LP3925 Datasheet, PDF (37/51 Pages) Texas Instruments – High Performance Power Management Unit for Handset Applications

Backup Battery Charger
Backup battery charger (BBC) is intended for charging an ex-
ternal coin battery. Its output is connected to VCOIN-pin. It
consists of Voltage Limited Current Source with 1kΩ output
resistor. By default it is always on. It is possible to turn backup
battery charger off via registers. VCOIN – voltage limit and
ICOIN – current source values are also programmable via
registers (the possible values are stated in Backup battery
charger selection table). BBC has a reverse current protec-
tion. VCOIN-pin voltage can be measured by an internal ADC.
MOMENTARY POWER LOSS
When power is removed from the LP3925 PMU and MPL
function is enabled (register 0x81) then Momentary Power
Loss (MPL) timer starts as unexpected power down has oc-
curred. This timer is set to the maximum duration allowed for
a momentary power loss (register 0x81). If power is restored
before the timer expires, then MPL block informs LP3925
control logic about MPL-event.
The MPL circuit is powered from VCOIN-pin. MPL timer uses
32.786 kHz RTC crystal oscillator for time base. Host proces-
sor needs to enable the MPL function every time the device
is powered up.
FIGURE 19. BBC IU Characteristics
30120419
32.768 kHz CRYSTAL OSCILLATOR
There are two options for implementing the 32.768 kHz os-
cillator:
1. 1. An external crystal that is connected between XIN and
XOUT. The external crystal ESR must not exceed 100
kΩ; if this value is exceeded the circuit may never start
oscillating.
2. 2. An external oscillator module could be used by
connecting the module output directly into XIN. When
using an external oscillator module, the XOUT pin should
be unconnected.
Pins XIN and XOUT are not able to drive external load. Os-
cillator output is buffered to pin OSC_32KHZx Note. Oscillator
is powered by LDO1, thus before it is up, the oscillator will not
start.
BACKUP BATTERY CHARGER ELECTRICAL CHARACTERISTICS
Typical values and limits appearing in normal type apply for T_J = 25°C. Limits appearing in boldface type apply over the entire
junction temperature range for operation, T_J = −40°C to +125°C. Unless otherwise specified, the following applies for V_BATT = 3.6V.
(Note 6)
Symbol
Parameter
Conditions
Typ.
Limits
Min
Max
Units
V_COIN
Voltage Limit
I_LOAD = 1µA; V_COIN programmed to
3.00V
−3
+3
%
I_COIN
R_VCOIN
I_LEAK
I_GND
Charging Current
Internal Series Resistor
Reverse leakage current
Charger Ground Current
Load Regulation
V_COIN pin shorted to GND
I_COIN programmed to 200 µA
VRTC-pin current = 0
I_LOAD = 0.
I_LOAD = 0.
−20
+20
%
1.0
0.5
1.6
kΩ
10
2.5
µA
0.5
37
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