LTC4012-3_15 Datasheet, PDF (17/28 Pages) Linear Technology – High Efficiency, Multi-Chemistry Battery Charger with PowerPath Control

LTC4012-3
Applications Information
The value of R_PROG controls the maximum value of charge
current which can be programmed (Q1 continuously on).
PWM of the Q1 gate voltage changes the value of R_PROG
to produce lower currents. The frequency of this modula-
tion should be higher than a few kHz, and C_PROG must be
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_PROG or other compensation values. Charge
current will be proportional to the duty cycle of the PWM
input on the gate of Q1.
Programming LTC4012-3 Output Voltage
Figure 5 shows the external circuit for programming the
charger output voltage. The voltage is then governed by
the following equation:
V_BAT
=
1.2085V • ₍R1+
R2
R2₎
,R2
=
R2A
+
R2B
85Ω
TYPICAL
BAT
11
FBDIV
10
LTC4012-3
V_FB 9
GND
(EXPOSED PAD) 21
R1
C_Z
+
R2A
R2B*
*OPTIONAL TRIM RESISTOR
4012-3 F05
Figure 5. Programming Output Voltage
See Table 2 for approximate resistor values for R2.
R1= R2^{}_1.2V^B₀^A₈^T_5V – 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 im-
pedance are not included in the specifications given in the
Electrical Characteristics. This leads to the possibility that
very accurate (0.1%) external resistors might be required.
Actually, the temperature rise of the LTC4012-3 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 Output Voltage
V_BAT
R1 (0.25%) R2A (0.25%)
(V)
(k Ω)
(k Ω)
4.1
165
69
4.2
167
67.3
8.2
162
28
8.4
169
28.4
12.3
301
32.8
12.6
294
31.2
16.4
284
22.6
16.8
271
21
20.5
316
19.8
21
298
18.2
24.6
298
15.4
25.2
397
20
R2B (1%)*
(Ω)
–
200
–
–
–
–
–
–
–
–
–
–
*To Obtain Desired Accuracy Requires Series Resistors For R2.
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