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AN-CM-246 Using Dialog's GreenFET Load Switches in Super Capacitor Applications

Contents

Terms and Definitions

ACL Active Current Limit
IC Integrated Circuit
PCB Printed Circuit Board

References

  1. SLG59M1717V, Datasheet, Dialog Semiconductor.
  2. AN-1068 GreenFET and High Voltage GreenFET Load Switch Basics, Application Note, Dialog Semiconductor

Author: Andrii Hrypa and Petro Zeykan

Introduction

Capacitance is the ability of a system to store an electric charge (Figure 1). Capacitance value is the ratio of the charge accumulated on conductive plates to the applied voltage:

Figure 1: Two Charged Conductive Plates Separated with Dielectric Layer

The capacitance of a parallel plate system is proportional to the plate area S and inversely proportional to the distance d between these two conductive plates.

where

ε - an absolute permittivity of the dielectric material;

S - a square of conductive plate;

d - a distance between two conductive plates.

A component designed to store electric charge is called capacitor. In electronic circuits, capacitors can be used in a variety of different ways: coupling and decoupling effects, smoothing output signal of rectifiers, as a timing element, etc.

A capacitor which can store a great amount of charge is called a supercapacitor or ultracapacitor. Supercapacitor's electrodes are made of the materials that have a high surface area. This allows to hold much more charge without increasing capacitor's size.

Supercapacitors have an electrolyte inside and when the electrodes are charged, the electrolyte polarizes according to the charge of the electrodes (Figure 2). This charge separation creates two oppositely charged layers. Such system behaves like two conventional capacitors attached in series. These capacitors have plate separation distances that are a few nanometers thick.

Figure 2: A Supercapacitor Structure and Charge Distribution

The combination of the increased surface area and the small plate separation distance increases the capacitance and the energy stored in the supercapacitor.

Design Description

In this application note, we will describe how to apply Dialog’s nFET SLG59M1717V load switch to power up into super-capacitor or very high capacitive loads. To implement a very large capacitive load, (3) 5 V, 0.22 F supercapacitors were connected in parallel. The SLG59M1717V is a high-performance, 4 mΩ, 5 A single-channel, feature-rich nFET load switch designed for all 0.8 V to 5.5 V power rail applications. In addition to its world-class RDSON, the SLG59M1717V incorporates two-stage overcurrent protection: (1) resistor-adjustable Active Current Limit and (2) a fixed 1.6 A Short-circuit Current Limit.

A typical connection of Dialog's SLG59M1717V in capacitive/resistive load applications is illustrated in Figure 3.

Figure 3: Typical Application Diagram of SLG59M1717V

Start-up Inrush Current Considerations

When a voltage is applied to a discharged (or an uncharged) capacitor, a large initial current may flow through a GreenFET load switch. This current is called inrush current and can be calculated by equation below:

where

C - is the total load capacitance;

dV
dt
- the load switch VOUT slew rate during voltage ramp up.

To reduce inrush current for a given load capacitance, it is necessary to decrease the VOUT slew rate of the load switch. An equivalent circuit of the SLG59M1717V’s slew rate control loop with capacitors at its VIN and VOUT pins is shown in Figure 4:

Figure 4: SLG59M1717V’s Equivalent Slew-rate Control Loop Circuit

For a desired VOUT slew rate (VOUT(SR)), a corresponding CSLEW value is selected. At the VOUT pin and with ON = LOW, the internal FET is OFF, VOUT is initially at 0 V, and there is no stored charge on CLOAD. When a low-to-high transition is applied to the IC’s ON pin, an internal current source (I1) is enabled which, in turn, charges the external slew rate capacitor, CSLEW. The SLG59M1717V’s internal micropower op amp sets the circuit’s VOUT(SR) based on the slew rate of the nodal voltage at its non-inverting pin (the voltage at the CAP pin).

Capacitive Load Operation

In Figure 5, a typical SLG59M1717's power up behavior for resistive and capacitive load is illustrated. As can be observed, output voltage and current through the GreenFET load switch are changing linearly.

Figure 5: Typical Power up Operation Waveform for VDD = VIN = 4 V, RSET = 40 kΩ, CSLEW = 10 nF,
RLOAD = 100 Ω, CLOAD = 10 µF

In this example, a 10 nF CSLEW capacitor sets the slew rate to approximately 1.1 V/ms, thus inrush current is only 11 mA. In using a 0.66F supercapacitor, the resultant inrush current is dramatically increased and a non-linear ramp is observed during power up (see Figure 6). Since the inrush current is higher than the GreenFET load switch's IACL threshold (set by external RSET resistor), the load switch's internal overcurrent protection is activated. To keep the load capacitor's charging current from exceeding the load switch’s IACL threshold, the ACL monitor increases the FET resistance. However, if the overload condition persists where the die temperature rises because of the increased FET resistance, the GreenFET load switch’s internal Thermal Shutdown Protection circuit will be activated. If the die temperature exceeds 150 °C the load switch's internal Thermal Shutdown Protection circuit shuts the FET completely OFF, thereby allowing the die to cool. When the die cools to the load switch's lower 130 °C temperature threshold, the FET is automatically turned back on. This process may repeat as long as the output current overload condition persists.

According to the above, charging supercapacitors with high currents leads to continuously switching ON/OFF load switch and a "saw" appears on the VOUT (see Figure 6). To charge up such a large load capacitor, a 2 µF capacitor was chosen for CSLEW. It sets the slew rate to 5.75 V/s and thus limits the inrush current to 3.8 A (see Figure 7).

Even if CSLEW is chosen and protection circuits are not triggered during power up, we recommend to check package-case temperature as well. If during power up, the package case temperature rises higher than 20 °C to 30 °C, we recommend increasing CSLEW value until case temperature rise is less than 30°C (or lower), otherwise frequent overheating cycles may impact the load switch's long-term reliability.

Figure 6: Active Current Limit and Thermal Protection Operation During Power Up on High Capacitive Load Caused by Improper GreenFET Load Switch Slew Rate Settings for VDD = VIN = 4 V, IACL = 5.7 A, RSET = 40 kΩ, CSLEW = 10 nF, CLOAD = 0.66 F, RLOAD = 100 Ω
Figure 7: Typical Power Up for High Capacitive Load with Proper GreenFET Load Switch Slew Rate Settings for VDD = VIN = 4 V, IACL = 5.7 A, RSET = 40 kΩ, CSLEW = 2 µF, CLOAD = 0.66 F, RLOAD = 100 Ω

Conclusion

Charging high capacitive loads without any control may cause large inrush currents which can severely damage a load switch as well as any circuits downstream of the load switch. In addition, the charging of large capacitive loads may cause frequent over-heating cycles that can affect any load switch's long-term reliability. To solve this problem, Dialog Semiconductor offers a wide range of high-performance nFET load switches with at least one or more protection features like adjustable inrush current control, active current limit, thermal shutdown protection. For more information on Dialog’s family of load switches with advanced features, please, visit www.dialog-semiconductor.com