LT1620 Linear Technology, LT1620 Datasheet - Page 8

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

LT1620/LT1621
APPLICATIONS
8
reduces battery charging current until the external load
subsides.
In Figure 4 the LT1620 is coupled with an LT1513 SEPIC
battery charger IC to create an input overcurrent protected
charger circuit.
The programming voltage (V
through a resistor divider (R
supply to ground. In this configuration, if the input current
drawn by the battery charger combined with the system
load requirements exceeds a current limit threshold of 3A,
the battery charger current will be reduced by the LT1620
such that the total input supply current is limited to 3A.
Refer to the LT1513 data sheet for additional information.
PROGRAMMING ACCURACY CONSIDERATIONS
PWM Controller Error Amp Maximum Source Current
In a typical battery charger application, the LT1620 con-
trols charge current by servoing the error amplifier output
pin of the associated PWM controller IC. Current mode
control is achieved when the LT1620 sinks all of the
current available from the error amplifier. Since the LT1620
has finite transconductance, the voltage required to gen-
erate its necessary output current translates to input
offset error. The LT1620 is designed for a typical I
current of 130 A to help reduce this term. Knowing the
current source capability of the associated PWM control-
ler in a given application will enable adjustment of the
required programming voltage to accommodate the de-
sired charge current. A plot of typical V
vs PWM source capability is shown in Figure 5a. For
example, the LTC1435 has a current source capability of
about 75 A. This translates to about –15mV of induced
programming offset at V
PROG pin must be 15mV lower).
V
The LT1620 sense amplifier circuit has an inherent input
referred 3mV offset when IN
loop operation during light load conditions. This offset vs
input voltage has a linear characteristic, crossing 0V as
IN
output (times a factor of 10), and thus to the programming
CC
+
– IN
– V
PROG
–
= 80mV. The offset is translated to the AVG
Programmed Voltage 0.8V
U
PROG
INFORMATION
U
P1
+
– IN
CC
(the absolute voltage at the
and R
– V
–
= 0V to insure closed-
W
PROG
P2
PROG
) from the 5V input
) is set to 1.0V
voltage offset
U
OUT
sink
voltage V
IN
desired load current corresponds to 100mV across the
sense resistor, the typical offset, at V
absolute voltage at the PROG pin must be 7.5mV higher).
This error term should be taken into consideration when
using V
V
(LT1620GN Only)
The offset term described above for V
V
tional factor of 2). However, V
well below the zero offset point of 1.6V, so adjustment for this
term is usually required. A plot of typical V
voltage vs IN
setting the V
typically requires an additional –50mV offset (the absolute
voltage at the PROG2 pin must be 50mV lower).
Sense Amplifier Input Common Mode < (V
The LT1620 sense amplifier has additional input offset
tolerance when the inputs are pulled significantly below
the V
referred offset of up to 11mV when the inputs are at 0V
common-mode. This additional offset term reduces roughly
linearly to zero when V
applications, this offset increases the charge current tol-
erance for “cold start” conditions until V
from ground. The resulting output current shift is generally
negative; however, this offset is not precisely controlled.
Precision operation should not be attempted with sense
amplifier common mode inputs below V
referred offset tolerance vs V
V
The LT1620 sense amplifier induces a small additional
offset when V
a linear characteristic and amounts to about 0.33mV
(input-referred) over the recommended operating range
of V
and AVG2 outputs (times factors of 10 and 20), and thus
to the programming voltages. A plot of programming
offsets vs V
CC
PROG2
CC
+
CC
– IN
– V
CC
, centered at 5V. This offset is translated to the AVG
5V
programming voltage proportionally (times an addi-
ID
PROG2
supply. The amplifier can induce additional input
–
PROG
values significantly away from 80mV.
is pictured in Figure 5b. For example, if the
PROG2
CC
+
CC
. A plot of typical V
Programmed Voltage 1.6V
– IN
is shown in Figure 5e.
moves away from 5V. This offset follows
voltage to correspond to IN
–
is pictured in Figure 5c. For example,
CM
is about V
CM
PROG2
is shown in Figure 5d.
PROG
voltage is typically set
PROG
CC
PROG
– 0.5V. In typical
offset voltage vs
BAT
CC
also affects the
+
is 7.5mV (the
– 0.5V. Input
CC
– IN
moves away
PROG2
– 0.5V)
–
= 15mV
offset