Linear Technology DC164 Demo Manual page 4

DEMO MANUAL DC164
U
OPERATIO
The circuit in Figure 1 highlights the capabilities of the
LTC1624 configured as a step-down switching regulator.
The application circuit is set up for a variety of output
voltages. Output voltages from 2.5V to 5V are available by
selecting the appropriate jumper position. An additional
jumper position is also available for a user-selectable
output voltage by adding the appropriate feedback divider
resistor at R5.
The LTC1624 is a current mode switching regulator con-
troller that drives an external N-channel power MOSFET
using a fixed-frequency architecture. Burst Mode opera-
tion provides high efficiency at low load currents. Operat-
ing efficiencies typically exceed 90% over two decades of
load current range. A maximum duty cycle limit of 95%
provides low dropout operation that extends operating
time in battery-operated systems.
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 LTC1624 switching regulator IC and was not designed
for any other purpose. Further detailed information and
alternate topology applications are shown in the LTC1624
data sheet.
4
I
IN
A
V
+ +
IN
V V J1
IN
–
–
GND
J2
D C B A
STEP-DOWN CONVERTER
DEMO CIRCUIT 164A
LTC1624CS
Figure 3. Proper Measurement Setup
LTC1624 CONTROLLER DESCRIPTION
Main Control Loop
The LTC1624 uses a constant-frequency, current mode
architecture. During normal operation, the top MOSFET is
turned on during each cycle when the oscillator sets a
latch, and turned off when the main current comparator
resets the latch. The peak inductor current that resets the
latch is controlled by the voltage on the I
is the output of the error amplifier. V
amplifier to receive an output feedback voltage from an
external resistive divider. When the load current increases,
it causes a slight decrease in V
reference, which in turn causes the I
increase until the average inductor current matches the
new load current. While the top MOSFET is off, an internal
bottom MOSFET is turned on for approximately 300ns to
400ns to recharge the bootstrap capacitor C
The top MOSFET driver is biased from the floating boot-
strap capacitor C , which is recharged during each off
B
cycle. The dropout detector counts the number of oscilla-
tor cycles that the top MOSFET remains on and periodi-
cally forces a brief off period to allow C
The main control loop is shut down by pulling I
below its 1.19V clamp voltage. Releasing I
an internal 2.5µA current source to charge compensation
capacitor C . When the I
C TH
I
OUT
V
A
OUT
+
V
OSENSE
V LOAD
V
OUT
–
GND
LINEAR TECHNOLOGY
(408) 432-1900
/RUN pin, which
TH
allows the error
FB
relative to the 1.19V
FB
/RUN voltage to
TH
.
B
to recharge.
B
/RUN
TH
/RUN allows
TH
/RUN pin voltage reaches 0.8V,
OPTIONAL
REMOTE V
OUT
SENSE CONNECTION