LTC1701(1999) 데이터 시트보기 (PDF) - Linear Technology
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LTC1701 Datasheet PDF : 12 Pages
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LTC1701
U
OPERATIO
The LTC1701 uses a contant off-time, current mode
architecture. The operating frequency is then determined
by the off-time and the difference between VIN and VOUT.
To optimize efficiency, the LTC1701 automatically switches
between continuous and Burst Mode operation.
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
When the load is relatively light, the LTC1701 automati-
cally switches to Burst Mode operation in which the
internal PMOS switch operates intermittently based on
load demand. The main control loop is interrupted when
the output voltage reaches the desired regulated value.
The hysteretic voltage comparator trips when ITH/RUN is
below 1.5V, shutting off the switch and reducing 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.
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
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
=
VIN − VOUT
VIN + VD
1
TOFF
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:
∆IL
=
VIN
− VOUT
fL
VOUT + VD
VIN + VD
4
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