LTC1612 데이터 시트보기 (PDF) - Linear Technology
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LTC1612 Datasheet PDF : 8 Pages
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LT1612
BLOCK DIAGRA
VIN 3
VC 1
FB 2
MODE 7
–
A1
V/I
+
0.62V
–+
A3
0.7V
ENABLE
RSENSE
0.08Ω
SLOPE
A2
COMPENSATION
FLIP-FLOP
R
Q
S
SWITCH
DRIVER
RECTIFIER
DRIVE
BOOST DIODE
6 BOOST
SWITCH
5 SW
RECTIFIER
SHDN 8
SHUTDOWN
OSCILLATOR
U
OPERATIO
The LT1612 employs fixed frequency, current mode con-
trol. This type of control uses two feedback loops. The
main control loop sets output voltage and operates as
follows: A load step causes VOUT and the FB voltage to be
perturbed slightly. The error amplifier responds to this
change in FB by driving the VC pin either higher or lower.
Because switch current is proportional to the VC pin
voltage, this change causes the switch current to be
adjusted until VOUT is once again satisfied. Loop compen-
sation is taken care of by an RC network from the VC pin
to ground.
Inside this main loop is another that sets current limit on
a cycle-by-cycle basis. This loop utilizes current compara-
tor A2 to control peak current. The oscillator runs at
800kHz and issues a set pulse to the flip-flop at the
beginning of each cycle, turning the switch on. With the
switch now in the ON state the SW pin is effectively
connected to VIN. Current ramps up in the inductor linearly
4 GND
1612 BD
at a rate of (VIN – VOUT)/L. Switch current is set by the VC
pin voltage and when the voltage across RSENSE trips the
current comparator, a reset pulse will be generated and the
switch will be turned off. Since the inductor is now loaded
up with current, the SW pin will fly low and trigger the
rectifier to turn on. Current will flow through the rectifier
decreasing at a rate of VOUT/L until the oscillator issues a
new set pulse, causing the cycle to repeat.
If the load is light and VC decreases below A3’s trip point,
the device will enter the Burst Mode operation region (the
MODE pin must be at ground or floating). In this state the
oscillator and all other circuitry except the reference and
comparator A3 are switched on and off at low frequency.
This mode of operation increases efficiency at light loads
but introduces low frequency voltage ripple at the output.
For continuous switching and no low frequency output
voltage ripple, pull the MODE pin high. This will disable
comparator A3 which forces the oscillator to run
continuously.
4
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