ISL81801 Bi-Directional Operation

Question

Hello, 
 I have an application that allowing current flow in both directions in a buck/boost fashion. It includes a battery and a charger/load. I have a few questions related to this: 
 - Is it recommended to have the battery on the input side or the output side of the ISL81801 circuit? 
 - How does the ISL81801 regulate forward vs reverse current? The spec sheet isn't very clear on how it would know which way the current should flow. My application needs to regulate input CV/CC as well as output CV/CC (not all simultaneously), so how does the chip know what takes precendence? For example - if I need the battery voltage to stay at or below 30v, and the battery charge current to not go above 20A, how do I set the other CV/CC values so that the battery regulation is correct? 
 Please let me know if something is unclear or needs clarification. 
 Thank you in advance, 
 Ryan M.

AK_Renesas · Accepted Answer

Hi Ryan, 
 Thank you for your inquiry regarding the ISL81801, related to battery connections and current direction. Here is the answer. 
 The placement of the battery in your bidirectional buck-boost design with the ISL81801 depends on your application&rsquo;s requirements for power flow and regulation. Here&rsquo;s an explanation to help clarify the reasoning behind placing the battery on either the input or output side of the circuit: 
 Appendix A: 
 1. Battery on the Input Side 
 When the battery is on the input side , the ISL81801 will manage power as follows: 
 
 Boost Mode for Discharging : The ISL81801 will boost the battery voltage (e.g., 12V from a battery) up to a higher voltage to supply the load. For example, if your load requires 48V, the controller will increase (boost) the battery&rsquo;s lower voltage to 48V. 
 Buck Mode for Charging : When the system provides power to the battery for charging, the ISL81801 will reduce (buck) the incoming higher voltage to charge the battery at a lower voltage. 
 
 This configuration is useful when: 
 
 You want the battery to directly supply the load at a higher voltage. 
 The power source charges the battery when needed, and the battery provides power when the source is unavailable. 
 
 2. Battery on the Output Side 
 When the battery is placed on the output side , the ISL81801 will control power flow as follows: 
 
 Buck Mode for Charging : The controller reduces (bucks) a higher input voltage down to the battery&rsquo;s charging voltage (e.g., from 48V to 12V) and controls the current to charge the battery safely. 
 Boost Mode for Discharging : When the load draws power, the ISL81801 will draw power from the battery and boost the battery&rsquo;s voltage if necessary to meet the load&rsquo;s requirements. 
 
 This configuration is useful when: 
 
 You need direct control of the battery&rsquo;s charge and discharge. 
 You want the battery to supply power only to specific loads or during certain conditions, while also managing charging directly. 
 
 How to Decide Where to Place the Battery

Battery on Input Side : Choose this if the battery acts as a primary input power source and you need to boost its voltage to power higher-voltage systems.

Battery on Output Side : This is preferred when the battery is meant to be managed as a chargeable storage system, where the system regulates both the charge (buck) and the discharge (boost) of the battery.

Appendix B: 
 To ensure that the battery voltage stays at or below 30V and the battery charge current does not exceed 20A , you will need to configure the ISL81801 for both constant voltage (CV) and constant current (CC) regulation. Here's how to approach it: 
 Key Steps to Set the CV and CC Values:

Set the Constant Voltage (CV) for the Battery : The ISL81801 uses feedback from the output to regulate the voltage. To keep the battery voltage at or below 30V, you&rsquo;ll set up a voltage feedback loop using a resistive divider. 
 
 FB_OUT Pin : This is the pin that monitors the output voltage for regulation. 
 Resistor Divider : Use a resistor divider from the battery&rsquo;s positive terminal to the FB_OUT pin to set the desired output voltage (30V in this case).

Set the Constant Current (CC) for the Battery Charge : The ISL81801 has current sensing for both the input and output, which allows for current regulation. To limit the battery charge current to 20A, you&rsquo;ll need to configure the current sense resistors .

ISEN+ and ISEN- Pins : These pins sense the output current using an external sense resistor.

Sense Resistor Selection : The value of the sense resistor Rsense ​ determines the current limit. 
 
 The differential voltage between ISEN+ and ISEN- is typically 51mV for constant current.

Input CV/CC Configuration : If you're also concerned with regulating the input voltage and current (e.g., from a charger or power supply), the ISL81801 allows for independent input-side regulation using the FB_IN and CS+ / CS- pins. 
 
 FB_IN Pin : Set a voltage divider here to regulate the input voltage, similar to the output. 
 CS+ and CS- Pins : These pins handle current sensing for the input. You can use another sense resistor to limit the input current if required.

Prioritizing Battery CV/CC Regulation : In your case, where the battery regulation is the most critical (i.e., keeping the battery voltage below 30V and the charge current under 20A): 
 
 Configure the output-side feedback (FB_OUT) and current sense (ISEN+ and ISEN-) to prioritize battery regulation. 
 The ISL81801 will reduce the output current if the battery reaches its maximum voltage (30V), maintaining a constant voltage while regulating current.

If you have any further query or need more detailed equations or component suggestions, then kindly give me an update. 
 Regards, 
 Avinash Kumar

AK_Renesas · Answer

Hi Ryan, 
 Thank you for your response. Here is the detailed explanation for your question. 
 1. Bidirectional Current Feedback on CS+/- Pins: 
 The ISL81801 does indeed have the capability to sense current bidirectionally using the CS+/- pins . This is essential in applications where current can flow in both directions, such as in buck/boost converters with bidirectional energy flow (e.g., battery charging/discharging). 
 
 2. Adjusting Voltage and Current Setpoints with a Microcontroller: 
 To adjust the voltage and current setpoints dynamically with a microcontroller, the recommended approach is to use digital potentiometers (digipots) or DACs (Digital-to-Analog Converters) controlled by the microcontroller. You can feed these into the feedback pins of the ISL81801 to change the voltage and current limits. 
 
 If you have any further query, please feel free to ask. 
 
 Thanks, and regards, 
 Avinash Kumar

AK_Renesas · Answer

Hi Ryan, 
 For detailed product specifications on digital potentiometers, you can refer to the following link: 
 Renesas Digital Potentiometers Product Selector 
 This resource will help you select the appropriate digital potentiometer for your application.

Thanks, and regards, 
 Avinash Kumar

ISL81801 Bi-Directional Operation

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