LTC4001 Datasheet, PDF (13/20 Pages) Linear Technology – 2A Synchronous Buck Li-Ion Charger

APPLICATIO S I FOR ATIO
V_INSENSE
9
R_NOM
121k
NTC
11
0.74 • V_{INSENSE –}
+
LTC4001 NTC BLOCK
TOO COLD
R1
13.3k
R_NTC
100k
–
0.29 • V_{INSENSE +}
TOO HOT
LTC4001
+
0.02 • V_INSENSE
–
NTC ENABLE
GNDSENS
4
4001 F04
Figure 4. Extending the Delta Temperature
where R_NOM is the value of the bias resistor, R_HOT and
R_COLD are the values of R_NTC at the desired temperature
trip points. Continuing the example from before with a
desired hot trip point of 50°C:
R_NOM
=
R_COLD – R_HOT
2.815 – 0.4086
=
100k • (3.2636 – 0.3602)
2.815 – 0.4086
= 120.8k, 121k is nearest 1%
R1
=
100k
•
⎛⎝⎜
0.4086
2.815 – 0.4086
•
(3.266
–
0.3602)
–
0.3602^{⎞⎠⎟}
= 13.3k, 13.3k is nearest 1%
The final solution is as shown if Figure 4 where R_NOM =
121k, R1 = 13.3k and R_NTC = 100k at 25°C.
Input and Output Capacitors
The LTC4001 uses a synchronous buck regulator to pro-
vide high battery charging current. A 10µF chip ceramic
capacitor is recommended for both the input and output
capacitors because it provides low ESR and ESL and can
handle the high RMS ripple currents. However, some high
Q capacitors may produce high transients due to self-
resonance under some start-up conditions, such as con-
necting the charger input to a hot power source. For more
information, refer to Application Note 88.
EMI considerations usually make it desirable to minimize
ripple current in the battery leads, and beads or inductors
may be added to increase battery impedance at the 1.5MHz
switching frequency. Switching ripple current splits be-
tween the battery and the output capacitor depending on
the ESR of the output capacitor and the battery impedance.
If the ESR of the output capacitor is 0.1Ω and the battery
impedance is raised to 2Ω with a bead or inductor, only 5%
of the ripple current will flow in the battery. Similar
techniques may also be applied to minimize EMI from the
input leads.
4001f
13