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参数资料
| 型号： | LTC1727ES5-5 |
| 英文描述： | Industrial Control IC |
| 中文描述： | 工业控制IC |
| 文件页数： | 8/12页 |
| 文件大小： | 178K |
| 代理商： | LTC1727ES5-5 |
5
LTC1701/LTC1701B
OPERATIO
U
The LTC1701 uses a contant off-time, current mode archi-
tecture. The operating frequency is then determined by the
off-time and the difference between VIN and VOUT.
The output voltage is set by an external divider returned to
the VFB pin. An error amplfier compares the divided output
voltage with a reference voltage of 1.25V and adjusts the
peak inductor current accordingly.
Main Control Loop
During normal operation, the internal PMOS switch is
turned on when the VFB voltage is below the reference
voltage. The current into the inductor and the load in-
creases until the current limit is reached. The switch turns
off and energy stored in the inductor flows through the
external Schottky diode into the load. After the constant
off-time interval, the switch turns on and the cycle repeats.
The peak inductor current is controlled by the voltage on
the ITH/RUN pin, which is the output of the error
amplifier.This amplifier compares the VFB pin to the 1.25V
reference. When the load current increases, the FB voltage
decreases slightly below the reference. This decrease
causes the error amplifier to increase the ITH/RUN voltage
until the average inductor current matches the new load
current.
The main control loop is shut down by pulling the ITH/RUN
pin to ground. When the pin is released an external resistor
is used to charge the compensation capacitor. When the
voltage at the ITH/RUN pin reaches 0.8V, the main control
loop is enabled and the error amplifier drives the ITH/RUN
pin. Soft-start can be implemented by ramping the voltage
on the ITH/RUN pin (see Applications Information sec-
tion).
Low Current Operation
To optimize efficiency when the load is relatively light, the
LTC1701 automatically switches to Burst Mode operation
in which the internal PMOS switch operates intermittently
based on load demand. The main control loop is inter-
rupted when the output voltage reaches the desired regu-
lated value. The hysteretic voltage comparator trips when
ITH/RUN is below 1.5V, shutting off the switch and reduc-
ing the power consumed. The output capacitor and the
inductor supply the power to the load until the output
voltage drops slightly and the ITH/RUN pin exceeds 1.5V,
turning on the switch and the main control loop which
starts another cycle.
For reduced output ripple, the LTC1701B doesn't use
Burst Mode operation and operates continuously down to
very low currents where the part starts skipping cycles.
Dropout Operation
In dropout, the internal PMOS switch is turned on continu-
ously (100% duty cycle) providing low dropout operation
with VOUT at VIN. Since the LTC1701 does not incorporate
an under voltage lockout, care should be taken to shut
down the LTC1701 for VIN < 2.5V.
APPLICATIO S I FOR ATIO
WU
U
The basic LTC1701 application circuit is shown in
Figure 1. External component selection is driven by the
load requirement and begins with the selection of L1. Once
L1 is chosen, the Schottky diode D1 can be selected
followed by CIN and COUT.
L Selection and Operating Frequency
The operating frequency is fixed by VIN, VOUT and the
constant off-time of about 500ns. The complete expres-
sion for operating frequency is given by:
fO =
+
VV
T
IN
OUT
IN
D
OFF
1
Although the inductor does not influence the operating
frequency, the inductor value has a direct effect on ripple
current. The inductor ripple current
IL decreases with
higher inductance and increases with higher VIN or VOUT:
=
+
I
VV
fL
VV
L
IN
OUT
D
IN
D
where VD is the output Schottky diode forward drop.
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