Low-Battery Detection - Linear Technology DC193 Demo Manual

OPERATIO

LOW-BATTERY DETECTION

The low-battery detector senses the input voltage through
an external resistor divider. This divided voltage connects
to the (–) input of a voltage comparator (LB
compared to an internal 1.25V reference voltage. Neglect-
ing LB input bias current, the following expression is
IN
used for setting the trip voltage threshold:
V = 1.25
LB_TRIP
The LB is an N-channel open-drain that goes low when
OUT
the battery voltage drops below the threshold voltage. In
shutdown, the comparator is disabled and LB
high impedance state. Figure 3 is a schematic diagram
detailing the low-battery comparator connection and
operation.
V
IN
R
LB2
1%
LB
C R
FILTER LB1
0.01µF 1%
Figure 3. Low-Battery Comparator
HOW TO MEASURE VOLTAGE REGULATION
When measuring voltage regulation, remember that all
measurements must be taken at the point of regulation,
that is, where the LTC1626's control loop looks for the
information to keep the output voltage constant. This
information occurs between Pin 7 and Pin 11 of the
LTC1626. These points correspond to the output termi-
nals of the demonstration board. Test leads should be
attached to these terminals. Measurements should not be
taken at the end of test leads at the load. Refer to Figure 4
for proper monitoring equipment configuration.
Arrow.com. Arrow.com. Arrow.com. Arrow.com. Arrow.com.
Downloaded from Downloaded from Downloaded from Downloaded from Downloaded from
U
) )
R
LB2
1 +
R
LB1
LTC1626
+
1.25V
IN
–
) and is
IN
V
IN
LB
OUT
LB
IN
SHDN
is in the
OUT
When performing line or load regulation tests, always look
at the input voltage across the input terminals. For the
purposes of these tests, the demonstration circuit should
be fed from a regulated DC bench supply, so additional
variations on the DC input do not add errors to the
regulation measurements.
LB
OUT
RIPPLE MEASUREMENT
To measure output ripple voltage, it is best to measure
directly across the output terminals.
As in the previous regulation tests, the supply must be a
DC193 • F03 regulated DC source so that the ripple on the input to the
circuit under test does not add to the output ripple, causing
errors in the measurement.
The technique used to measure the output ripple is also
important. Here is a list of things to do and not do when
using a scope probe:
1. DO NOT USE THE GROUND LEADS/CLIPS THAT ARE
ATTACHED TO THE SCOPE PROBE!
2. DO ATTACH THE SHIELD OF THE PROBE BODY TO THE
NEGATIVE SIDE OF THE OUTPUT CAPACITOR! DO NOT
USE WIRE!
3. DO PUT THE TIP OF THE SCOPE PROBE DIRECTLY
ON THE POSITIVE TERMINAL OF THE OUTPUT
CAPACITOR
4. DO NOT USE A PROBE WHOSE BODY IS NOT COM-
PLETELY SHIELDED.
DEMO MANUAL DC193
NO DESIGN SWITCHER
+
A
+
V
GND
V
LTC1626
OUT
SINGLE CELL
LI-ION, STEP-DOWN
CONVERTER
DC193
GND
Figure 4. Proper Measurement Setup
+
A
+
V
+
LOAD
DC193 • F04
5