LT1620 Linear Technology, LT1620 Datasheet - Page 5

• Manufacturer Part Number: LT1620
• Description: Rail-to-Rail Current Sense Amplifier
• Manufacturer: Linear Technology

APPLICATIONS
OPERATION
used as integration nodes to facilitate averaging of the
current sense amplifier signal. (Note: filter capacitors on
these pins should bypass to the V
of these signals enables direct sensing and control of DC
load current, eliminating the inclusion of ripple current in
load determination.
Transconductance Amplifier
The transconductance amplifier converts the difference
between the current programming input voltage (V
and the average current sense output (V
at the amplifier output pin (I
unidirectional and only sinks current. The amplifier is
designed to operate at a typical output current of 130 A
In Figure 2, an LT1620MS8 is coupled with an LTC1435
switching regulator in a high performance lithium-ion
battery charger application. The LTC1435 switching regu-
lator delivers extremely low dropout as it is capable of
approximately 99% duty cycle operation. No additional
power supply voltage is required for the LT1620 in this
application; it is powered directly from a 5V local supply
generated by the LTC1435. The DC charge current control
and high common mode current sense range of the
LT1620 combine with the low dropout capabilities of the
LTC1435 to make a 4-cell Li-Ion battery charger with over
96% efficiency, and only 0.5V input-to-output drop at 3A
charging current. Refer to the LTC1435 data sheet (available
from the LTC factory) for additional information on IC func-
tionality, performance and associated component selection.
This LT1620/LTC1435 battery charger is designed to yield
a 16.8V float voltage with a battery charge current of 3.2A.
The V
switch MOSFETs). The charger provides a constant 3.2A
charge current until the battery voltage reaches the pro-
grammed float voltage. Once the float voltage is achieved,
a precision voltage regulation loop takes control, allowing
the charge current to fall as required to complete the
battery charge cycle.
IN
supply can range from 17.3V to 28V (limited by the
U
U
(Refer to the Functional Block Diagram)
INFORMATION
U
OUT
). The amplifier output is
CC
W
supply.) Integration
AVG
) into a current
U
PROG
)
with V
tions, the I
loop on the mated PWM controller IC to maintain a
programmed load current.
Comparator
The comparator circuit (available only in the LT1620GN)
may be used as an end-of-cycle sensor in a Li-Ion battery
charging system. The comparator detects when the charg-
ing current has fallen to a small value (typically 20% of the
maximum charging current). The comparator drives an open
collector output (MODE) that pulls low when the V
voltage is more positive than V
the programmed threshold).
R
The LT1620 will operate throughout a current program-
ming voltage (V
V
current setting program voltage of – 0.8V, corresponding
to 80mV differential voltage across the current sense
amplifier inputs. Given the desired current requirement,
selection of the load current sense resistor R
possible. For the desired 3.2A charge current;
At the programmed 3.2A charge current, the sense resis-
tor will dissipate (0.08V)(3.20A) = 0.256W, and must be
rated accordingly.
Current Sense
The current sense inputs are connected on either side of
the sense resistor with IN
given average charging current flow. The sense resistor to
IN
pair or minimum PC trace spacing for noise immunity.
Keep lead lengths short and away from noise sources for
best performance.
SENSE
CC
+
R
, IN
), however, optimum accuracy will be obtained with a
SENSE
AVG
–
Selection
input paths should be connected using twisted
= V
OUT
= 80mV/3.2A or 0.025
PROG
current is used to servo the current control
PROG
. In typical PWM/charger type applica-
) range of 0V to – 1.25V (relative to
+
at the more positive potential,
PROG2
LT1620/LT1621
(output current below
SENSE
AVG2
5
is