Datasheet

7
LTC1435A
(Refer to Functional Diagram)
OPERATION
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on and periodically forces a brief off period to allow C
B
to
recharge.
The main control loop is shut down by pulling the RUN/SS
pin low. Releasing RUN/SS allows an internal 3µA current
source to charge soft start capacitor C
SS
. When C
SS
reaches
1.3V, 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 released 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.
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
2
detects current reversal and turns off
the bottom MOSFET. If the voltage across R
SENSE
does not
exceed the hysteresis of I
2
(approximately 20mV) for one
full cycle, then on following cycles the top and bottom drives
are disabled. This continues until an inductor current peak
exceeds 20mV/R
SENSE
or the I
TH
voltage 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
+
and SENSE
–
pins is below 1.4V and
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
CC
/EXTV
CC
Power
Power for the top and bottom MOSFET drivers and most
of the other LTC1435A circuitry is derived from the INTV
CC
pin. The bottom MOSFET driver supply pin is internally
connected to INTV
CC
in the LTC1435A. When the EXTV
CC
pin is left open, an internal 5V low dropout regulator
supplies INTV
CC
power. If EXTV
CC
is taken above 4.8V, the
5V regulator is turned off and an internal switch is turned
on to connect EXTV
CC
to INTV
CC
. This allows the INTV
CC
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.
APPLICATIONS INFORMATION
WUU
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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
SENSE
. Once R
SENSE
is known, C
OSC
and L can be chosen. Next, the power MOSFETs and D1 are
selected. Finally, C
IN
and C
OUT
are selected. The circuit
shown in Figure 1 can be configured for operation up to an
input voltage of 28V (limited by the external MOSFETs).
R
SENSE
Selection for Output Current
R
SENSE
is chosen based on the required output current. The
LTC1435A current comparator has a maximum threshold
of 150mV/R
SENSE
and an input common mode range of
SGND to INTV
CC
. The current comparator threshold sets
the peak of the inductor current, yielding a maximum av-
erage output current I
MAX
equal to the peak value less half
the peak-to-peak ripple current ∆I
L
.
Allowing a margin for variations in the LTC1435A and
external component values yields:
R
mV
I
SENSE
MAX
=
100
The LTC1435A works well with R
SENSE
values ≥ 0.005Ω.
C
OSC
Selection for Operating Frequency
The LTC1435A uses a constant frequency architecture with
the frequency determined by an external oscillator capaci-
tor C
OSC
. Each time the topside MOSFET turns on, the
voltage C
OSC
is reset to ground. During the on-time, C
OSC
is charged by a fixed current. When the voltage on the ca-
pacitor reaches 1.19V, C
OSC
is reset to ground. The process
then repeats.