SLVSA32C – OCTOBER 2009 – REVISED MAY 2010
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Step 4. Calculate the Output Capacitor and ESR (C4)
Calculate capacitance
To calculate the capacitance of the output capacitor, minimum load current must be first determined. Typically, in
standby mode the load current is 100 A, however this really depends on the application. With this value of
To allow wider operating conditions and improved performance in low-power mode, it is recommended to use a
100 F capacitor. Higher value of output capacitor allows improved transient response during load stepping.
Calculate ESR
Using Equation 38, ESR is calculated to be, RESR < 660 m. Capacitors with lowest ESR values should be selected. To meet both the requirements, capacitance and low
ESR, several low ESR capacitors may be connected in parallel. In this example, we will select a capacitor with
ESR value as 30 m
.
Filter capacitor (C12) of value 0.1F can be added to filter out the noise in the output line.
Step 5. Calculate the Feedback Resistors (R4, R5)
To keep the quiescent current low and avoid instability problems, it is recommended to select R4 and R5 such
that, R4 + R5 ~ 250 k
.
Using
Equation 1 and using a fixed standard value of R4 = 187 k
, R5 is calculated to be, R5 = 59.84 k .
Step 6. Calculate Type 3 Compensation Components
Resistances (R6, R9)
Using Equation 19, for VINTyp = 14 V, VRamp is calculated to be, VRamp = 1.4 V. Using Equation 15, fLC is calculated to be, fLC = 2.95 kHz. Using VRamp, fLC from above, assuming fc as 1/10th of fsw and Equation 46, R6 is calculated to be, R6 = 316.96 k
.
.
Capacitors (C5, C8, C7)
Using
Equation 48, C5 is calculated to be, C5 = 340.43 pF.
Using Equation 16, fESR is calculated to be, fESR = 53.05 kHz. Using
Equation 50, C8 is calculated to be, C8 = 9.74 pF.
Using
Equation 49, C7 is calculated to be, C7 = 285.1 pF.
Step 7. Calculate Soft-Start Capacitor (C6)
The recommended value of soft-start capacitor is 100nF (typical).
Step 8. Calculate Bootstrap Capacitor (C3)
The recommended value of bootstrap capacitor is 0.1 F (typical).
Step 9. Calculate Power-On Reset Delay Capacitor (C2)
To achieve 2.2 ms delay, the reset delay capacitor can be calculated using
Equation 9 to be C2 = 2.2 nF.
Step 10. Calculate Input Capacitor (C1, C11)
Typical values for C11 are 0.1 F and 0.01 F.
Input capacitor (C1) should be rated more than the maximum input voltage (VINMax). The input capacitor should
be big enough to maintain supply in case of transients in the input line. Using
Equation 34, C1 is calculated to
be, C1 = 6.25 F. For improved transient response, a higher value of C1 such as 220 F is recommended.
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