Out3; Vout (E3); Vosense (E4) - Linear Technology DC164 Demo Manual

DEMO MANUAL DC164
U
OPERATIO
response. Compensation is provided by R
operating current level is user-programmable via an exter-
nal current sense resistor (R
circuit current limit is set to approximately 4A.
This demo board is optimized for 3.3V outputs. A wide
input supply range allows operation from 4.8V to 28V for
V voltages of 3.3V and 2.5V. For 5V outputs the
OUT
minimum input voltage is 5.4V at full load.
The lowest operating input voltage is limited by the exter-
nal MOSFET M1. For operation below 4.8V, subthreshold-
level MOSFETs should be substituted. The minimum input
voltage of the LTC1624 is 3.5V.
Remote Output Voltage Sensing
Remote output voltage sensing can be accomplished by
externally connecting a sense lead from V
to the load. To prevent the output from overshooting in
case of a sense-lead fault, a 10Ω resistor (R6) is con-
nected on the printed circuit board across the V
V terminals. This prevents V
OSENSE
Connect the external load only to V
surface mount 10Ω resistor mentioned above cannot
handle the load current that would pass though it should
the load be incorrectly connected to V
How to Measure Voltage Regulation
When trying to measure voltage regulation, remember
that all measurements must be taken at the point of
regulation. This point is where the LTC1624's control loop
looks for the information to keep the output voltage
constant. In this demonstration board this information
point occurs between the signal ground and the output
side of R1. These points correspond to the GND (E5) and
V (E4) terminals of the board. Output voltage test
OSENSE
leads should be attached directly to these terminals. The
load should be placed across V
Measurements should not be taken at the end of test leads
at the load; refer to Figure 3 for the proper monitoring
equipment configuration.
This applies to line regulation (input to output voltage
regulation) as well as load regulation tests. In doing line
regulation tests always look at the input voltage across the
input terminals.
6
and C . The
C C
) and is set to 3.0A. Short-
S
directly
OSENSE
and
OUT
from floating.
OSENSE
, not to V . The
OUT OSENSE
.
OSENSE
(E3) to GND (E5).
OUT
For the purposes of these tests the demonstration circuit
should be fed from a regulated DC bench supply so
additional variation on the DC input does not add an error
to the regulation measurements.
Output Voltage Programming
The jumper (J2) selects the output voltage according to
Table 1. Output voltages of 5V, 3.3V, 2.5V and one user
programmable output voltage are jumper selectable. Re-
sistor R5 (see Figure 1) is left unstuffed so a user select-
able output voltage can easily be programmed.
The output voltage is set by a resistive divider according
to the following formula (refer to Figure 1):
 
R 1
= +
 
V 1 19 1 . V
OUT
 
RX
R1 is set to 35.7k; jumper J2 selects the value of RX. If no
jumpers are in place for J2 or if only jumper J2D is selected
without a resistor in place for R5, the output voltage will be
1.19V (since the equivalent value of RX will be infinite). Be
careful not to exceed the output capacitor's maximum
voltage rating of 10V when selecting R5.
At high input-to-output differential voltages, the on-time
becomes very small. Due to internal gate delays and
response times of the internal circuitry, the minimum
recommended on-time is 450ns. Because this board
allows for a wide output voltage range and the operating
frequency remains constant at 200kHz, a potential duty
cycle limitation exists when low output voltages are
selected (V < 2.5V). When the duty cycle is less than
OUT
9%, cycle skipping may occur; this increases the inductor
ripple current but does not cause V
Avoiding cycle skipping imposes a limit on the input
voltage for a given output voltage only when V
V = 11.1V
IN(MAX) OUT
Modification For 5A Output Current
The DC164 demo board has provisions for higher output
currents. Additional pad locations are available for adding
one extra input and output capacitor together with a larger
footprint for a Schottky diode. The following list shows the
to lose regulation.
OUT
< 2.2V.
OUT
+ 5V For DC > 9%.