BQ51025 Datasheet, PDF (12/49 Pages) Texas Instruments – Compliant Single Chip Wireless Power Receiver

bq51025
SLUSBX7B – SEPTEMBER 2014 – REVISED MARCH 2015
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9.3 Feature Description
9.3.1 Dynamic Rectifier Control
The Dynamic Rectifier Control algorithm offers the end-system designer optimal transient response for a given
maximum output current setting. This is achieved by providing enough voltage headroom across the internal
regulator (LDO) at light loads in order to maintain regulation during a load transient. The WPC system has a
relatively slow global feedback loop where it can take up to 150 ms to converge on a new rectifier voltage target.
Therefore, a transient response depends on the loosely-coupled transformer's output-impedance profile. The
Dynamic Rectifier Control allows for a 1.5-V change in rectified voltage before the transient response is observed
at the output of the internal regulator (output of the bq51025 device). A 1-A application allows up to a 2-Ω output
impedance. Figure 13 shows the Dynamic Rectifier Control behavior during active power transfer.
9.3.2 Dynamic Power Scaling
The Dynamic Power Scaling feature allows for the loss characteristics of the bq51025 device to be scaled based
on the maximum expected output power in the end application. This effectively optimizes the efficiency for each
application. This feature is achieved by scaling the loss of the internal LDO based on a percentage of the
maximum output current. Note that the maximum output current is set by the K_ILIM term and the R_ILIM resistance
(where R_ILIM = K_ILIM / I_ILIM). The flow diagram in Figure 13 shows how the rectifier is dynamically controlled
(Dynamic Rectifier Control) based on the voltage level at the ILIM pin (V_ILIM). This voltage represents a fixed
percentage of the I_ILIM setting. Table 1 summarizes how the rectifier behavior is dynamically adjusted based on
two different R_ILIM settings. Table 1 is shown for I_MAX, which is the maximum operating output current and is
typically lower than I_ILIM (about 20% lower). See RILIM Calculations for more details on how to set I_ILIM.
Table 1. Dynamic Rectifier Regulation⁽¹⁾
Output Current
Percentage
(Low-Power Mode)
Output Current
Percentage
(Proprietary Mode)
0 to 10%
10 to 20%
20 to 40%
>40%
0 to 5%
5 to 10%
10 to 20%
>20%
Low Power (5-W)
Mode
R_ILIM = 700 Ω
I_ILIM = 0.6 A
(I_MAX = 0.5 A)
0 to 0.05 A
0.05 to 0.1 A
0.1 to 0.2 A
>0.2 A
Low Power (5-W)
Mode
R_ILIM = 700 Ω
I_ILIM = 1.2 A
(I_MAX = 1 A)
0 to 0.05 A
0.05 to 0.1 A
0.1 to 0.2 A
>0.2 A
Proprietary 10-W
Mode
R_ILIM = 495 Ω
I_ILIM = 1.7 A
(I_MAX = 1.4 A)
0 to 0.070 A
0.070 to 0.14 A
0.14 to 0.28 A
>0.28 A
V_RECT ⁽²⁾
V_OUT + 2.0
V_OUT + 1.6
V_OUT + 0.6
V_OUT + 0.12
(1) R_OS = Open. The relation between V_ILIM and I_LIM has some dependency on the R_OS value.
(2) V_RECT is regulated to a maximum of 11 V.
Table 1 shows the shift in the Dynamic Rectifier Control behavior based on the two different R_ILIM settings. With
the rectifier voltage (V_RECT) as the input to the internal LDO, this adjustment in the Dynamic Rectifier Control
thresholds dynamically adjusts the power dissipation across the LDO where,
  P_DIS V_RECT  V_OUT ˜I_OUT
(1)
Figure 22 shows how the Dynamic Power Scaling feature reduces the V_RECT with increased load, allowing the
post-regulation LDO to have maximum headroom at low load conditions for better load transient performance
and minimal power dissipation at high loads. Note that this feature balances efficiency with optimal system
transient response.
9.3.3 VO_REG Calculations
The bq51025 device allows the designer to set the output voltage by setting a feedback resistor divider network
from the OUT pin to the VO_REG pin, as seen in Figure 7. Select the resistor divider network so that the voltage
at the VO_REG pin is 0.5 V (default setting) at the desired output voltage. The target VO_REG voltage can be
changed through I²C by changing Table 4
12
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