LTC1435 Linear Technology, LTC1435 Datasheet - Page 7

• Manufacturer Part Number: LTC1435
• Description: High Efficiency Low Noise Synchronous Step-Down Switching Regulator
• Manufacturer: Linear Technology

Main Control Loop
The LTC1435 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
comparator I1 resets the RS latch. The peak inductor
current at which I1 resets the RS latch is controlled by the
voltage on the I
EA. The V
section, allows EA to receive an output feedback voltage
V
current increases, it causes a slight decrease in V
relative to the 1.19V reference, which in turn causes the I
voltage to increase until the average inductor current
matches the new load current. 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 I2, or the beginning of the next cycle.
The top MOSFET driver is biased from floating bootstrap
capacitor C
cycle. However, when V
V
the top MOSFET continuously. The dropout detector counts
the number of oscillator cycles that the top MOSFET
remains on and periodically forces a brief off period to
allow C
The main control loop is shut down by pulling the RUN/SS
pin low. Releasing RUN/SS allows an internal 3 A current
source to charge soft start capacitor C
reaches 1.3V, the main control loop is enabled with the I
voltage clamped at approximately 30% of its maximum
value. As C
leased allowing normal operation to resume.
Comparator OV guards against transient overshoots
> 7.5% by turning off the top MOSFET and keeping it off
until the fault is removed.
OPERATION
FB
OUT
from an external resistive divider. When the load
, the loop may enter dropout and attempt to turn on
B
to recharge.
OSENSE
B
SS
, which normally is recharged during each off
TH
continues to charge, I
pin , which is the output of error amplifier
U
pin, described in the Pin Functions
(Refer to Functional Diagram)
IN
decreases to a voltage close to
TH
is gradually re-
SS
. When C
SS
TH
TH
FB
Low Current Operation
The LTC1435 is capable of Burst Mode operation in which
the external MOSFETs operate intermittently based on
load demand. The transition to low current operation
begins when comparator I2 detects current reversal and
turns off the bottom MOSFET. If the voltage across R
does not exceed the hysteresis of I2 (approximately 20mV)
for one full cycle, then on following cycles the top and
bottom drives are disabled. This continues until an induc-
tor current peak exceeds 20mV/R
exceeds 0.6V, either of which causes drive to be returned
to the TG pin on the next cycle.
Two conditions can force continuous synchronous opera-
tion, even when the load current would otherwise dictate
low current operation. One is when the common mode
voltage of the SENSE
the other is when the SFB pin is below 1.19V. The latter
condition is used to assist in secondary winding regulation
as described in the Applications Information section.
INTV
Power for the top and bottom MOSFET drivers and most
of the other LTC1435 circuitry is derived from the INTV
pin. The bottom MOSFET driver supply pin is internally
connected to INTV
pin is left open, an internal 5V low dropout regulator
supplies INTV
the 5V regulator is turned off and an internal switch is
turned on to connect EXTV
INTV
external source such as the output of the regulator itself
or a secondary winding, as described in the Applications
Information section.
CC
CC
/EXTV
power to be derived from a high efficiency
CC
CC
Power
power. If EXTV
CC
+
in the LTC1435. When the EXTV
and SENSE
CC
to INTV
CC
SENSE
–
pins is below 1.4V and
is taken above 4.8V,
CC
or the I
. This allows the
LTC1435
TH
voltage
SENSE
7
CC
CC