
Analog Integrated Circuit Device Data
Freescale Semiconductor
29
PC34708
Functional Block Requirements and Behaviors
Battery Management Block Description and Application Information
and then the system will power up. VBUS and VAUX are used to sense when the VBUS and AUX charge path are within range to
charge, and as an internal supply to power up internal detectors and circuitry in a dead battery case.
7.1.3
Building Blocks and Functions
The battery management interface consists of several building blocks and functions, as depicted in
Figure 5. These building
blocks and functions are described by the following, while the charger operation is described in the next section.
7.1.3.1
Buck Charger Operation
The charger buck takes its feedback from two sources, depending on the state of the battery:
MBATT present and VBATT is less than the LOWBATT threshold. MBATT will be opened and BP will be set to the VCHRCV[5:0]
settings (set to 3.6 by default) to supply the system loads. The feedback for the buck charger is taken from a conventional
internal resistor divider connected at the BP pin. This supports dead battery operation and allows the system to operate if the
battery does not exist or is being trickle charged through a separate path.
For batteries above LOWBATT[1:0] threshold, BP tracks the battery voltage level VBATT. If MBATT is present, MBATT will be
closed when the PC34708 detects when the BP voltage is at the same potential as the VBATT node. When the VBATT node is
below this threshold and MBATT is present, the feedback is taken from the BP node. When the MBATT is closed, the feedback
is taken from the BATT node. If MBATT is not populated, then the feedback node will always be taken from the BATT node.
The buck regulator core implements PWM in normal operation and auto-PFM modes PWM-PS mode during soft start. The
charger buck needs to be able to support 100% duty operation for cases when the input falls close to the output voltage. In this
mode, the high side PMOS is switched fully on, and the low side NMOS switch is disabled. The regulator will stay in 100% (or
close to it) duty-cycle mode until either BP rises above the CHRCV[5:0], under which case BP is clamped to a maximum of
CHRCV[5:0], or when the loop feedback demands a lower duty cycle. While in 100% duty cycle mode, the output current limit is
still operational. The CHRRCV is the point at which the battery charger enters the constant voltage mode from the constant
current mode. This is set to 3.6 V by default.
The buck charger can be disabled by software by setting the CHREN=0. It defaults to 1 on power up and is reset by RESETB,
so every time the PMIC goes into an off mode, the charger is re enabled. The CHRGEN signal disables linear and buck charging.
Table 12. Buck Charger Constant Voltage Settings
CHRCV[5:0]
Buck Charge
Output Voltage (V)
CHRCV[5:0]
Buck Charge
Output Voltage (V)
000000
3.50
011001
4.00
000001
3.52
011010
4.02
000010
3.54
011011
4.04
000011
3.56
011100
4.06
000100
3.58
011101
4.08
000101
3.60 (default)
011110
4.10
000110
3.62
011111
4.12
000111
3.64
100000
4.14
001000
3.66
100001
4.16
001001
3.68
100010
4.18
001010
3.70
100011
4.20
001011
3.72
100100
4.22
001100
3.74
100101
4.24
001101
3.76
100110
4.26
001110
3.78
100111
4.28
001111
3.80
101000
4.30