4
DEMO MANUAL DC094
DESIGN READY SWITCHER
I TRODUCTIO
U U
The circuit in Figure 1 highlights the capabilities of the
LTC1435. The application circuit is set up for a variety of
output voltages. Output voltages from 1.8V to 5V are
available by selecting the appropriate jumper position.
The LTC1435 is a synchronous step-down switching
regulator controller which drives external N-channel power
MOSFETs using a fixed frequency architecture. Burst
Mode operation provides high efficiency at low load cur-
rents. Operating efficiencies typically exceed 90% over
three decades of load current range. A maximum high duty
cycle limit of 99% provides low dropout operation which
extends operating time in battery-operated systems.
The use of small spring-clip leads are very convenient for
small-signal bench testing and voltage measurements,
but should not be used with the high currents associated
with this circuit. Soldered wire connections are required to
properly ascertain the performance of the PC board.
This demonstration unit is intended for the evaluation of
the LTC1435 switching regulator IC and was not designed
for any other purpose.
OPERATIO
U
The operating frequency is set by an external capacitor
C
OSC
, allowing maximum flexibility in optimizing effi-
ciency. In this application the frequency is set to 170kHz.
A secondary winding feedback control pin SFB inhibits
Burst Mode which reduces noise and RF interference.
Soft start is provided by an external capacitor C
SS
which
can be used to properly sequence supplies. The operating
current level is user-programmable via an external current
sense resistor and is set to 3A. Short-circuit current limit
is set approximately to 4A.
This demo board is optimized for 3.3V outputs. A wide
input supply range allows operation from 4.5V to 28V for
V
OUT
voltages of 3.3V and 5V. Because this board allows
for a wide output voltage range (1.8V to 5V) and the
operating frequency remains constant at 170kHz, there is
a duty cycle induced limit on the maximum input voltage
when low output voltages are selected (V
OUT
< 2.9V). This
is necessary for an adequate turn-on time for the top
MOSFET with the required duty cycle at a given frequency.
If a higher input supply voltage is required together with
low output voltage, the operating frequency can be de-
creased by increasing C
OSC
.
Main Control Loop
The LTC1435 uses a constant frequency, current mode
step-down architecture. Current mode operation was
judged to be mandatory for its well-known advantages of
clean start-up, accurate current limit and excellent line and
load regulation.
During normal operation, the top MOSFET is turned on
each cycle when the oscillator sets a latch and turned off
when the main current comparator resets the latch. The
peak inductor current is controlled by the voltage on the
I
TH
pin, which is the output of error amplifier EA.
The V
OSENSE
pin 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 1.19V reference, which in turn causes the I
TH
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 or the beginning of the
next cycle.
The top MOSFET driver is biased from floating bootstrap
capacitor C4, which normally is recharged during each off
cycle. However, when V
IN
decreases to a voltage close to
V
OUT
, the loop may enter dropout and attempt to turn on
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 C4 to recharge.
A built-in comparator guards against transient overshoots
> 7.5% by turning off the top MOSFET and keeping it off
until the fault is removed.
