LTC1435A Linear Technology, LTC1435A Datasheet - Page 7

• Manufacturer Part Number: LTC1435A
• Description: High Efficiency Low Noise Synchronous Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIONS
OPERATION
The basic LTC1435A application circuit is shown in Figure
1, High Efficiency Step-Down Converter. External compo-
nent selection is driven by the load requirement and begins
with the selection of R
and L can be chosen. Next, the power MOSFETs and D1 are
selected. Finally, C
shown in Figure 1 can be configured for operation up to an
input voltage of 28V (limited by the external MOSFETs).
R
R
LTC1435A current comparator has a maximum threshold
of 150mV/R
SGND to INTV
the peak of the inductor current, yielding a maximum av-
erage output current I
the peak-to-peak ripple current I
on and periodically forces a brief off period to allow C
recharge.
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
1.3V, the main control loop is enabled with the I
clamped at approximately 30% of its maximum value. As
C
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.
Low Current Operation
The LTC1435A 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 I
the bottom MOSFET. If the voltage across R
exceed the hysteresis of I
full cycle, then on following cycles the top and bottom drives
are disabled. This continues until an inductor current peak
exceeds 20mV/R
SS
SENSE
SENSE
continues to charge, I
is chosen based on the required output current. The
Selection for Output Current
SENSE
CC
. The current comparator threshold sets
SENSE
U
and an input common mode range of
2
U
IN
detects current reversal and turns off
MAX
SENSE
and C
(Refer to Functional Diagram)
or the I
INFORMATION
2
TH
U
equal to the peak value less half
(approximately 20mV) for one
. Once R
OUT
is gradually released allowing
TH
are selected. The circuit
L
voltage exceeds 0.6V,
.
W
SENSE
SS
. When C
is known, C
SENSE
SS
TH
U
does not
reaches
voltage
B
OSC
to
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 LTC1435A 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
5V regulator is turned off and an internal switch is turned
on to connect EXTV
power to be derived from a high efficiency external source
such as the output of the regulator itself or a secondary
winding, as described in the Applications Information
section.
Allowing a margin for variations in the LTC1435A and
external component values yields:
The LTC1435A works well with R
C
The LTC1435A uses a constant frequency architecture with
the frequency determined by an external oscillator capaci-
tor C
voltage C
is charged by a fixed current. When the voltage on the ca-
pacitor reaches 1.19V, C
then repeats.
OSC
R
SENSE
CC
OSC
Selection for Operating Frequency
/EXTV
. Each time the topside MOSFET turns on, the
OSC
is reset to ground. During the on-time, C
CC
CC
100
I
MAX
power. If EXTV
Power
mV
CC
CC
+
in the LTC1435A. When the EXTV
and SENSE
to INTV
OSC
is reset to ground. The process
CC
CC
. This allows the INTV
SENSE
–
is taken above 4.8V, the
pins is below 1.4V and
LTC1435A
values 0.005 .
7
OSC
CC
CC
CC