Datasheet
17
4012fa
LTC4012/
LTC4012-1/LTC4012-2
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 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:
V
V R R
R
R R A R B
BAT
=
+
( )
= +
1 2085 1 2
2
2 2 2
. •
,
See Table 2 for approximate resistor values for R2.
R R
V
R R A R B1 2
1 2085
1 2 2 2=
= +
V
BAT
.
– ,
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
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
Z
may
improve transient response and loop stability. A value of
10pF is normally adequate.
Table 2. Programming Output Voltage
V
BAT
(V)
R1 (0.25%)
(kΩ)
R2A (0.25%)
(kΩ)
R2B (1%)*
(Ω)
4.1 165 69 –
4.2 167 67.3 200
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 –
*To Obtain Desired Accuracy Requires Series Resistors For R2.
Figure 5. Programming Output Voltage
11
10
BAT
FBDIV
85Ω
TYPICAL
9
V
FB
LTC4012
R1
R2A
R2B*
4012 F05
C
Z
21
GND
(EXPOSED PAD)
*OPTIONAL TRIM RESISTOR
+