LTC4071
8
4071fc
operaTion
In cases where the input supply may be shorted to GND
when not supplying power, for example with a solar cell, add
a diode in series with R
IN
to prevent the input from loading
the battery. For more information, refer to the photovoltaic
charger example in the Applications Information section.
Adjustable Float Voltage, V
FLOAT
A built-in 3-state decoder connected to the ADJ pin pro-
vides three programmable float voltages: 4.0V, 4.1V, or
4.2V. The float voltage is programmed to 4.0V when ADJ
is tied to GND, 4.1V when ADJ is floating (disconnected),
and 4.2V when ADJ is tied to V
CC
. The state of the ADJ
pin (and NTC pins) is sampled for about 36µs about once
every 1.2 seconds when HBO is high, and when HBO is
low the sampling rate reduces to about once every 3.6
seconds with the same duty cycle. If V
CC
falls below
V
LBD
, the sampling stops. When it is being sampled, the
LTC4071 applies a relatively low impedance voltage at the
ADJ pin. This technique prevents low level board leakage
from corrupting the programmed float voltage.
NTC Qualified Float Voltage, ∆V
FLOAT(NTC)
The NTC pin voltage is compared against an internal
resistor divider tied to the NTCBIAS pin. This divider has
tap points that are matched to the NTC thermistor resis-
tance/temperature conversion table for a Vishay curve 2
thermistor at temperatures of 40°C, 50°C, 60°C, and 70°C.
The curve 2 thermistor is also designated by a B25/85
value of 3490.
Battery temperature conditioning adjusts the float voltage
down to V
FLOAT_EFF
when the NTC thermistor indicates
that the battery temperature is too high. For a 10k curve 2
thermistor and a 10k NTCBIAS resistor, each 10°C increase
in temperature above 40°C causes the float voltage to drop
by a fixed amount, ∆V
FLOAT(NTC)
, depending on ADJ. If ADJ
is at GND, the float voltage steps down by 50mV for each
10°C temperature increment. If ADJ is floating, the step
size is 75mV. And if ADJ is at V
CC
, the step size is 100mV.
Refer to Table 1 for the range of V
FLOAT_EFF
programming.
Table 1. NTC Qualified Float Voltage
ADJ ∆V
FLOAT(NTC)
TEMPERATURE
V
NTC
AS % OF
NTCBIAS V
FLOAT_EFF
GND 50mV T < 40°C
40°C ≤ T < 50°C
50°C ≤ T < 60°C
60°C ≤ T < 70°C
70°C < T
V
NTC
> 36.5
29.0 < V
NTC
≤ 36.5
22.8 < V
NTC
≤ 29.0
17.8 < V
NTC
≤ 22.8
V
NTC
≤ 17.8
4.000
3.950
3.900
3.850
3.800
Floating 75mV T < 40°C
40°C ≤ T < 50°C
50°C ≤ T < 60°C
60°C ≤ T < 70°C
70°C < T
V
NTC
> 36.5
29.0 < V
NTC
≤ 36.5
22.8 < V
NTC
≤ 29.0
17.8 < V
NTC
≤ 22.8
V
NTC
≤ 17.8
4.100
4.025
3.950
3.875
3.800
V
CC
100mV T < 40°C
40°C ≤ T < 50°C
50°C ≤ T < 60°C
60°C ≤ T < 70°C
70°C < T
V
NTC
> 36.5
29.0 < V
NTC
≤ 36.5
22.8 < V
NTC
≤ 29.0
17.8 < V
NTC
≤ 22.8
V
NTC
≤ 17.8
4.200
4.100
4.000
3.900
3.800
For all ADJ pin settings the lowest float voltage setting is:
3.8V = V
FLOAT_MIN
= V
FLOAT
– 4 • ∆V
FLOAT(NTC)
.
This occurs at NTC thermistor temperatures above 70°C,
or if the NTC pin is grounded.
To conserve power in the NTCBIAS and NTC resistors, the
NTCBIAS pin is sampled at a low duty cycle at the same
time that the ADJ pin state is sampled.
High Battery Status Output: HBO
The HBO pin pulls high when V
CC
rises to within V
HBTH
of the programmed float voltage, V
FLOAT_EFF
, including
NTC qualified float voltage adjustments assuming V
CC
has risen above V
LBC_VCC
.
If V
CC
drops below the float voltage by more than V
HBTH
+
V
HBHY
the HBO pin pulls low to indicate that the battery is
not at full charge. The input supply current to the LTC4071
drops to less than 550nA (typ) as the LTC4071 no longer
shunts current to protect the battery. And the NTCBIAS
sample clock slows to conserve power.
For example, if the NTC thermistor requires the float voltage
to be dropped by 100mV (ADJ = V
CC
and 0.29•V
NTCBIAS
< V
NTC
< 0.36•V
NTCBIAS
) then the HBO rising threshold
is detected when V
CC
rises past:
V
FLOAT
– ∆V
FLOAT(NTC)
– V
HBTH
= 4.2V – 100mV – 40mV = 3.96V.
