LT1620 Linear Technology, LT1620 Datasheet - Page 7

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

APPLICATIONS
between the IN
supply rails is also imperative in these types of circuits, as
large current transients are the norm. Power supply
decoupling should be placed as close as possible to the
ICs, and each IC should have a dedicated capacitor.
Design Equations
Sense resistor: R
Current limit programming voltage:
V
Voltage feedback resistors:
R
End-of-Cycle Flag Application
Figure 3 illustrates additional connections using the
LT1620GN, including the end-of-cycle (EOC) flag feature.
The EOC threshold is used to notify the user when the
required load current has fallen to a programmed value,
usually a given percentage of maximum load.
The end-of-cycle output (MODE) is an open-collector pull-
down; the circuit in Figure 3 uses a 10k pull-up resistor on
the MODE pin, connected to V
The EOC flag threshold is determined through program-
ming V
sponds to 20 times the voltage across the sense amplifier
inputs.
PROG
F1
Figure 3. End-of-Cycle Flag Implementation with LT1620GN
/R
F2
= V
PROG2
= (V
CC
BATT(FLOAT)
. The magnitude of this threshold corre-
CONNECTED AS IN FIGURE 2
– [(10)(V
+
SENSE
I
V
MODE
IN
OUT
EE
–
and IN
SENSE
LT1620GN
U
PROG2
ID
PROG
= V
AVG2
–
AVG
– 1.19)/1.19
V
INFORMATION
)]
IN
CC
inputs. Effective decoupling of
U
+
ID
/I
CC
MAX
.
W
R3
10k
+
END-OF-CYCLE
(ACTIVE LOW)
C2
3.3 F
C1, 3.3 F
R1
5.5k
R2
50k
U
LT1620/21 • F03
As mentioned in the previous circuit discussion, the
charging current level is set to correspond to a sense
voltage of 80mV. The circuit in Figure 3 uses a resistor
divider to create a programming voltage (V
0.5V. The MODE flag will therefore trip when the charging
current sense voltage has fallen to 0.5V/20 or 0.025V.
Thus, the end-of-cycle flag will trip when the charging
current has been reduced to about 30% of the maximum
value.
Input Current Sensing Application
Monitoring the load placed on the V
system is achieved by placing a second current sense
resistor in front of the charger V
useful for systems that will overstress the input supply
(wall adapter, etc.) if both battery charging and other
system functions simultaneously require high currents.
This allows use of input supply systems that are capable
of driving full-load battery charging and full-load system
requirements, but not simultaneously. If the input supply
current exceeds a predetermined value due to a combina-
tion of high battery charge current and external system
demand, the input current sense function automatically
22 F
5V
+
RUN
1
2
3
4
7
6
4
Figure 4. Input Current Sensing Application
SENSE
I
GND
IN
V
S/S
GND
OUT
IN
–
GND
TAB
LT1620MS8
8
LT1513
+
V
22 F
C
PROG
1
V
AVG
V
V
IN
SW
I
0.1 F
X7R
C1
1 F
CC
FB
FB
+
8
7
6
5
5
2
3
0.22 F
24
R1
0.033
L1B
10 H
4.7 F
LT1620/LT1621
R
3k
1%
R
12k
1%
IN
P1
P2
R
0.1
L1A
10 H
MBRS340
IN
SENSE
input. This function is
1620/21 • F04
TO
SYSTEM LOAD
supply of a charging
C2
1 F
57k
6.4k
CC
+
–V
22 F
V
BATT
2
PROG2
= 12.3V
7
Li-ION
)of