SLUSDB2A August 2018 – December 2021 UCC28951
PRODUCTION DATA
The resistor R_{EF} from the DELEF pin to GND along with the resistor divider R_{AEFHI} from CS pin to ADELEF pin and R_{AEF} from ADELEF pin to GND sets equal delays T_{AFSET} and T_{BESET} between outputs OUTA or OUTB going low and related output OUTF or OUTE going low Figure 7-4. The total resistance of this resistor divider should be in the range between 10 kΩ and 20 kΩ.
These delays gradually increase as function of the CS signal from T_{AFSET1}, which is measured at V_{CS} = 0.2 V, to T_{AFSET2}, which is measured at V_{CS} = 1.8 V. This is opposite to the DELAB and DELCD behavior and this delay is longest (T_{AFSET2}) when the signal at CS pin is maximized and shortest (T_{AFSET1}) when the CS signal is minimized. This approach will reduce the synchronous rectifier MOSFET body diode conduction time over a wide load current range thus improving efficiency. The ratio between the longest and shortest delays is set by the resistor divider R_{AEFHI} and R_{AEF}. If CS and ADELEF are tied, the ratio is maximized. If ADELEF is connected to GND, then the delay is fixed, defined only by resistor R_{EF} from DELEF to GND.
The delay time T_{AFSET} is defined by the following Equation 6. Equation 6 also defines the delay time T_{BESET}.
where
Equation 6 is an empirical approximation of measured data, thus, there is no unit agreement in it. As an example, assume R_{EF} = 15 kΩ, CS = 1 V and K_{EF} = 0.5. Then the T_{AFSET} is going to be 41.7 ns. K_{EF} is defined as Equation 7:
R_{AEF} and R_{AEFHI} define the portion of voltage at pin CS applied to the pin ADELEF (see Figure 8-3). K_{EF} defines how significantly the delay time depends on CS voltage. K_{EF} varies from 0, where ADELEF pin is shorted to ground (R_{AEF} = 0) and the delay does not depend on CS voltage, to 1, where ADELEF is tied to CS (R_{AEFHI} = 0).
The allowed resistor range on DELEF, R_{EF} is 13 kΩ to 90 kΩ.
The plots in Figure 7-5 and Figure 7-6 show delay time settings as function of CS voltage and K_{EF} for two different conditions: R_{EF} = 13 kΩ (Figure 7-5) and R_{EF} = 90kΩ (Figure 7-6)