8.2.2.5 Dropout

With a constant 1.4-MHz switching frequency, there is a minimum input voltage limit for a given output voltage to be regulated. This is due to the minimum off time limit. If the input voltage less than the minimum input voltage limit, the output voltage drops accordingly, which is called dropout condition. Figure 7 and Figure 8 show the typical dropout curve for 3.3 V and 5 V output voltage with 3 A and 1.5 A load respectively. Equation 7 can be used to estimate this minimum input voltage limit.

Equation 7.

where

• V_OUT = target output voltage
• F_SW = maximum switching frequency including tolerance
• t_off(min) = minimum off time including tolerance
• R_dsl = low side FET on resistance
• R_dsh = high side FET on resistance
• R_L = inductor DC resistance
• I_O = maximum load current
• t_d1 = dead time between high side FET off and low side FET on, 15nS typical
• t_d2 = dead time between low side FET off and high side FET on, 10nS typical
• V_d = forward voltage of low side FET body diode