LTC4009-2_15 Datasheet, PDF (16/28 Pages) Linear Technology – High Efficiency, Multi-Chemistry Battery Charger

LTC4009
LTC4009-1/LTC4009-2
Applications Information
LTC4009
PROG
13
R_PROG
R_MAX
511k
5V
C_PROG
Q1
0V
2N7002
4009 F04
Figure 4. Programming PWM Current
compensation capacitance connected to ITH to maintain
stability or prevent large current overshoot during start-
up. Selecting a higher Q1 PWM frequency (≈10kHz) will
reduce the need to change C_PROG or other compensation
values.Charge current will be proportional to the duty cycle
of the PWM input on the gate of Q1.
Programming LTC4009 Output Voltage
Figure 5 shows the external circuit for programming the
charger voltage when using the LTC4009. The voltage is
then governed by the following equation:
V_BAT
=
1.2085V • (R1+
R2
R2) _,
R2
=
R2A
+
R2B
See Table 2 for approximate resistor values for R2.
R1=
R2^{}
V_BAT
1.2085V
–
1^{}
,
R2
=
R2A
+
R2B
Selecting R2 to be less than 50k and the sum of R1 and
R2 at least 200k or above, achieves the lowest possible
error at the V_FB sense input. Note that sources of error
such as R1 and R2 tolerance, FBDIV R_ON or V_FB input
impedance are not included in the specifications given in
the Electrical Characteristics. This leads to the possibil-
ity that very accurate (0.1%) external resistors might be
95Ω
TYPICAL
BAT
11
FBDIV
9
LTC4009
V_{FB 10}
GND
(EXPOSED PAD) ²¹
*OPTIONAL TRIM RESISTOR
R1
C_Z
+
R2A
R2B*
4009 F05
Figure 5. Programming LTC4009 Output Voltage
required. Actually, the temperature rise of the LTC4009 will
rarely exceed 50°C at the end of charge, because charge
current will have tapered to a low level. This means that
0.25% resistors will normally provide the required level of
overall accuracy. Table 2 gives recommended values for
R1 and R2 for popular lithium-ion battery voltages. For
values of R1 above 200k, addition of capacitor C_Z may
improve transient response and loop stability. A value of
10pF is normally adequate.
Table 2. Programming LTC4009 Output Voltage
V_BAT VOLTAGE R1 (0.25%) R2A (0.25%) R2B (1%)*
4.1V
165k
69.0k
–
4.2V
167k
67.3k
200
8.2V
162k
28.0k
–
8.4V
169k
28.4k
–
12.3V
301k
32.8k
–
12.6V
294k
31.2k
–
16.4V
284k
22.6k
–
16.8V
271k
21.0k
–
20.5V
316k
19.8k
–
21.0V
298k
18.2k
–
24.6V
298k
15.4k
–
25.2V
397k
20.0k
–
*To obtain required accuracy requires series resistors for R2.
4009fd
16