4V
500
0 10 20 30 40 50 60 70 80 90 100
DUTY CYCLE (%)
1707 F03
Figure 3. Maximum Inductor Peak Current vs Duty Cycle
7
Share Link: 