8.7 Input Voltage Limitations

Although the high switching frequency has advantages when it comes to the requirement of external passive components and possible transient response, when operating in these conditions, there are certain limitations that need to be considered when designing the application:

Dropout operation
Minimum on-time

To determine the two ends of the operating range, Equation 8-7 can be used.

Equation 8-7. Input Voltage Range

$V_{I N} = \frac{V_{O U T} + I_{O U T} \times (R_{D C R} + R_{D S O N}) + V_{FW} \times (1 - D)}{D}$

Where:


V_OUT	=	Output voltage
I_OUT	=	Output current
L	=	Inductance value
R_DCR	=	DCR of the selected inductor
R_DSON	=	On-resistance of the internal high-side switch
D	=	Duty cycle
V_FW	=	Forward voltage drop of the diode

To determine the minimum input voltage required for proper regulation, the duty cycle must be replaced by the maximum duty cycle; whereas, to determine the maximum voltage, the duty cycle must be calculated to avoid going below the minimum on-time.

Figure 8-1. Input Voltage Range vs. Output Voltage