LTC1735 Linear Technology, LTC1735 Datasheet - Page 9

• Manufacturer Part Number: LTC1735
• Description: High Efficiency Synchronous Step-Down Switching Regulator
• Manufacturer: Linear Technology

OPERATIO
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
which I
Pin 3 (I
(V
receive an output feedback voltage V
resistive divider. When the load current increases, it
causes a slight decrease in V
ence, which in turn causes the I
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
the next cycle.
The top MOSFET driver is powered from a floating boot-
strap capacitor C
from INTV
MOSFET is turned off. As V
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 A
current source to charge soft-start capacitor C
C
I
value. As C
leased allowing normal operation to resume. If V
not reached 70% of its final value when C
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
An overvoltage comparator, OV, guards against transient
overshoots (>7.5%) as well as other more serious
TH
SS
OSENSE
voltage clamped at approximately 30% of its maximum
reaches 1.5V, the main control loop is enabled with the
OSC
1
TH
1
.
resets the RS latch is controlled by the voltage on
), described in the pin functions, allows EA to
), which is the output of error amplifier EA. Pin 6
resets the RS latch. The peak inductor current at
CC
SS
through an external diode when the top
continues to charge, I
B
U
. This capacitor is normally recharged
(Refer to Functional Diagram)
IN
decreases towards V
FB
TH
relative to the 0.8V refer-
voltage to increase until
2
, or the beginning of
FB
TH
from an external
is gradually re-
SS
has charged
SS
OUT
OUT
. When
, the
has
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
the buffered I
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
Mode operation, if the error amplifier drives the I
below 0.86V, the buffered I
parator will be clamped at 0.86V. The inductor current
peak is then held at approximately 20mV/R
1/4 of maximum output current). If I
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
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.
TH
input to the current comparator is gradu-
TH
input to the current com-
OSENSE
TH
drops below 0.6V,
then drops below
LTC1735
SENSE
CC
). In Burst
TH
TH
voltage
(about
rises
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