SLUSBT8 July   2014

PRODUCTION DATA.  

  1. Features
  2. Applications
  3. Description
  4. Application Information
  5. Revision History
  6. Pin Configuration and Functions
  7. Specifications
    1. 7.1 Absolute Maximum Ratings
    2. 7.2 Handling Ratings
    3. 7.3 Recommended Operating Conditions
    4. 7.4 Thermal Information
    5. 7.5 Electrical Characteristics
    6. 7.6 Timing Requirements
    7. 7.7 Typical Characteristics
  8. Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1 Power Down
      2. 8.3.2 Power-On Reset
    4. 8.4 Device Functional Modes
      1. 8.4.1 Operation
        1. 8.4.1.1 Input Overvoltage Protection
        2. 8.4.1.2 Input Overcurrent Protection
        3. 8.4.1.3 Battery Overvoltage Protection
        4. 8.4.1.4 Thermal Protection
        5. 8.4.1.5 Enable Function
        6. 8.4.1.6 Fault Indication
  9. Application and Implementation
    1. 9.1 Typical Application Circuit
      1. 9.1.1 Design Requirements
        1. 9.1.1.1 Selection of RILIM
        2. 9.1.1.2 Selection of RBAT
        3. 9.1.1.3 Selection of R(CE), R(FAULT), and R(PU)
        4. 9.1.1.4 Selection of Input and Output Bypass Capacitors
      2. 9.1.2 Detailed Design Procedures
        1. 9.1.2.1 Powering Accessories
      3. 9.1.3 Application Curves
  10. 10Power Supply Requirements
  11. 11Layout
    1. 11.1 Layout Guidelines
    2. 11.2 Layout Example
  12. 12Device and Documentation Support
    1. 12.1 Trademarks
    2. 12.2 Electrostatic Discharge Caution
    3. 12.3 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

9 Application and Implementation

9.1 Typical Application Circuit

VOVP = 5.85 V, IOCP = 125 mA, V(BOVP) = 4.35 V.

typ_app_lusbt8.gifFigure 11.

9.1.1 Design Requirements

9.1.1.1 Selection of RILIM

The overcurrent threshold is programmed by a resistor, RILIM, connected from the ILIM pin to VSS. Figure 4 shows the OCP threshold as a function of RILIM, and may be approximated by the following equation:

Equation 1. IOCP = 25 ÷ RILIM (current in A, resistance in kΩ)

Choose a IOCP between 50 mA and 300 mA and apply the above equation to select a RILIM resistor value from 500 kΩ to 83.3 kΩ respectively. However, at lower OCP limits, approaching 50 mA, the precision of the current protection circuit decreases the achievable accuracy of the OCP threshold.

9.1.1.2 Selection of RBAT

It is strongly recommended that the battery not be tied directly to the VBAT pin of the device, as under some failure modes of the IC, the voltage at the IN pin may appear on the VBAT pin. This voltage can be as high as 30 V, and applying 30 V to the battery in case of the failure of the bq24311 can be hazardous. Connecting the VBAT pin through R(BAT) prevents a large current from flowing into the battery in case of a failure of the IC. In the interests of safety, RBAT should have a high value. The problem with a large R(BAT) is that the voltage drop across this resistor because of the VBAT bias current I(VBAT) causes an error in the V(BOVP) threshold. This error is over and above the tolerance on the nominal 4.35V V(BOVP) threshold.

Choosing RBAT in the range 100 kΩ to 470 kΩ is a good compromise. In the case of an IC failure, with RBAT equal to 100kΩ, the maximum current flowing into the battery would be (30 V – 3 V) ÷ 100 kΩ = 246 μA, which is low enough to be absorbed by the bias currents of the system components. R(BAT) equal to 100 kΩ would result in a worst-case voltage drop of R(BAT) × I(VBAT) = 1 mV. This is negligible to compared to the internal tolerance of 50mV on V(BOVP) threshold.

If the Bat-OVP function is not required, the VBAT pin should be connected to VSS.

9.1.1.3 Selection of R(CE), R(FAULT), and R(PU)

The CE pin can be used to enable and disable the IC. If host control is not required, the CE pin can be tied to ground or left un-connected, permanently enabling the device.

