LT1620_15 Datasheet, PDF (6/12 Pages) Linear Technology – Rail-to-Rail Current Sense Amplifier

LT1620/LT1621
APPLICATIONS INFORMATION
RUN
C13
0.033µF
X7R
C14 R1
1nF 1k
C17, 0.01µF
5 _IN+
IN^{– 4}
C18
0.1µF
6
V_CC
GND ³
LT1620MS8
7
PROG
I_OUT ²
R_P1
C16
8
AVG
1
SENSE
3k
0.1µF ^C15
1%
0.1µF
R_P2
15.75k
1%
R2
1.5M
C11, 56pF
C12, 0.1µF
C_OSC
RUN/SS
TG
BOOST
C9, 100pF
C10
100pF
I_TH
SW
SFB
LTC1435
V_IN
SGND
INTV_CC
V_OSENSE
SENSE^–
BG
PGND
SENSE⁺ EXTVCC
C8, 100pF
R_F2
110k
0.1%
R_F1
1.44M
0.1%
C4
0.1µF
C5, 0.1µF
D1*
D2*
C6
0.1µF
+ C1 + C2
22µF
22µF
35V
35V
V_IN
17.3V TO 28V
Si4412DY
L1
27µH
Si4412DY
R_SENSE
0.025Ω
+
C3
22µF
35V
V_BATT
16.8V
Li-ION
+
C7
4.7µF
* D1, D2: CENTRAL
SEMICONDUCTOR CMDSH-3
LT1620/21 • F02
Figure 2. LT1620/LTC1435 Battery Charger
Charge Current Programming
Output current delivered during current mode operation is
determined through programming the voltage at the PROG
pin (V_PROG). As mentioned above, optimum performance
is obtained with (V_CC – V_PROG) = 0.8V. The LT1620 is
biased with a precision 5V supply produced by the LTC1435,
enabling use of a simple resistor divider from V_CC to
ground for a V_PROG reference. Using the desired 2.5kΩ
Thevenin impedance at the PROG pin, values of R_P1 = 3k
and R_P2 = 15.75k are readily calculated. The PROG pin
should be decoupled to the V_CC supply.
Different values of charging current can be obtained by
changing the values of the resistors in the V_PROG setting
divider to raise or lower the value of the programming
voltage, or by changing the sense resistor to an appropri-
ate value as described above.
Output Float Voltage
The 3.2A charger circuit is designed for a 4-cell Li-Ion
battery, or a battery float voltage of 16.8V. This voltage is
programmed through a resistor divider feedback to the
LTC1435 V_OSENSE pin, referencing its 1.19V bandgap
voltage. Resistor values are determined through the rela-
tion: R_F1 = (V_BATT – 1.19)/(1.19/R_F2). Setting R_F2 = 110k
yields R_F1 = 1.44M.
Other Decoupling Concerns
The application schematic shown in Figure 2 employs
several additional decoupling capacitors. Due to the inher-
ently noisy environment created in switching applications,
decoupling of sensitive nodes is prudent. As noted in the
schematic, decoupling capacitors are included on the
current programming pin (PROG) to the V_CC rail and
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