LTC4002_15 Datasheet, PDF (14/16 Pages) Linear Technology – Standalone Li-Ion Switch Mode Battery Charger

LTC4002
APPLICATIO S I FOR ATIO
Next, choose the P-channel MOSFET. The Si6435ADQ in
a TSSOP-8 package with R_DS(ON) = 42mΩ (nom), 55mΩ
(max) offers a small solution. The maximum power dissi-
pation with V_IN = 5V and V_BAT = 4V at 50°C ambient
temperature is:
P_D
=
(^1.5A)²
(^55m
5V
Ω₎₍4V₎
=
0.099
W
T_J = 50°C + (0.099W)(65°C/W) = 56.5°C
C_IN is chosen for an RMS current rating of about 0.8A at
85°C. The output capacitor is chosen for an ESR similar to
the battery impedance of about 100mΩ. The ripple voltage
on the BAT pin is:
( ) _VOUT(RIPPLE)
=
∆I_L(MAX)
2
ESR
= ^{(0.96A)(0.1Ω)} = 48mV
2
C1: Taiyo Yuden TMK325BJ106MM
C2: Taiyo Yuden JMK325BJ226MM
L1: TOKO B952AS-6R8N
The Schottky diode D2 shown in Figure 2 conducts current
when the pass transistor is off. In a low duty cycle case, the
current rating should be the same or higher than the
charge current. Also it should withstand reverse voltage as
high as V_IN.
Board Layout Suggestions
When laying out the printed circuit board, the following
considerations should be taken to ensure proper opera-
tion of the LTC4002.
GATE pin rise and fall times are 20ns and 50ns respectively
(with C_GATE = 2000pF). To minimize radiation, the catch
diode, pass transistor and the input bypass capacitor
traces should be kept as short as possible. The positive
side of the input capacitor should be close to the source of
the P-channel MOSFET; it provides the AC current to the
pass transistor. The connection between the catch diode
and the pass transistor should also be kept as short as
possible. The SENSE and BAT pins should be connected
directly to the sense resistor (Kelvin sensing) for best
charge current accuracy. Avoid routing the NTC PC board
trace near the MOSFET switch to minimize coupling switch-
ing noise into the NTC pin.
The compensation capacitor connected at the COMP pin
should return to the ground pin of the IC or as close to it
as possible. This will prevent ground noise from disrupt-
ing the loop stability. The ground pin also works as a heat
sink, therefore use a generous amount of copper around
the ground pin. This is especially important for high V_CC
and/or high gate capacitance applications.
CHARGE
STATUS
BAT
R1
2k
V_IN
5V TO 22V
D1
B330
C3
0.1µF
CER
2
V_CC
3
GATE
M1
Si6435ADQ
C1
10µF
CER
LTC4002ES8-4.2
5
7
CHRG SENSE
L1
6.8µH
D2
B330
1
C_C
0.47µF
R_C
2.2k
COMP
6
BAT
R_SENSE
68mΩ
NTC GND
8
4
T
10k
NTC
C2
22µF
CER
+ ^4.2V
Li-Ion
BATTERY
4002 F02
NTC: DALE NTHS-1206N02
Figure 2. 1.5A Single Cell Li-Ion Battery Charger
4002f
14