LT1239
APPLICATIONS INFORMATION
3. Protection: Regulator 1 allows the use of current limit-
ing resistors to prevent overcharging lithium-ion cells.
A low-battery detector shuts down regulator 2 and the
error amplifier to prevent over discharging the lithium
cells. An error amplifier is included to provide voltage
equalization for two series connected lithium-ion cells.
Adjusting Output Voltage
Regulator 1 is an adjustable regulator. This allows the
output voltage to be set for various battery types and
voltages. The output voltage is adjustable from 3.75V up
to 20V. The regulator will servo its output voltage in order
to maintain the adjust pin at 3.75V with respect to ground.
The output voltage is set with a resistor divider from output
to ground as shown in Figure 1. The resistor values should
be chosen so that the current in the divider is approxi-
mately 5µA. This means that the impedance from the
adjust pin to ground should be approximately 750kΩ. The
bias current at the adjust pin is 50nA (typical) and will flow
into the adjust pin. The error in the output voltage, due to
the adjust pin bias current will be equal to the bias current
multiplied by the value of R2 ( I
ADJ
×
R2). This error is small
and is compensated for in the formulas shown in Figure 1.
IN 1
OUT 1
R2
50nA
5µA
REGULATOR 1
ADJ
R1
≈
750k
V
OUT
= 3.75 1 + R2 + I
ADJ
(R2)
R1
R2 =
(V
OUT
– 3.75V)
(3.75V/R1) + I
ADJ
)
)
CHOOSE: R1 = 750k
I
ADJ
= 50nA
Figure 1. Adjusting Output Voltage
Example: To set the output voltage to 6.8V for a 2-cell
lithium-ion system, use R1 = 750k and I
ADJ
= 50nA.
Then:
R2 =
6.8V – 3.75V
= 604k
(3.75V/750k) + 50nA
8
U
W
U
U
Equalizing Lithium-Ion Cells
The error amplifier on the LT1239 is used to equalize the
cell voltages in a 2-cell lithium-ion backup system. The
error amplifier is internally connected as a unity-gain
follower and is designed to sink or source about 3mA. The
bias current for the error amplifier will be approximately
3nA and will flow out of the pin. The output voltage of the
error amplifier can be set by connecting the input to a tap
point on the resistor divider used to set the output voltage
for regulator 1 as shown in Figure 2. The error amplifier
will then equalize the cell voltages by charging the cell with
the lowest output voltage. The output voltage of regulator
1 controls the total cell voltage and the error amplifier
forces the cell voltages to be equal. The error amplifier
output current will go to zero when the cell voltages are
equal and the total cell voltage is equal to the output
voltage of regulator 1.
6.8V
IN 1
OUT 1
R2 = 604k
3.75V
R3
69.8k
3.4V
+
10µF
REGULATOR 1
ADJ
–
E/A
E/A (IN)
R1
681k
LT1239 • F02
E/A (OUT)
+
Figure 2. Equalizing Lithium-Ion Cells
LT1239 • F01
For battery voltages greater than the low-battery detection
threshold the error amplifier is active. For battery voltages
lower than the low-battery detection threshold the output
of the error amplifier is inactive. When the error amplifier
is active it can source or sink approximately 3mA. When
the error amplifier is inactive its output is a high imped-
ance, as long as it is not forced above V
IN2
or below
ground.
The error amplifier is powered from the same supply pin
as regulator 2. In most applications the backup batteries
and the output of regulator 1 will provide power to this
point. This means that the protection resistors (R4 in
Figure 5) in series with the output of regulator 2 will limit
the output current capability of the error amplifier in a
fault condition.
LINER [ LINEAR TECHNOLOGY ]
