8.2.1 Application Curves

VIN = 10 V	VDD = 5 V
ƒSW = 130 kHz	LSW = 6 µH	Duty Cycle = 50%

Figure 3. Power Loss vs Output Current

VIN = 10 V	VDD = 5 V
ƒSW = 130 kHz	LSW = 6 µH	Duty Cycle = 50%

Figure 5. Safe Operating Area – PCB Horizontal Mount ^{LFM: Linear Feet per Minute (Air Flow Velocity)}

LFM: Linear Feet per Minute (Air Flow Velocity)

VIN = 10 V	VDD = 5 V
ISW = 5 A	LSW = 6 µH	Duty Cycle = 50%

Figure 7. Normalized Power Loss vs Frequency

	VIN = 10 V	VDD = 5 V	ISW = 5 A
	ƒSW = 130 kHz	Duty Cycle = 50%

Figure 9. Normalized Power Loss vs Output Inductance

VIN = 10 V	VDD = 5 V
ƒSW = 130 kHz	LSW = 6 µH	Duty Cycle = 50%

Figure 4. Power Loss vs Temperature

VIN = 10 V	VDD = 5 V
ƒSW = 130 kHz	LSW = 6 µH	Duty Cycle = 50%

Figure 6. Typical Safe Operating Area

ISW = 5 A	VDD = 5 V
ƒSW = 130 kHz	LSW = 6 µH	Duty Cycle = 50%

Figure 8. Normalized Power Loss vs Input Voltage

1. The Typical bq500101 System Characteristic curves are based on measurements made on a PCB design with dimensions of 4.0 inches (W) × 3.5 inches (L) × 0.062 inch (T) and 6 copper layers of 1-oz. copper thickness. See the System Example section for detailed explanation.

8.3.1 Power Loss Curves

MOSFET centric parameters such as ON-resistance and gate charges are primarily needed by engineers to estimate the loss generated by the devices. In an effort to simplify the design process for engineers, Texas Instruments has provided measured power loss performance curves. Figure 3 plots the power loss of the bq500101 as a function of load current. This curve is measured by configuring and running the bq500101 as the circuit shown in Figure 10. The measured power loss is the bq500101 device power loss which consists of both input conversion loss and gate drive loss. Equation 1 is used to generate the power loss curve.

The power loss curve in Figure 3 is measured at the maximum recommended junction temperature of
T_J = 125°C under isothermal test conditions.

Figure 10. Power Loss Test Circuit

8.3.2 Safe Operating Area (SOA) Curves

The SOA curves in the bq500101 datasheet give engineers guidance on the temperature boundaries within an operating system by incorporating the thermal resistance and system power loss. Figure 5 and Figure 6 outline the temperature and airflow conditions required for a given load current. The area under the curve dictates the safe operating area. All the curves are based on measurements made on a PCB design with dimensions of 4.0" (W) x 3.5" (L) x 0.062" (T) and 6 copper layers of 1-oz. copper thickness.

8.3.3 Normalized Curves

The normalized curves in the bq500101 data sheet give engineers guidance on the Power Loss and SOA adjustments based on their application specific needs. These curves show how the power loss and SOA boundaries will adjust for a given set of systems conditions. The primary Y-axis is the normalized change in power loss and the secondary Y-axis is the change is system temperature required in order to comply with the SOA curve. The change in power loss is a multiplier for the Power Loss curve and the change in temperature is subtracted from the SOA curve.

8.3.3.1 Calculating Power Loss and SOA

The user can estimate product loss and SOA boundaries by arithmetic means (see the Design Example below). Though the Power Loss and SOA curves in this datasheet are taken for a specific set of test conditions, the following procedure will outline the steps engineers should take to predict product performance for any set of system conditions.

8.3.3.1.1 Design Example

Operating Conditions: Output Current (l_SW) = 9 A, Input Voltage (V_IN ) = 8 V, Switching Frequency (ƒ_SW) = 300 kHz, Output Inductor (L_SW) = 5 µH, Duty Cycle = 50%.

8.3.3.1.2 Calculating Power Loss

• Typical Power Loss at 9 A = 1.78 W (Figure 3)
• Normalized Power Loss for switching frequency ≈ 1.03 (Figure 7)
• Normalized Power Loss for input voltage ≈ 0.96 (Figure 8)
• Normalized Power Loss for output inductor ≈ 1.075 (Figure 9)
• Final calculated Power Loss = 1.78 W × 1.03 × 0.96 × 1.075 ≈ 1.89 W

8.3.3.1.3 Calculating SOA Adjustments

• SOA adjustment for switching frequency ≈ 0.20°C (Figure 7)
• SOA adjustment for input voltage ≈ –0.30°C (Figure 8)
• SOA adjustment for output inductor ≈ 0.60°C (Figure 9)
• Final calculated SOA adjustment = 0.2 + (–0.3) + 0.6 ≈ 0.5°C

Figure 11. Power Stage bq500101 SOA, T_A = 25°C

In the design example above, the estimated power loss of the bq500101 would increase to 1.89 W. In addition, the maximum allowable board and/or ambient temperature would have to decrease by 0.5°C. Figure 11 graphically shows how the SOA curve would be adjusted accordingly.

1. Start by drawing a horizontal line from the application current to the SOA curve.
2. Draw a vertical line from the SOA curve intercept down to the board/ambient temperature.
3. Adjust the SOA board/ambient temperature by subtracting the temperature adjustment value.

In the design example, the SOA temperature adjustment yields a reduction in allowable board/ambient temperature of 0.5°C. In the event the adjustment value is a negative number, subtracting the negative number would yield an increase in allowable board/ambient temperature.