BQ25070 Datasheet, PDF (11/21 Pages) Texas Instruments – 1A, Single-Input, Single-Cell LiFePO4 Linear Battery Charger with 50mA LDO

bq25070
www.ti.com
SLUSA66 – JULY 2011
Fast Charge Mode
Once V_BAT > V_LOWV, the bq25070 enters constant current (CC) mode where charge current is regulated using the
internal MOSFETs between IN and OUT. The total current is shared between the output load and the battery.
Once the battery voltage charges up to V_BAT(OVCH), the CHG output goes high indicating the charge cycle is
complete and the bq25070 switches the battery regulation voltage to V_BAT(REG). The battery voltage is allowed to
relax down to V_BAT(REG). The charger remains enabled and regulates the output to V_BAT(REG). If at any time the
battery falls below V_REC, the charge cycle restarts.
CHARGE CURRENT TRANSLATOR (IMON)
When the charger is enabled, internal circuits generate a current proportional to the charge current at the IMON
input. The current out of IMON is 1/1000 (±10%) of the charge current. This current, when applied to the external
charge current programming resistor, R1 (Figure 12), generates an analog voltage that can be monitored by an
external host to calculate the current sourced from BAT. Connect a 1kΩ resistor from IMON to GND. The voltage
at IMON is calculated as:
V_{IM ON}
= I_IN
´
1
V
A
(1)
INPUT OVER VOLTAGE PROTECTION
The bq25070 contains an input over voltage protection circuit that disables the LDO output and charging when
the input voltage rises above V_OVP. This prevents damage from faulty adapters. The OVP circuitry contains an
115μs deglitch that prevents ringing on the input from line transients from tripping the OVP circuitry falsely. If an
adapter with an output greater than V_OVP is plugged in, the IC completes soft-start power up and then shuts
down if the voltage remains above V_OVP after 115μs. The LDO remains off and charging remains disabled until
the input voltage falls below V_OVP.
UNDER-VOLTAGE LOCKOUT (UVLO)
The bq25070 remains in power down mode when the input voltage is below the under-voltage lockout threshold
(V_UVLO). During this mode, the control input (CTRL) is ignored. The LDO, the charge FET connected between IN
and OUT are off and the status output (CHG) is high impedance. Once the input voltage rises above V_UVLO, the
internal circuitry is turned on and the normal operating procedures are followed.
EXTERNAL NTC MONITORING (TS)
The bq25070 features a flexible, voltage based external battery pack temperature monitoring input. The TS input
connects to the NTC thermistor in the battery pack to monitor battery temperature and prevent dangerous
over-temperature conditions. During charging, the voltage at TS is continuously monitored. If, at any time, the
voltage at TS is outside of the operating range (V_COLD to V_HOT), charging is suspended. When the voltage
measured at TS returns to within the operation window, charging is resumed. When charging is suspended due
to a battery pack temperature fault, the CHG output remains low and continues to indicate charging.
The temperature thresholds are programmed using a resistor divider from LDO to GND with the NTC thermistor
connected to the center tap from TS to GND. See Figure 14 for the circuit example. The value of R1 and R2 are
calculated using the following equations:
R1 = ^{-R2 ´ RHOT ´ (0.125 - 1)}
0.125 ´ (R2 + RHOT)
(2)
R2 =
-RHOT ´ RCOLD ´ (0.125 - 0.250)
RHOT ´ 0.250 ´ (0.125 - 1) + RCOLD ´ 0.125 ´ (1 - 0.250)
(3)
RHOT is the expected thermistor resistance at the programmed hot threshold; RCOLD is the expected thermistor
resistance at the programmed cold threshold.
Copyright © 2011, Texas Instruments Incorporated
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