In applications where external control is required, the CE pin can be controlled by a host processor. As in the case of the VBAT pin (see Selection of Rbat), the CE pin should be connected to the host GPIO pin through as large a resistor as possible. The limitation on the resistor value is that the minimum VOH of the host GPIO pin less the drop across the resistor should be greater than VIH of the bq24311 CE pin. The drop across the resistor is given by R(CE) × IIH.

The FAULT pin is an open-drain output that goes low during OV, OC, battery-OV, and OT events. If the application does not require monitoring of the FAULT pin, it can be left unconnected. But if the FAULT pin has to be monitored, it should be pulled high externally through R(PU), and connected through R(FAULT) to the host. R(FAULT) prevents damage to the host controller if the bq24311 fails (see Selection of Rbat). The resistors should be of high value, in practice values between 22 kΩ and 100 kΩ should be sufficient.

9.1.1.4 Selection of Input and Output Bypass Capacitors

The input capacitor CIN in Figure 11 is for decoupling, and serves an important purpose. Whenever there is a step change downwards in the system load current, the inductance of the input cable causes the input voltage to spike up. CIN prevents the input voltage from overshooting to dangerous levels. It is strongly recommended that a ceramic capacitor of at least 1μF be used at the input of the device. It should be located in close proximity to the IN pin.

COUT in Figure 11 is also important: If a fast (< 1 μs rise time) overvoltage transient occurs at the input, the current that charges COUT causes the device’s current-limiting loop to kick in, reducing the gate-drive to FET Q1. This results in improved performance for input overvoltage protection. COUT should also be a ceramic capacitor of at least 1 μF, located close to the OUT pin. COUT also serves as the input decoupling capacitor for the charging circuit downstream of the protection IC.

9.1.2 Detailed Design Procedures

9.1.2.1 Powering Accessories

In some applications, the equipment that the protection IC resides in may be required to provide power to an accessory (that is, a cellphone may power a headset or an external memory card) through the same connector pins that are used by the adapter for charging. Figure 12 and Figure 13 illustrate typical charging and accessory-powering scenarios:

chg_flow_lusbt8.gifFigure 12. Charging - The Red Arrows Show the Direction of Current Flow
pwr_accs_lusbt8.gifFigure 13. Powering an Accessory - The Red Arrows Show the Direction of Current Flow

In the second case, when power is being delivered to an accessory, the bq24311 device is required to support current flow from the OUT pin to the IN pin.

If VOUT > V(UVLO) + 0.7 V, FET Q1 is turned on, and the reverse current does not flow through the diode but through Q1. Q1 will then remain ON as long as VOUT > V(UVLO) – V(HYS-UVLO) + RDS(on) x I(ACCESSORY). Within this voltage range, the reverse current capability is the same as the forward capability, 0.5 A. It should be noted that there is no overcurrent protection in this direction.

support_lus763.gifFigure 14.

9.1.3 Application Curves

Scope_1_slusbt8.gif
ROUT = 50 Ω
Figure 15. Normal Power-On Showing Soft-Start
Scope_5_slusbt8.gif
VIN = 5 V to 12 V tr = 20 μs
Figure 17. OVP Response for Input Step
Scope_8_slusbt8.gif
A.
OCP Counter Counts to 15 Before Switching OFF the Device
Figure 19. OCP, Powering Up into a Short Circuit on OUT Pin
Scope_10_slusbt8.gif
Figure 21. OCP, ROUT Switches from 130 Ω to 30 Ω, Shows Current Limiting and Soft-Stop
Scope_12_slusbt8.gif
Figure 23. BAT-OVP, V(VBAT) Steps from 3.9V to 4.4V,
Shows BAT-OVP Counter
Scope_2_slusbt8.gif
VIN = 0 V to 10 V tr = 50 μs
Figure 16. OVP at Power-On
Scope_7_slusbt8.gif
VIN = 15 V to 5 V tr = 400 μs
Figure 18. Recovery from OVP
Scope_9_slusbt8.gif
Figure 20. OCP, Zoom-in on the First Cycle of Figure 19
Scope_11_slusbt8.gif
Figure 22. BAT-OVP, V(VBAT) Steps from 4.3 V to 4.4 V, Shows tDGL(BAT-OVP) and Soft-Stop