Detailed explanation of the BQ25618/9 dedicated switch charging chip for TWS true wireless headset charging compartment

alan000345 · Published on 2019-10-31 09:19

Detailed explanation of the BQ25618/9 dedicated switch charging chip for TWS true wireless headset charging compartment [Copy link]

BQ25618/9 is a three-in-one (protection, charging and boost) IIC control switch charging chip specially developed by TI for TWS earphone charging case.

BQ25618 and BQ25619 have the same specifications. The difference is that BQ25618 uses a miniaturized DSBGA package with a pin spacing of 0.4mm, which has higher requirements for the production process. BQ25619 uses a slightly larger WQFN package to facilitate circuit layout. The package size of the device is shown in Figure 1 below.

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We understand this chip from the following four perspectives:

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Figure 2

1: Buck charging function:

a) The input operating voltage range supports 4-13.5V, and the instantaneous surge voltage can support up to 22V, which can easily support 5V, 9V, and 12V working systems.

b) Input overvoltage is detected by the VAC pin. The default OVP value is 14.2V. Through IIC, there are four levels of OVP value adjustment: 5.7 V/6.4 V/11 V/14.2, which can be flexibly adjusted according to actual needs.

c) IIC programming sets the input over-current protection point, the range can be from 100mA to 3.2A, and the minimum step current is 100mA.

d) Dynamic Power Management (DPM-Dynamic Power Management), when the input current exceeds the input overcurrent protection point, the input voltage gradually decreases. When the set voltage drop value is reached, the default value is 4.5V (3.9V-5.4V, can be set using IIC, minimum step 100mV), the system begins to reduce the charging current to ensure that the input voltage remains at the set value. The DPM function can effectively prevent the entire system from being paralyzed due to insufficient load capacity of the input adapter.

e) With automatic input detection function, when it detects that the input adapter is inserted, the system will automatically shut down the built-in boost circuit.

2: Boosting function:

a) When the adapter is plugged in, the PMID can still be used to charge the headset. At this time, the voltage is the input voltage and the boost stops working. PMID_GOOD is designed for input overvoltage and overcurrent protection.

b) PMID pin is a boost output pin, with a maximum output current of 1A, and the output voltage supports 4 levels (4.6V/4.75V/5V/5.15V) IIC adjustable, which is convenient for reducing the output voltage according to actual needs and reducing losses. When there is only battery, the built-in boost starts to work and outputs the corresponding set voltage through PMID.

c) PMID_GOOD pin is the detection pin of PMID voltage. When PMID voltage rises to 3.8V, PMID_GOOD level changes from low to high, indicating that the boost system is working properly. If overvoltage (over 5.8V) or overcurrent (over 6A) occurs on the battery side, PMID_GOOD will change from high to low. According to PMID_GOOD indication, it is easy to drive the external PMOS to cut off the boost system when overvoltage or overcurrent occurs, thereby adding double protection to the system. The external protection circuit can be shown in Figure 3.

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Figure 3

3 : Output Introduction

a) The maximum charging current is 1.5A, and it has a charging cut-off current of 20mA step (20mA-1.5A programmable) and a step charging voltage of 10mV, which can make the battery in the charging compartment charge more fully.

b) In order to compensate for the accuracy of the cut-off current and allow the battery to be charged more fully, BQ25618/9 adds a TOP-OFF Timer function, as shown in the four red boxes in the figure below. After the system detects that the charging is cut off, the charging time can be selectively increased through IIC (0, 15min, 30min, 45min).

c) The QON pin supports shipping mode. In shipping mode, the leakage current can be as low as 7uA.

d) The optimized path management function can provide power to the system even when the battery is over-discharged or disconnected.

e) The typical value of the static current at room temperature is 9.5uA, which is more than 70% lower than the leakage current of the switch charging chip. It can greatly increase the standby time when powered by batteries.

f) It has system overvoltage protection, battery overvoltage and overcurrent protection, battery over-discharge protection, in addition to the TS pin overtemperature protection, there are also complete protection mechanisms such as chip internal overtemperature protection (150C shutdown).

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Figure 4

4 : Introduction to PCB wiring

BQ25619 optimizes the location of the pins, making the input to output more concise. The following is the PCB layout guide for BQ25619:

a) The input capacitor should be as close to the PMID pin and GND pin as possible and connected using the shortest trace. Add a 1nF small size (such as 0402 or 0201) decoupling capacitor to remove high frequency noise and improve EMI.

b) The inductor input pin should be placed as close to the SW pin as possible. Minimize the copper area of the trace to reduce the electric and magnetic field radiation.

Note that the routing must meet the maximum current (see the power loop indicated by the yellow arrow in Figure 5). Do not use multiple layers for SW routing to prevent parasitic capacitance from causing excessive peak voltage at the switch moment.

c) Place the output capacitor close to the inductor and chip, and the grounding needs to be connected to the IC ground with the shortest trace.

d) Separate the small signal ground from the power ground, ground at a single point, or use a 0 resistor to connect the small signal ground to the power ground.

e) Decoupling capacitors must be placed close to the IC pins using the shortest wires possible.

f) Solder the exposed thermal pad on the back of the IC to the PCB, making sure there are adequate thermal vias directly under the IC and connected to the ground plane to help dissipate heat.

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Figure 5

Here are the references for BQ25619 :

http://www.ti.com.cn/cn/lit/ug/sluuc27a/sluuc27a.pdf

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