參數(shù)資料
型號(hào)： LTC1735-1
廠商： Linear Technology Corporation
元件分類(lèi)： 基準(zhǔn)電壓源/電流源
英文描述： RADIATION HARDENED HIGH EFFICIENCY, 5 AMP SWITCHING REGULATORS
中文描述： 抗輻射高效，5安培開(kāi)關(guān)穩(wěn)壓器
文件頁(yè)數(shù)： 9/32頁(yè)
文件大小： 373K
代理商： LTC1735-1
9
LTC1735
OPERATIOU
Main Control Loop
The LTC1735 uses a constant frequency, current mode
step-down architecture. During normal operation, the top
MOSFET is turned on each cycle when the oscillator sets
the RS latch and turned off when the main current com-
parator I
1
resets the RS latch. The peak inductor current at
which I
1
resets the RS latch is controlled by the voltage on
Pin 3 (I
TH
), which is the output of error amplifier EA. Pin6
(V
OSENSE
), described in the pin functions, allows EA to
receive an output feedback voltage V
FB
from an external
resistive divider. When the load current increases, it
causes a slight decrease in V
FB
relative to the 0.8V refer-
ence, which in turn causes the I
TH
voltage to increase until
the average inductor current matches the new load cur-
rent. While the top MOSFET is off, the bottom MOSFET is
turned on until either the inductor current starts to reverse,
as indicated by current comparator I
2
, or the beginning of
the next cycle.
The top MOSFET driver is powered from a floating boot-
strap capacitor C
B
. This capacitor is normally recharged
from INTV
CC
through an external diode when the top
MOSFET is turned off. As V
IN
decreases towards V
OUT
, the
converter will attempt to turn on the top MOSFET continu-
ously (“dropout’’). A dropout counter detects this condi-
tion and forces the top MOSFET to turn off for about 500ns
every tenth cycle to recharge the bootstrap capacitor.
The main control loop is shut down by pulling Pin 2
(RUN/SS) low. Releasing RUN/SS allows an internal 1.2
μ
A
current source to charge soft-start capacitor C
SS
. When
C
SS
reaches 1.5V, the main control loop is enabled with the
I
TH
voltage clamped at approximately 30% of its maximum
value. As C
SS
continues to charge, I
TH
is gradually re-
leased allowing normal operation to resume. If V
OUT
has
not reached 70% of its final value when C
SS
has charged
to 4.1V, latchoff can be invoked as described in the
Applications Information section.
The internal oscillator can be synchronized to an external
clock applied to the FCB pin and can lock to a frequency
between 90% and 130% of its nominal rate set by capaci-
tor C
OSC
.
An overvoltage comparator, OV, guards against transient
overshoots (>7.5%) as well as other more serious
(Refer to Functional Diagram)
conditions that may overvoltage the output. In this case,
the top MOSFET is turned off and the bottom MOSFET is
turned on until the overvoltage condition is cleared.
Foldback current limiting for an output shorted to ground
is provided by amplifier A. As V
OSENSE
drops below 0.6V,
the buffered I
TH
input to the current comparator is gradu-
ally pulled down to a 0.86V clamp. This reduces peak
inductor current to about 1/4 of its maximum value.
Low Current Operation
The LTC1735 has three low current modes controlled by
the FCB pin. Burst Mode operation is selected when the
FCB pin is above 0.8V (typically tied to INTV
CC
). In Burst
Mode operation, if the error amplifier drives the I
TH
voltage
below 0.86V, the buffered I
TH
input to the current com-
parator will be clamped at 0.86V. The inductor current
peak is then held at approximately 20mV/R
SENSE
(about
1/4 of maximum output current). If I
TH
then drops below
0.5V, the Burst Mode comparator B will turn off both
MOSFETs to maximize efficiency. The load current will be
supplied solely by the output capacitor until I
TH
rises
above the 60mV hysteresis of the comparator and switch-
ing is resumed. Burst Mode operation is disabled by
comparator F when the FCB pin is brought below 0.8V.
This forces continuous operation and can assist second-
ary winding regulation.
When the FCB pin is driven by an external oscillator, a low
noise cycle-skipping mode is invoked and the internal
oscillator is synchronized to the external clock by com-
parator C. In this mode the 25% minimum inductor
current clamp is removed, providing constant frequency
discontinuous operation over the widest possible output
current range. This constant frequency operation is not
quite as efficient as Burst Mode operation, but provides a
lower noise, constant frequency spectrum.
The FCB pin is tied to ground when forced continuous
operation is desired. This is the least efficient mode, but is
desirable in certain applications. The output can source or
sink current in this mode. When sinking current while in
forced continuous operation, current will be forced back
into the main power supply potentially boosting the input
supply to dangerous voltage levels—BEWARE.
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