LTC4012-2 Linear Technology Corporation, LTC4012-2 Datasheet - Page 17

• Manufacturer Part Number: LTC4012-2
• Description: Multi-Chemistry Battery Charger
• Manufacturer: Linear Technology Corporation

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The value of R
current which can be programmed (Q1 continuously on).
PWM of the Q1 gate voltage changes the value of R
to produce lower currents. The frequency of this modula-
tion should be higher than a few kHz, and C
increased to reduce the ripple caused by switching Q1. In
addition, it may be necessary to increase loop compensa-
tion 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
current will be proportional to the duty cycle of the PWM
input on the gate of Q1.
Programming LTC4012 Output Voltage
Figure 5 shows the external circuit for programming the
charger voltage when using the LTC4012. The voltage is
then governed by the following equation:
APPLICATIONS INFORMATION
V
BAT
=
1 2085
Figure 5. Programming Output Voltage
*OPTIONAL TRIM RESISTOR
.
PROG
PROG
TYPICAL
85Ω
LTC4012
(EXPOSED PAD)
or other compensation values.Charge
V R
controls the maximum value of charge
R
•
2
(
1
FBDIV
+
GND
BAT
V
FB
R
2
11
10
21
)
9
,
R
2
R2A
R1
=
R2B*
R A R B
4012 F05
2
C
Z
+
+
PROG
2
must be
PROG
See Table 2 for approximate resistor values for R2.
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
such as R1 and R2 tolerance, FBDIV R
impedance are not included in the specifi cations given in
the Electrical Characteristics. This leads to the possibil-
ity that very accurate (0.1%) external resistors might be
required. Actually, the temperature rise of the LTC4012 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
improve transient response and loop stability. A value of
10pF is normally adequate.
Table 2. Programming Output Voltage
*To Obtain Desired Accuracy Requires Series Resistors For R2.
R
1
V
12.3
12.6
16.4
16.8
20.5
24.6
25.2
(V)
4.1
4.2
8.2
8.4
=
21
BAT
R
2
⎛
⎜
⎝
1 2085
LTC4012-1/LTC4012-2
.
FB
V
BAT
sense input. Note that sources of error
R1 (0.25%)
(kΩ)
V
165
167
162
169
301
294
284
271
316
298
298
397
– ,
1
⎞
⎟
⎠
R
2
R2A (0.25%)
=
R A R B
(kΩ)
67.3
28.4
32.8
31.2
22.6
19.8
18.2
15.4
2
69
28
21
20
LTC4012/
+
ON
2
or V
R2B (1%)*
200
(Ω)
FB
–
–
–
–
–
–
–
–
–
–
–
17
Z
input
may
4012f