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ZHCSNG6 November 2021 UCC28781-Q1

PRODUCTION DATA

8.2.2.4 Calculation of BUR Pin Resistances

Referring back to Section 7.3.1, it is recommended that ABM is entered at no higher than 50% to 60% of full load. Equation 1 and Equation 2, or Equation 1 and Equation 4, provide two equations for calculating two unknowns for the BUR-pin resistor values. However, choose the target values of V_CST(BUR), ΔV_BUR(AAM), and ΔV_BUR(LPM) first. Because the ratio of I_BUR(AAM) to I_BUR(LPM) is fixed at 1.852 (5 µA / 2.7 µA), it is necessary to target ΔV_BUR(AAM) = 185 mV to ensure that ΔV_BUR(LPM) = 100 mV, per guidance in Section 7.3.1.

The procedure to determine the value of V_CST(BUR) is quite complex and is not provided in this datasheet. Instead, a soon-to-be-released UCC28781 Excel Calculator Tool automatically calculates this value based on user input and determines the V_BUR target voltage V_{BUR_tgt}. Using this target value, it further determines the appropriate values for R_BUR2 and R_BUR1 to meet the BUR pin targets based on user selections for the following set of equations.

Expected values are used to determine recommended resistances, then actual resistances are selected from standard value series and the resulting actual voltages are calculated from the selected resistor values. Actual voltage results should be close to the targeted values.

Calculate expected ΔV_BUR(LPM) value based on ΔV_BUR(AAM) target value.

Equation 38.

{∆ V}_{B U R (L P M)} = I_{B U R (L P M)} \times (\frac{R_{B U R 1} \times R_{B U R 2}}{R_{B U R 1} + R_{B U R 2}})

Calculate the expected value for the parallel combination of R_BUR1 with R_BUR2.

Equation 39.

R_{B U R 1} | | R_{B U R 2} = \frac{{∆ V}_{B U R (A A M)}}{I_{B U R (A A M)}}

Calculate the recommended value for R_BUR1 and choose a standard 1% tolerance value for R_{BUR1_act} that is close to the recommended value.

Equation 40.

R_{B U R 1_r e c} = \frac{{∆ V}_{B U R (A A M)}}{I_{B U R (A A M)}} \times (\frac{V_{R E F}}{V_{B U R_t g t} + {∆ V}_{B U R (A A M)}})

Calculate the recommended value for R_BUR2 using R_{BUR1_act} and choose a standard 1% tolerance value for R_{BUR2_act} that is close to the recommended value.

Equation 41.

R_{B U R 2_r e c} = R_{B U R 1} \times [\frac{(V_{B U R_t g t} + {∆ V}_{B U R (A A M)})}{V_{R E F} - (V_{B U R_t g t} + {∆ V}_{B U R (A A M)})}]

Calculate the actual values for V_BUR, ΔV_BUR(AAM), and ΔV_BUR(LPM) using R_{BUR1_act} and R_{BUR2_act}.

Equation 42.

V_{B U R_a c t} = V_{R E F} \times (\frac{R_{B U R 2}}{R_{B U R 1} + R_{B U R 2}}) - I_{B U R (A A M)} \times (\frac{R_{B U R 1} \times R_{B U R 2}}{R_{B U R 1} + R_{B U R 2}})

Equation 43.

{∆ V}_{B U R (A A M)} = I_{B U R (A A M)} \times (\frac{R_{B U R 1} \times R_{B U R 2}}{R_{B U R 1} + R_{B U R 2}})

Equation 44.

{∆ V}_{B U R (L P M)} = I_{B U R (L P M)} \times (\frac{R_{B U R 1} \times R_{B U R 2}}{R_{B U R 1} + R_{B U R 2}})

Finally, verify that the total summation of the BUR voltage with hysteresis does not exceed the BUR-pin upper clamp voltage of 2.4 V.

Equation 45.

V_{B U R_a c t} + {∆ V}_{B U R (A A M)} + {∆ V}_{B U R (L P M)} \leq 2.4 V