Datasheet

LTC4009
LTC4009-1/LTC4009-2
16
4009fd
applicaTions inForMaTion
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
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 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.
Figure 4. Programming PWM Current
Figure 5. Programming LTC4009 Output Voltage
13
Q1
2N7002
4009 F04
PROG
LTC4009
R
PROG
R
MAX
511k
C
PROG
0V
5V
11
9
BAT
FBDIV
95Ω
TYPICAL
10
V
FB
LTC4009
R1
R2A
R2B*
4009 F05
C
Z
*OPTIONAL TRIM RESISTOR
21
GND
(EXPOSED PAD)
+